Daphniopsis australis is a small euryhaline cladoceran. D. australis is a cladoceran species found in lakes of southeastern Australia . Lakes in these regions have drastic fluctuation of temperature and salinity due to dry, hot weather and water evaporation in summer (Timms, 2007). This makes the availability of D. australis highly seasonal, with abundance high in spring, low in winter and sparse in summer and autumn (Campbell, 1994). Genus Daphniopsis is widely distributed in saline waters in Asia, Australia, North America and South America (Ismail et al., 2011). Brackish waters with salinities of 4-30 PSU have reported to be the best salinity range where Daphniopsis australis can be found (Sergeev & Williams, 1985).
1.2 Life cycle of Daphniopsis australis
D. australis population was resulting from the progenies of cyclic parthenogenesis consisting of an alternation of an asexual phase and a sexual phase (Ismail et al., 2011). Studies by Ismail et al., (2011) have reported that in the rearing environment, the asexual phase occurrs in a condition similar to the stock culture. Female individuals are only produced during the asexual phase. D. australis females reach maturity after 6-7 days (from birth to sexual maturation) with four to five juvenile instars. Moreover, adult parthenogenetic females can reproduce approximately 10-12 clutches in the life time with embryonic development of 3-4 days. On top of reaching the senescent stage, the reproduction usually ceases, and the female continue to live with an empty brood chamber until death. Parthenogenetic female of D. australis can live up to 20-30 days but on a rare occasion, some of females have life span up to 60 days. Sexual phase occurred under unfavourable culture conditions, and was triggered by low food density and overcrowding when temperature, salinity and photoperiod were set up as a constant variables at 20-22 °C, 22-23 PSU and 12 h light and 12 h dark (Ismail et al.,2011). Throughout the sexual phase, parthenogenetic females produce a brood carrying either diploid males or females and some of these females are able to transform into sexual females. The sexual female carrying an ephippium is called an ephippial female. Both the male and ephippial female individuals were involved in a mating process, and the fertilized sexual eggs would then develop into diapausing eggs. The ephippial female of D. australis could produce a maximum of two diapausing eggs which were encased in an ephippium and detached from the mother during moulting. Ephippial females usually are formed in the parthenogenetic mode, and an ephippium was produced before the sexual eggs were released and only appeared for a short period because they would die soon after releasing the ephippium. Thus, the lifespan of ephippial females is much shorter than that of the parthenogenetic females (Ismail et al., 2011).
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Figure 1.0 : The life cycle of most cladoceran species Harris et al. (2012)
2.0 Feeding of cladoceran on microalgae
2.1 Ingestion rates
A study done by Pagano (2008) on two species of cladoceran; Diaphanosoma excisum and Moina micrura have indicated that D. excisum ingested only the small particles (Monoraphidium, Chlorella) while M. micrura fed efficiently on a wide ranges size of phytoplankton particles, from unicellular picoplankton Chlorella sp. (2-4 Âµm equivalent spherical diameter, ESD) to a large Coelastrum reticulatum coenobia (20-40 Âµm ESD), but the selectivity depended on the nature and size distribution of the phytoplankton. The ingestion rates for M. micrura and and D. excisum displayed a classic linear increase of their specific ingestion rates; increasing algal concentration will increase the ingestion rates of the cladoceran species until it reaches the saturation point and then the rate remain relatively constant. Furthermore, another study done by Katechakis and Stibor (2004) on three species of marine cladoceran (Penilia avirostris, Podon intermedius and Evadne nordmanni ) have reported that P. intermedius reached highest grazing coefficients at food sizes between 2.5 and < 135 Âµm (diatoms and dinoflagellates). In contrast, E. nordmanni filtered the entire food size range offered <210Âµm, but preferred organisms between 7.5 and <15Âµm (Prymnesiophyceae and different kinds of unidentified nanoplankton) and particles between 100 and <150 Âµm long (diatoms, 5-15 Âµm in diameter). Ingestion rates increased linearly with increasing food supply for P. avirostris until a concentration threshold (around 270 Âµg C -1, equivalent to 2.8 mm3 1-1), while ingestion rates for P. intermedius and E. nordmanni at 0.4 mm3 1-1 were 0.8Â±0.2 and 0.9 Â± 0.2 Âµg C ind. -1 day -1 respectively. This is supported by Mueller-Navarra et al. (2000), who reported a size range of food organisms from 4 to 115 Âµm for marine cladocerans in general. Previous studies on the brackish cladoceran, Daphniopsis australis have showed that D. australis are able to ingest particles with the following size range of 3-10 Âµm which is the three common algae in aquaculture, i.e., Tetraselmis suecica, Nannochloropsis oculata and Isochrysis tahitian (Ismail et al., 2011a). The size range falls in the range studied by most researchers on cladoceran food size selectivity (Ismail et al., 2011; Pagano, 2008 and Katechakis and Stibor,2004).
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2.2 Grazer preference
Phytoplankton has been shown to be a valuable food source for Daphnia, a congener of Daphniopsis, in both temperate and polar latitudes (Atienza et al., 2007). Size-selective filter-feeding by Daphnia is primarily determined by the structure and mesh width of the filtration apparatus and the size of the animal. Thus, the ability to utilize a particular food source differs among species and also size classes within a species. Studies have shown that one of the cladoceran species, Daphnia magna could grazed more efficiently on Clamydomonas sajao than Chlorella pyrenoidosa regardless of the cell abundance of each alga in the algal assemblage (Yin et al., 2010). Compared to Chlorella, Clamydomonas was assumed to be a more adequate food item in respect of size, digestibility and handling time. First, previous studies have revealed that intersetular distance of limbs ranged from 3.4 to 8.7 Âµm in adult Daphnia (Porter et al., 1983; Hartmann & Kunkel, 1991), indicating that small-sized Chlorella (3.7 Âµm) in this study may approach the lower limits of filterability of D. magna and are more difficult to collect than large-sized Clamydomonas. Second, Clamydomonas cells have two flagella, which are assumed to help them adhere to the feeding appendages of Daphnia during the collection process (Knisely & Geller, 1986), while smooth walled Chlorella may slip through feeding limbs of Daphnia. Thirdly, Chlorella usually has thick cell walls while Clamydomonas often do not have sturdy cell walls but have flagellates (Borowitzka & Borowitzka, 1988; Wehr & Sheath, 2003). Therefore, Clamydomonas cells can be digested more easily than Chlorella. Last, during the feeding process of Daphnia, algal cells are collected and compacted into boluses of fixed size (Hartmann & Kunkel, 1991), and then the boluses are transferred to the mouth and eaten. The size of the bolus must have two constraints: (1) maximal energy content per bolus; (2) below the size range of the mouth of Daphnia. Therefore, Daphnia might take more time to collect a bolus of Chlorella cells, which would increase the food handling time and decrease the filter efficiency. Daphnia will only discriminate food quality based on cell size and digestibility. Another study by Xuwang et al. (2011) has proved that starved cladoceran Daphnia magna would graze more efficiently on Chlorella pyrenoidosa which have more nutritional content than on Chlamydomonas sajao. The mechanisms used by cladoceran in feeding selectivity lies on two processes: gathering information and collecting food particle, involving analysis of optical or chemical cues and rejecting unwanted food particles, respectively (Hansen et al., 1991). Foraging behavior of filter zooplankton is always evaluated in the conditions that the animals are fed with diets differed in food palatability. However, food nutritional content is also an important criterion to evaluate food quality.
2.3 Behavior selection
Filter feeding zooplankton face a variety of food resources, varying greatly in size, digestibility, nutritional values and abundance (Porter and Orcutt, 1980). According to the optimal foraging theory, diet selection by feeders depend on food size, searching time, handling time, nutrition content and assimilation efficiency, all of which is to maximize net energy gains by feeders (Pyke et al., 1977). The ability of zooplankton to select optimal food items in the face of a mixture food resources comprising high and low quality food will maximize the fitness of grazers, and help them persist the populations in competing with other aquatic organisms (DeMott, 1993). Phytoplankton is the major food item for filter zooplankton, and food quality of algae usually lies on the palatability and nutritional value. Food palatability can also be defined as physical makeup, involving digestibility and particle size, and moreover, food nutritional value is also regarded as chemical composition, including lipid composition, total carbon content and stoichiometric concentration of carbon and phosphorous (Ahlgren et al., 1990). On one hand, numerous data documented the capability of filter zooplankton (e.g. copepods and cladocera) to discriminate food value based on physical makeup of algal particles, which reveals that copepods and cladocera graze more efficiently on naked and relative large but with intermediated size algae, e.g. Cryptomonas and Chlamydomonas, since energy expenditure of feeders to collect and digest these algae is lower than that of small sized and thick cell wall packaged algae, e.g. Chlorella (Yin et al., 2010). On the other hand, some studies also verified the ability of filter zooplankton (e.g. cladoceran) to feed preferentially on algae with higher total carbon content, because algae, superior in chemical composition (e.g. eicosapentaenoic acid, docosahexaenoic acid and total organic carbon), could afford more metabolic energy and would support higher survival and reproduction for feeders (Meise et al., 1985). Obviously, it should be advantageous for feeder to select good food particles valuable in both physical makeup and chemical composition. In terms of behavior during feeding selectivity, an example of a cladoceran species, Daphnia magna which is a filter feeding cladoceran has shown to swim more slowly upon encountering a region of rich food, and thus optimize their food intake in a patchy environment (Smith & Baylor, 1953). The observation that Daphnia reduce the vertical component of their swimming speeds if the suspended food particle concentration is increased can be explained as an adaptive response, optimizing food intake in a patchy environment. There is evidence of vertically segregated layers of phytoplankton less than 1 m thick in small freshwater bodies, which form a typical habitat for D. magna (Reynolds, 1976; Moss, 1980), so this suggestion is plausible. Zooplankton populations nearly always exhibit a nonrandom vertical distribution, which is often linked with diurnal vertical migration behavior (Meester, 1994). The bulk of the evidence suggests that the main adaptive significance of the residence in deeper water during the day is a reduction of visual predation pressure (Lampert, 1993). Diurnal vertical migration can thus be viewed as the result of a trade-off between reduced predation risk and reduced reproductive output. They observed that it is the bigger individuals that migrate, and that the nutritional state of the animals remaining in the upper strata is better than that of those migrating to deeper strata during the day. Although the pelagic habitat is often considered to be relatively homogeneous and unstructured (Hutchinson 1961), the environmental conditions that migrating and non-migrating animals encounter are very different. As they are subjected to different selection regimes, this may result in their life histories and other traits being co-adapted with their habitat preference. Schatz and McCauley (2007) have clearly indicated that in both marine and freshwater systems, the ratio of key elements in algal food supplies e.g., C: P: N has been shown to be a major determinant of food quality for herbivores, such as Daphnia (DeMott, 2003), rotifers (Rothhaupt, 1995; Jensen & Verschoor, 2004), and some copepods (Kiorboe, 1989; Jones and Flynn 2005). A large mismatch between elemental ratios of resources and consumers can have significant effects on growth, reproduction, and mortality of consumers (Sterner and Hessen 1994). Recent reviews have highlighted the role that variation in elemental ratios can play in explaining features of both populations and communities (e.g., Andersen et al. 2004; Moe et al. 2005). Schatz and McCauley (2007) in their study, they concluded that adults exposed to a food quality gradient with uniform cell density showed significantly different behavior. Individuals orient themselves around the section of higher food quality very quickly. A comparison between individuals exposed to a homogeneous food environment and individuals exposed to a food quality gradient with uniform cell density demonstrates that individual foraging is significantly different between these two conditions. Deviations in movement around the high food quality sections are greater for juveniles, but not significantly different from those of the adults. Five adult individuals of Daphnia have also rapidly located the high food quality patch as in the gradient trials and demonstrated a distribution significantly different from uniform (Schatz & McCauley, 2007). When presented with spatial gradients in food quality, individuals rapidly located (i.e.,<5-10min) spatial regions of high food quality and altered their foraging behavior to remain in these regions. It seems that both juveniles and adults of Daphnia located the high-quality regions on approximately the same time-scale, even though swimming speeds for adults are higher than for juveniles (Ryan and Dodson, 1998). In contrast, individuals exposed to a homogeneous environment of food quality and quantity foraged randomly, showing no clear preference for any region of the chamber (i.e, physical location). These individuals foraged in a manner that is significantly different from individuals found in environments with a food quality gradient. This observations on foraging by individuals exposed to homogeneous food concentrations are consistent with previous work investigating foraging patterns that did not attempt to manipulate food quality (Schatz & McCauley, 2007). In terms of foraging preference through manipulation of ingestion rates have shown that the differences among ingestion studies could be accounted for by different acclimation procedures reflecting internal stimuli for foraging (Plath, 1998). Experiments have shown that individuals acclimated under poor food quality conditions respond by accelerating feeding when presented with high-quality food, whereas individuals not acclimated behave differently (Plath and Boersma,2001; Darchambeau & Thys, 2005). In addition, a large body of evidence exists demonstrating the important role that chemical cues play in the behavioral response of Daphnia to such things as food and predators (Larsson and Dodson, 1993). It remains to be tested whether variation in food quality significantly influences vertical migratory responses by Daphnia, but results have suggested that the timescale for behavioral responses could play an appropriate role in vertical migratory behavior or in responses to externally imposed spatial gradients in food quality (Schatz & McCauley, 2007). Studies however have concluded that suitable habitat for Daphnia is determined by such things as food quantity, predators, and temperature. Individuals appear to perceive these factors and optimize fitness; however, the effect of food quality has not been considered.
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2.4 Temperature and salinity on reproduction of D. australis
Temperature is an essential factor initiating the change of the life history of aquatic poikilotherms because its flux can induce a range of physiological responses in small aquatic animals such as cladocerans (Smith, 1990). Above the optimal range, warm temperature increases an animal's metabolism and activities, resulting in faster development, growth and reproduction (Vijverberg, 2006). Warm temperature also accelerates egg development and shortens the parthenogenetic reproductive cycle (Goss & Bunting, 1983). This has also been supported by a study on Daphniopsis australis, where results have proven to show that temperature had a stronger impact on the number of total egg clutch and offspring production (Ismail et al., 2011b). The highest offspring production for D. australis occurred at 16Â°C and 17 ppt, but the egg clutch number was similar in temperature from 16 to 20Â°C and in salinity from 17 to 22 ppt. In contrast, the maximum reproductive output in a saline cladoceran, Moina mongolica, occurs at 20Â°C and salinity between 5 and 15 (He et al., 2001), while M. salina seems to be adapted to different temperature and salinity with the maximum reproduction at 30Â°C and 36 ppt (Gordo et al., 1994). In other studies, the temperature of 25Â°C was considered the upper threshold limiting the fecundity of temperate Daphnia spp. (Goss and Bunting, 1983; Moore et al., 1996). In comparison, D. australis produced the lowest egg production due to egg degeneration, and the lowest offspring production was observed at 25Â°C at all salinities. Among species in the Daphniidae, Daphnia magna can tolerate salinity Ë‚12ppt, but its reproductive performance is adversely affected when the salinity reaches 8 ppt (Arner and Koivisto, 1993). Studies have reveals that even though the reproductive performance of D. australis is adversely affected at a salinity of 27ppt, this species has a greater salinity tolerance than other species in the same family. D. australis have also shown to have drawbacks of fast development at high temperature such as low reproductive output due to short longevity. Increasing temperature and salinity produced negative impacts on longevity, survivorship, growth and reproductive variables of D. australis. However, the positive effect on maturation and development by increasing temperature was count-balanced by the increased of salinity. The optimal conditions for D. australis should be less than 27Â°C and 27ppt (Ismail et al., 2011b)
2.5 Chemical mediate
Chemical cues signify much of the language of life in the sea (Hay, 2009). In most marine species, chemical cues control whether they consume, ï¬ght with, run from, or mate with the creature next to them-as well as whether they are eaten by, infected by, or overgrown by natural enemies. Individuals can assess sex, social status, and even whether a potential mate is sperm depleted or sperm sufï¬cient using chemical cues. These cues can be so influential that male crustaceans will guard, carry, and attempt to mate with sponges, air-stones, rocks, or golf balls if these have been treated with the correct pheromone (Asai et al. 2000, Hardege et al. 2002, Breithaupt & Thiel 2008). In aquatic systems, chemical cues control feeding, habitat, and mating choices (Breithaupt & Thiel 2008). Chemical cues alleviate dominance hierarchies (Breithaupt & Thiel 2008) and determine whether animals forage to obtain lunch or stay hidden and endure hunger to avoid becoming lunch (Peacor & Werner 2001). One species of crustaceans, a turtle weed crab Caphyra rotundifrons, feeds only on the chemically defended tropical seaweed Chlorodesmis fastigiata. The cytotoxic diterpenoid chlorodesmin, the main secondary metabolite of Chlorodesmis, prevents fishes from consuming these algae but stimulates feeding by the specialist crab (Hay et al. 1989).
There are several analogous cases concerning other small, specialist mesograzers (amphipods, isopods, or nudibranchs, for example) that live on the hosts they consume and are stimulated to feed, live in, or decorate with prey by the specific prey metabolites that deter larger consumers (Hay,1992, 1996; Stachowicz & Hay 1999; Lindquist et al. 2005). In the simplest case, this cascade could involve one cue to locate the host from a distance, a second contact cue that initiates feeding, and possibly a third gustatory cue that stimulates continued feeding.
Recently, Coleman et al. (2007) found that a fish and a crab that consume snails were attracted more to the odor of seaweeds that have been attacked by snails rather than to the odor of un-attacked or artificially damaged plants. This is probably due to the seaweed evolving to call in consumers of snails or due to the consumers evolving to sense the evidence of past feeding as an indication of prey presence is unclear, but it may provide an advantage for the seaweed under either circumstances. The brown seaweed studied by Coleman et al. (2007) is known to induce increased phlorotannin-based chemical defenses when attacked by gastropods that are deterred by these compounds, but not when attacked by an isopod that is not deterred by these compounds (Pavia & Toth 2000). These induced defenses lower gastropod fitness when they are forced to feed on induced individuals (Toth et al. 2005), and in the field, gastropods colonizing induced plants move from them more rapidly than those colonizing uninduced plants (Borell et al. 2004). Since increased movement and increased time on primary substrate instead of in the plant canopy is associated with increased predation risk (Schmitt et al. 1983; see also discussion in Wakefiled & Murry 1998), this chemically mediated emigration is likely to be of both direct (less grazing) and indirect (increased predation on gastropods) benefit to the seaweed. Van Donk (2007) reviews several instances in planktonic systems where chemical cues from consumers reduced zooplankton activity, feeding, gut fullness, and thus visibility to visual consumers.
3.0 Algae as a prominent feeding source for cladocerans
3.1 Nutrient content in microalgae: Growth trends under different nutrient composition
Nutrients are one of the most important factors regulating phytoplankton growth. Among other factors, growth can be limited by the supply of nitrogen and phosphorus. Nutrients are usually spatially and temporally dynamic in terms of both concentrations and structure in marine environments (Lai et al., 2011). Phytoplankton will alter their cell physiology in response to environmental stress, including changes in light, temperature and nutrients (Cade-Menun & Paytan, 2010). Nutrient stress has been reported to affect carbon (C) forms: nitrogen (N) starvation may enhance lipid storage (Shifrin and Chisholm, 1981) and may decrease protein content (Kilham et al., 1997; Lynn et al., 2000; Heraud et al., 2005), while increased carbohydrate: protein ratios are thought to be indicative of phosphorus (P) deficiency (Dean et al., 2008). Phosphorus stress can also alter the response of algae to light and temperature stress (Sterner et al., 1997).
Many researchers agree that algal food quality is important. Nitrogen and phosphorus are major nutrients that are likely to limit growth of phytoplankton in the natural environment (Bougaran et al., 2010). Three cladoceran species that have been studied before, Ceriodaphnia cornuta, Moina micrura, and Scenedesmus spinosus have responded differently towards the different nitrogen (N) and phosphorus (P) ration. M. micrura had the highest growth rate, even with nutrient deficient algae. C. cornuta had moderate growth rates while D. gessneri had the lowest growth rates even in the N/P-sufficient algae. The nutrient deficiency affected most cladocerans, decreasing growth rates relative to nutrient sufficient algae (Ferrão-Filho et al., 2003). Ferrão-Filho et al. (2003) have indicated that cladocerans were differentially affected by nutrient deficient algae, in contrary with previous studies by Urabe et al. (1997) and Sterner (1993) where their results showed that Daphnia was affected only by phosphorus (P) limited algae. Cell nitrogen is mainly used to build proteins, amino acids and nucleic acids while phosphorus is mostly a constituent of nucleic acids and phospholipids (Geider and LaRoche, 2002). Some laboratory experiments on monospecific cultures also gave evidence of more complex N (nitrogen)-P (phosphorus) interactions. For example, Davies and Sleep (1989) demonstrated that carbon fixation of N and P starved Skeletonema costatum could be significantly stimulated only when both NO3 -and PO4 3- were added simultaneously, but was repressed when only NO3- was added. Terry (1980) suggested that, while the minimum law prevailed for most N: P ratios, a synergetic effect on Pavlova lutheri could occur between the two nutrients in a narrow range of N: P ratio. Since no clear biochemical process of interaction could be identified between nitrogen and phosphorus metabolism, these two nutrients were previously classified as biochemically independent nutrients (Arrigo, 2005; Saito et al., 2008). Recently, however, Agren (2004) hypothesized that the P-content of ribosomes could perform the main link between N-assimilation and P quota.
Algal growth has known to be associated with the nutrient composition of the algae. Furthermore, Lai et al. (2011) have emphasized that algae growth rate were significantly correlated with nitrogen (N) concentrations in all N-added treatments over the incubation period which proved to have a strong correlation between algal growth and N concentrations, and the same results was obtained for Phosphorus (P) added where growth rates of algae were significantly correlated with the phosphorus levels. Specific growth rates and maximum cell densities increased with the increase of initial N-added and P-added concentrations, which is consistent with other related research (Touzet et al., 2007; Wang et al., 2008). The reason for this was that the increased nutrient supply could increase the substrate concentration of a series of physiological processes of the algae, which, in turn, enhanced its assimilation in a certain scope and accelerated the growth of the algae (Lai et al., 2011). In these studies, P. donghaiense took up PO43âˆ’ more rapidly than NO3âˆ’, which is in agreement with previous findings (Hou et al., 2007). This likely represents a form of luxury consumption and several phytoplankton species are able to store excess P in intracellular pools for use during time of limitation. Therefore, the initial nutrient concentrations (nutrient supply) may play a more important role in affecting the growth of algal cell in batch culture experiments.
3.2 Biochemical composition in particular the fatty acid composition of algae under different nutrient manipulation
Food quality for grazers has been related to mineral (nitrogen, phosphorus) and biochemical (amino acids, fatty acids) constituents (Ahlgren & Hyenstrand, 2003). As has been previously reported for phosphorus (P)-limited algae, in many cases nitrogen (N)-limited algae can also be low quality food for grazers (Mitchell et al. 1992, Kilham et al. 1997, Weers and Gulati 1997, Ahlgren et al. 2000). For an example, analyses of 18 freshwater and 11 marine algal species showed in most cases an increased lipid content at nitrogen (N) limitation, often two to three times higher than cultures with replete N (Shifrin and Chisholm 1981). Mainly the saturated lipids, including saturated fatty acids (SAFA), seemed to increase, whereas phosphorus (P) lipids and polyunsaturated fatty acids (PUFA) decreased (Parrish and Wangersky,1990).
The lipid and fatty acid (FA) composition of the green algae, but not of the cyanobacteria, seemed to be effected by N nutrition (Piorreck et al. 1984).The lipid and fatty acid (FA) composition of the green algae, but not of the cyanobacteria, seemed to be effected by N nutrition (Piorreck et al. 1984). High quality of food has been connected with both high content of P (Sterner,1993) and high contents of PUFA, particularly of Ï‰á´ˆ type (Mueller-Navarra et al., 2000). Ahlgren and Hyenstrand (2003) have focused on changing the Nitrogen source and see the effects on biochemical composition of algae particularly on fatty acid composition of the green algae, Scenedesmus quadricauda and cyanobacterium, Synechococcus. The most important Ï‰á´ˆ Fatty acid (FA), Î±-linolenic acid (ALA: 18:3Ï‰3) have shown to increase with growth rate in both N sources. The most important Ï‰3 FA, ALA (18:3Ï‰3) increased with growth rate in both N sources. The Ï‰6 PUFA, linoleic acid (LA, 18:2Ï‰6) showed much lower levels with decreasing tendency with growth rate. N limitation affected the FA content differently in the two organisms:Scenedesmus did not follow the common pattern with higher SAFA at N limitation as some Synechococcus data indicated, but PUFA were lower in both organisms. In Scenedesmus the Ï‰3 PUFA (mainly ALA) was reduced at N limitation in both N sources, whereas no influence could be seen in the Ï‰6 PUFA (mainly LA). The opposite was found in Synechococcus, with no reduction in ALA but reduction in LA to undetectable levels. Common FA in green algae are ALA (18:3 Ï‰ 3), palmitic acid (16:0), and 16:4 Ï‰ 3 or the iso FA 18:0I (Volkman et al. 1989, Ahlgren et al. 1992, Dunstan et al. 1992). Many cyanobacteria contain low levels of FA, particularly PUFA. N limitation clearly influenced fatty acid composition and content: MUFA increased and some PUFA decreased. Results of Lynn et al. (2000) also showed that N limitation induced higher content of triglycerides and a corresponding lower content of phospholipids. The shortage of amino anid (aa) may block the production of enzymes involved in the desaturation processes. The elevated levels of oleic acid in N-limited condition agree with the results from the prymnesiophyte Isochrysis galbana by Flynn et al. (1992). However, the corresponding reduction of Ï‰3 PUFA, as we found in Scenedesmus, was not notable in Isochrysis where, instead, both Ï‰3 and Ï‰6 PUFA increased at the two limited N sources. In summary, C content was stable in both the green alga Scenedesmus quadricauda and the cyanobacterium Synechococcus sp., under replete and N-limited conditions. two limited N sources. N-limitation affected fatty acid content differently in the two organisms: in Scenedesmus the Ï‰3 PUFA (Ï‰-linolenic acid) was reduced whereas Ï‰6 PUFA (linoleic acid) was stable. Several researchers suggest that the low food quality of P-deficient algae might be because of an indirect effect via alterations in biochemical composition, such as reduced algal EPA or linolenic acid content (Ahlgren et al., 1998).
Studies showed that P-limitation reduced Ï‰3 PUFA in the diatom species Stephanodiscus hantzschii and green algae S. quadricauda (Ahlgren et al., 1998) and Chlamydomonas reinhardtii (Weers & Gulati, 1997), while Ï‰3 PUFA increased under P-limitation in S. acutus (Muller-Navarra, 1995a). However Park et al. (2002) have identified that Ï‰3 PUFA content did not decrease with Phosphorus (P) limitation. The same results was also obtained by few other researchers which emphasized that P-limitation is real and not an indirect effect of changes in algal fatty acid composition (Urabe et al., 1997;Weers & Gulati,1997; Demott,1998; Boersma, 2000). This was contrary with results by Ferrão-Filho et al. (2003) in his recent work using green algae, Scenedesmus spinosus; where two important PUFA (linoleic and linolenic acids) was increased under Phosphorus (P) deficient algae. Therefore, further research need to be done to see the fluctuation trends on biochemical composition of algae since algal taxonomy seems to largely determined the results obtain for the fatty acid levels under different nutrient manipulation (Park et al., 2002).
3.3 Growth of cladoceran fed algae with different nutrient composition
Elements such as Nitrogen (N) and Phosphorus (P) are essential nutrients by definition and must be obtained from the diet. Nitrogen is important in the amino acid and protein synthesis while phosphorus is important as a component of phospholipids in energy storage metabolism (i.e. Adenosine Triphosphate - ATP) and in nucleic acid synthesis. Studies have suggested that high levels of P in cladocerans are associated with high content of Ribonucleic acid (RNA) (Filho et al., 2003). Thus, fast-growing species may have a high demand for phosphorus and are likely to be more affected by P-deficient algae than slow-growing species (Main et al., 1997; Sterner & Schulz, 1998). Difference in the elemental ratios between zooplankton and their foods suggests that cladocerans must concentrate N (nitrogen) or P (phosphorus) relative to C (carbon) in their body tissue through feeding and metabolic processes in order to grow and produce offspring (Urabe and Watanabe, 1992).
Cladocerans can easily obtain the necessary amount of N, P, C, if N: C or P: C ratios in food are high (Urabe and Watanabe, 1992). In general, under P-limitation, algae tend to decrease essential PUFA (Polyunsaturated fatty acids) and increase saturated fatty acids' content (Morris, 1981; Piorreck & Pohl, 1984; Harrison, Thompson & Calderwood, 1990). As a result, it has been thought that growth limitation of cladocerans by P-deï¬cient algae might be an indirect effect of altered algal biochemical composition (Ferrão-Filho et al., 2003). For instance, Moina micrura was most sensitive to P-deficient algae and, remarkably grew better and produced more eggs in N-deficient algae than in Nâ„P sufficient algae. A previous study on the effects of nutrient limitation on growth of three zooplankton species (Moina micrura, Ceriodaphnia cornuta, and Daphnia gessneri) has provided tremendous results in responding towards different elemental ratio. Ceriodaphnia cornuta was less sensitive, growing well in both N and P-deficient algae. This species, however, had a lower clutch size in N-deficient algae. On the other hand, Daphnia gessneri was the most sensitive to mineral limitation, showing decreased growth and clutch size in both nutrient-deficient algae (Ferrão-Filho et al., 2003). Polyunsaturated fatty acids and Phosphorus seem to be not the most important factors in controlling growth and reproduction on cladocerans (Fileto et al., 2007).
3.4 Fatty acid content of cladoceran in response to different nutrient manipulation
Essential fatty acids play an important role in cellular metabolism, as a precursor of eicosanoids (i.e. prostaglandins) in animals, and also as part of cell membranes, regulating membrane fluidity, and acting as an antifreeze in organisms that live in low temperatures (Ferrão-Filho et al., 2003). Eicosanoids play an important role in the physiology of invertebrates, regulating egg production, egg-laying, spawning and hatching, among other functions (Stanley-Samuelson, 1994a, b). Some studies on the effect of PUFA additions to mono-algal diets have shown dietary dependence of cladocerans, especially temperate Daphnia species, on essential fatty acids (DeMott & Muller-Navarra, 1997; Sundbom & Vrede, 1997), but few studies have shown the same responses in tropical cladocerans (Ferrão-Filho et al., 2003). In general, under P-limitation, algae tend to decrease essential PUFA and increase saturated fatty acids' content (Morris, 1981; Piorreck & Pohl, 1984; Harrison, Thompson&Calderwood, 1990). Thus, it has been thought that growth limitation of cladocerans by P-deficient algae might be an indirect effect of altered algal biochemical composition (Muller-Navarra, 1995b; Sundbom & Vrede, 1997; Weers & Gulati, 1997). Ferrão-Filho et al. (2003) have reported that two important PUFA (Linoleic and linolenic acids) was increased in nutrient-deficient algae. Linoleic and linolenic acids are essential for almost all animals because they are not capable of synthesizing either (Stanley-Samuelson, 1994a). This was supported by an evident increase in lipid reserves in M. micrura tissues in relation to increase in linoleic and linolenic acids in N-deficient algae, which are both known to be essential to cladocerans (Ferrão-Filho et al., 2003). Furthermore, studies on one of the important species of cladoceran have also shown that D. magna growth is predicted to be depressed if Ï‰3-PUFA (linolenic acid) is below about 30nmol FA C-1.
3.5 Life table of zooplankton under different nutrient composition of algae
Development of modern ecology involves the studies on life-history traits of animals. A life-history consists of a schedule of multiple parameters, such as somatic growth, development time, age and size at maturation, egg and brood size, and survivorship (Urabe & Sterner, 2001). Demography at the population level and physiology at the individual level can be link through information gained from these parameters. Laboratory manipulations and field observations on plankton have shown that life-history parameters determine the population dynamics of consumers and prey (Gurney et al. 1990; McCauley et al. 1990). The populations dynamics of a single species in a wide range of conditions can be predicted through the information gained on the life-history study (Murdoch et al. 1992; McCauley et al. 1996).Food quality and quantity, temperature, competition and predation are among the important factors that cladocerans particularly respond rapidly to (Xi et al., 2005). Life table demography is an essential approach in understanding the life history strategies and population dynamics of zooplankton under continuously changing environmental conditions. Survivorship, fecundity, average lifespan, and generation time and population growth rate are the vital parameters that can provide valuable insight into the suitability of the ambient conditions for the zooplankton (Stearns, 1976). Life table studies are essential for the optimization of conditions for the mass cultivation of the experimental species, Daphniopsis australis.
Insufficiency of only a subset of all vital resources is the main characterization of poor food quality. Energy may be in surplus, while a single material such as protein is in deficient supply. To date, little is known about the response of multiple life-history traits of planktonic herbivores to changes in algal food quality (Sterner & Schulz 1998), although poor food quality is thought to be a major factor in the ecology of many animals. Planktonic herbivores may not perform optimally or in a way that is predicted from food quantity alone if some important elements or biochemical are deficient in the food. Previous studies have reported that one species of Daphnia, a common cladocerans in lakes and ponds, grew slowly and produced a smaller number of offspring when they fed on algae with a high C: P or C: N ratio (Sterner 1993; Sterner et al. 1993; Schulz & Sterner 1999). Furthermore, if the suite of elements or biochemical required for reproduction differs from somatic growth requirements, animals may alter their growth and reproduction patterns differently depending on the chemical composition of food (Urabe & Sterner, 2001). Some Daphnia species are known to produce larger eggs when food quantity is low. Such changes in egg size are believed to be an adaptive response, particularly because neonates from larger eggs can survive longer without food (Tessier et al . 1983; Goulden, Henry & Berrigan 1987; Gliwicz & Guisande 1992). However, when they feed on food with sufficient energy but some nutrient deficiency, production of large eggs may not be advantageous because neonates can gain energy from surrounding food. Thus, food quality may differentially affect a number of life-history traits in addition to body growth and fecundity.
A study done by Urabe and Sterner (2001) have reported that Daphnia fed on LOP (Low phosphorus) or LON (Low Nitrogen) algae not only grow slowly but they also produce a substantial number of eggs that cease to develop, and they have reduced survivorship, especially before maturation. As a result, the intrinsic rate of population increases for Daphnia fed on N (Nitrogen) or P (Phosphorus) deficient algae was much lower than those fed on the same amount of HIP (High in Phosphate) algae. In addition, Daphnia in H-LOP (High-Low in Phosphate) produced smaller eggs than those in H-HIP (High- High in Phosphate), while eggs produced in L-HIP (Low-High in Phosphate) were larger. Apparently, N (Nitrogen) or P (Phosphorus) deficient algae are a low-quality food for Daphnia, and the effects of low-quality food on life-history traits are different from the effects of low food abundance (Urabe & Sterner, 2001). Nonetheless, the results from the study have indicates that in nature, owing to spatial and temporal variation in the nutritional quality of food, Daphnia may not perform optimally or in a way that is predicted from food quantity alone (Urabe and Sterner,2001; Elser et al. 2001; Sterner & Schulz 1998;). On the one hand, low-quality food in terms of chemical composition constrains the material basis of life-history traits in animals. On the other hand, if animals have frequently faced food with low quality, they may be able to adjust their life-history traits to partially overcome growth penalties. Since both field data on the effects of food quality and physiological knowledge of cladoceran nutrition are still limited, we could not ascertain whether any of the changes we observed were adaptive responses. Implications of the quality or chemical composition of food resources on population and trophic dynamics have been pointed out by several authors (Sterner, Elser & Hessen 1992; Hessen 1997; Elser & Urabe 1999); but theoretical study incorporating food quality into population and trophic dynamics is still limited. Jensen and Verschoor (2004) have also discovered that rotifer, B. calyciflorus fed N(Nitrogen)-depleted algae resulted in similar life history responses as P(Phosphorus)-depleted algae; where both fecundity and somatic growth was decreased, but no reduction in lifespan. The results suggested that both types of algal nutrient limitation could have limited protein synthesis in B. calyciflorus: N(Nitrogen)-limitation directly reduces the (essential) amino acid and protein content in the algae, whereas P(Phosphorus)-limitation may affect rotifer protein synthesis through decreased contents of ribosomal RNA.P(Phosphorus) and PUFA-limitation in Daphnia, both elemental composition and PUFAs seem to be important and even substitutable (Boersma, Schops & McCauley, 2001).Generally P (Phosphorus) is most important for Daphnia under severe P (Phosphorus)-limitation of algae whereas PUFAs become more important under more moderate algal P(Phosphorus)-limitations. Furthermore, somatic growth rates of B. calyciflorus even differed after the first 24 hours, and these growth differences became more pronounced by the time of first reproduction because of the cumulative effect of food limitation. Both N (Nitrogen) and P (Phosphorus) algal limitation reduced somatic growth rates. This observation is in accordance with experiments where cladocerans fed nitrogen or phosphorus-limited algae also showed reduced somatic growth rates (Sterner,1993; Urabe & Sterner, 2001). Obviously, both algal N and P-limitation inhibit the formation of somatic (and reproductive) structures in herbivorous zooplankton. Nutrient-limited rotifer, B. calyciflorus did not abort eggs,as has been observed for Daphnia under algal nutrient limitation (Weers & Gulati, 1997; Urabe & Sterner,2001).Thus, reduction of somatic growth and reproduction appears to be a general pattern found in herbivorous zooplankton under both Nitrogen and Phosphorus limitation. Therefore, it is very important to study the life-history traits prior to changes in food quality and the implications of these responses for population and trophic dynamics of D. australis because with this kind of information we may be able to predict the population dynamics of a single species in a range of conditions (Murdoch et al . 1992; McCauley et al . 1996).
4.0 Live feed culture: A vital component in aquaculture
Feeding and food resources are an important component in aquaculture sector. In ensuring that the sustainable culture of most hatchery species can be obtain all year round, an adequate supply of live feed for first feeding in most hatchery cultured species is needed. Continuous supply of live feed organisms can only be obtained with a good culture condition primarily on its feeding requirements. Feeding of cladoceran requires a proper diet that can sustain the growth and reproduction. Although microalgae are the main food source for herbivorous zooplankton organisms, food quantity and quality are important factors to control cladoceran development growth and reproduction (Brown, 1991).
Food supply is one of the most important factors in the culture of any species, directly influencing survival and growth. If food supply is not sufficient, higher mortality rates can be expected (Martínez-Jerónimo & Gutierrez-Valdivia, 1991). Most cladoceran species are able to consume microalgae with size range from 2-25 Âµm.The type of algae that have been extensively used in aquaculture as live feed for zooplankton species are Chlorella,Tetraselmis, Isochrysis, Nannochloropsis, Pavlova and Rhodomonas (Ismail et al.,2011; Laverns and Sorgeloos 1996; Brown et al.,1997). A combination of physical and biochemical properties has been widely used as criteria in live food selection to feed zooplankton for larval fish rearing. However, different results were obtained in different zooplankton species on their food selectivity (Ismail et al., 2011). For instance, a brackish cladoceran, Diaphanosoma celebensis prefer Isochyris as their primary food, in contrast with calanoid copepods, Acartia sinjiensis where Isochrysis is not suitable for its growth and development (McKinnon et al., 2003).
5.0 Daphniopsis australis: from nature to aquaculture
Baseline study on the potential use of D. australis as a candidate to replace the use of Artemia cysts have been done and positive results on its population growth under laboratory condition have been obtained (Ismail et al., 2011). The use of a saline cladoceran as an alternative live food for marine aquaculture has been considered to reduce the dependence on the brine shrimp, Artemia sp. due to unreliable supply, poor natural biochemical composition, and Artemia have been considered the most expensive feed .D. australis maintains high growth at low temperature, which is an excellent feature to serve as a live food for cold-water ï¬sh larvae (Ismail et al., 2011).
6.0 Aims of study
The main aims of this study are to test the hypothesis that nutrient-limited algae can limit growth and reproduction of cladocerans and fatty acid composition are important for temperate cladoceran nutrition.
In summary, the main aims of the proposed PhD study are:
1) To investigate the effects of algae enrichment using different Nitrogen and Phosphorus ration on growth and fatty acid compositions of algae
2) To determine the optimize nutrient needed in algae for D. australis growth and fatty acid compositions
3) To determine the effect of food quality on life history of D. australis
4) To determine the suitability of D. australis as an alternative live food for rearing sea bass Lates calcarifer (Bloch) fry
Base on the literature review, summarise the major points you have discussed in this review. These points should be related to the questions (or aims) you raised in introduction.