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Many marine invertebrates of diverse and varied phyla, have a common reproductive strategy that involves a multi-phase life cycle that occupies two dramatically different environmental habitats. The separate mobile pelagic larval phase and the predominantly sedentary or sessile bottom dwelling phase is linked by a settlement event. Larval forms are usually very different from adult forms. Reproduction involves the eggs and sperm and/ or larvae being released in generally very large numbers into the water column. Here, most of the pelagic larvae are potentially capable of dispersing long distances from parental populations. They develop and grow for a certain period of time before metamorphosing into adults. During their time in the water column, the larvae feed on phytoplankton and small zooplankton, including other larvae. Their small size and abundance also make them susceptible to predation by other larger marine creatures. Their huge numbers probably increase their chances of survival. Overall the probability of successful recruitment is low. It used to be thought that this strategy represented an open system where chance settling out of larvae led to distribution and abundance of benthic phase adults governed mainly by post-recruitment effects (Thorson,1950, Caley,1960) Several recent studies on the dispersing larval phase of marine invertebrate life cycles suggest that patterns of larval supply are not the only factor influencing invertebrate population dynamics. It is now thought that the larvae themselves can influence their probability of success. Larval behaviours are responses to two challenges predator evasion and selection of a suitable settlement site. Supply side ecology ( Lewin. 1986) emphasizes the role of recruitment in limiting adult populations and structuring benthic communities. At the end of the dispersal phase, the abundance of larvae at settlement can be highly variable, both spatially and temporally. Variation in settlement can be caused by larval mortality and predation rates, transport mechanisms and larval behaviours. Moreover, coupling of larval supply with adult abundance seems also to effect distribution (Hughes et al 2000) The extent to which local larval recruitment depends on production by local adults is not clear and the extent of relative openness of marine benthic invertebrate populations is uncertain.(Strathmann et al, 2002, Swearer et al 2002) However, there seems to be significant planktonic processes in the pre-settlement stage that that influence population dynamics of settlement, recruitment and subsequent adult populations. Habitat selection by planktonic larvae can overcome patterns predicted by patterns of larval supply (Jenkins, 2005).
Factors influencing distribution and abundance of species
Biotic and abiotic interactions and dynamics within each phase of the life cycle have the potential to influence the distribution and abundance of the adult populations. Adult populations have specific environmental requirements and occupy habitats with particular constraints. These may be physical characteristics such as tidal gradient, exposure, rugosity, habitat complexity, depth. salinity and temperature gradients.There are also biotic factors. A readily available food supply is fundamental but distribution is also influenced by competition and predation. The need for reproduction and dispersal often favours clustering or at least close proximity of adults to ensure fertilization. Dispersal and recruitment of the larval stage ensures the continuance of the species but the role of the planktonic larval stage in invertebrate population dynamics is not yet fully understood. Differences in recruitment vary both both temporally and spatially but the extent that this effect has on adult abundance and distribution is still a matter of debate (Jenkins, 2005, Pawlik, 1968, Hughes et al, Grosberg and Levitan. 1992 and others). The influences of post settlement density related processes of predation and competition have to be considered in relation to influences effecting the larval stages of the invertebrate.
Reproductive strategies influencing dispersal and recruitment
A multi-phase life cycle is a reproductive strategy that can offer several advantages for benthic dwelling invertebrates especially those with sessile adult phases such as barnacles and tunicates or sedentary adult phases, such as mussels and crabs. The evolution of a larval stage allows adults with limited movement to disperse their young into new territories. The ability to disperse is an important adaptations of benthic marine invertebrates. The length of time the larvae spend in the water column can be hours days weeks or months depending on the species. Lecithotropic larvae are provided with a source of nutrition to use during their dispersal, usually in the form of a yolk sac, although some lecithotrophic larva can feed many, such as tunicatesare will not, and have to settle before their food source runs out. As a result, these species have short pelagic larval stages and generally do not disperse long distances.(Pawlik, 1986, Pawlik 1992) Planktotrophic development is the most common type of larval development, especially among benthic invertebrates. Many species have relatively long pelagic larval durations. During this time in the water column larvae feed and grow, and many species move through several stages of development. Barnacles, for example, undergo six moults before becoming a ciprid at which stage the stage they search for an appropriate substrate. to settle on (Molenock and Gomez, 1972). This strategy produces the potential of long distances dispersal and colonization of new territorys it also enables species to move away from any habitat that has become non-viable or overcrowded. Larval dispersal, or advection away from the spawning site may decrease competition between the different life stages as larvae use a different food source from the adults. Moreover, filter-feeding adults such as barnacles, are less likely to imbibe their own offspring and other benthic predators are also avoided. A pelagic larval phase that has a long duration is a strategy that could help some species break there parasite cycles.
Settlement and recruitment are the initial processes in determining adult population structures. The term settlement is used to describe the transition from a pelagic to a benthic way of life. This is the process where the larvae descend from the water column and take up a permanent abode on the sea bed. Metamorphic changes allow the larvae to acquire the features suitable for their new benthic life style. The settlement process starts with the onset of behaviour patterns associated with a phase of searching for suitable substratum, In the case of sessile invertebrates, this is succeeded by the initiation of permanent attachment to the substratum. This triggers morphogenic changes which culminate in metamorphosis into the juvenile form. Recruitment is generally a reference to newly settled individuals that have survived to a specified size after their settlement (Keough & Downes 1982).
Variability in recruitment to adult populations is a significant dynamic in the dispersal and abundance of marine invertebrates. The analysis of mechanisms which control settlement and recruitment and of the conditions under which recruitment variation affects adult distribution and abundance is complex but fundamental to understanding population and community variability.
Supply side ecology
Supply-side ecology is a term coined by Lewin (1986). This way of looking at population dynamics incorporates the potential role that variable larval input and variable recruitment plays in determining the size of local adult populations, Caley et al. 1996, Hughes 1984, 1990, Hughes et al. 2000, Gaines and Roughgarden 1985, Roughgarden et al. 1985) Larval. supply is influenced both by transport mechanisms and larval behaviours. Variation in settlement potential can impact on the distribution and abundance of adult invertebrate populations.
Influence of larval supply (or successful recruitment) on population or community structure and its importance relative to other factors.
The abundance of larvae in the water column affects the temporal variability of settlement. Post-settlement mortalities can be potentially replaced by new settlement from a plentiful larva supply where benthic mortality is caused by density - independent factors (Karlson and Levitan, 1990). Variability in larval supply can be associated with reproductive cycles of adult individuals (Roughgarden et al 1991). Pelagic larval forms are very susceptible to predation by various other marine animals. Rates of larval mortality can affect larval supply for settlement and recruitment. Mortality levels can be reduced by larval behaviour strategies aimed at predator avoidance . This is particularly significant in estuaries which often serve as nursery areas for fish and as a consequence are generally more abundant in predators (Dibacco et al 2001). Avoidance behaviour takes place on both small and large scales. Some larvae avoid predation at a small scale by sinking down the water column when threatened by a predator (Zaret and Suffern, 1976). More commonly a general large scale predator avoidance strategy used by many larvae is that of becoming nocturnally active. This limits fish predation as most fishes need light to find and hunt their prey. During the day the larvae are inactive and in shallow waters they remain hidden. Many invertebrate larvae may avoid predators by leaving the immediate coastal zone and developing in the open sea where their are fewer predators. In the open sea, in common with other planktonic species, invertebrate larvae can significantly reduce their risk of predation through diel vertical migrations (Marta-Almeida M, et al 2006) During the day they sink down to in the water colem were there is less light and fewer predators and come up to shallow waters where they feed at night on food such as micro-algae which are abundant in the photic zone. Variable predation at different depths may affect spatial variation of larvae within the water column. Varying mortality can be caused by retention in the water column for too long. The highest mortality in marine populations occurs during the larval stages, so mortality plays a significant though largely unquantified role in larval dispersal.
Abiotic effects such as wind patterns (Mc Quaid & Phillips, 2000) currents and other hydrographic factors (Gaines et al 1985, Pineda, 2007) can directly influence larval distribution and supply. as a means of larval transport Larval behaviours responding to local hydrographic features (Jackson 1986) can indirectly influence supply. Larval behaviours by positioning themselves strategically in the water column can utilise or avoid tidal flows or currents. (Gaines et al. 1985, Forward, R.B. Jr, and R.A. Tankersley 2001) This may be important in returning to find the restrictive habitat requirements needed for adult populations. Larvae are capable of. highly discriminative behaviours ,particularly on small scales where larval behaviour can be an important determinant not just of larval distribution and abundance but their behaviour patterns may also influence the subsequent adult distribution and abundance through local variations in settlement and / or recruitment.
Although some larvae can extend their survival for a short time if they do not find a suitable place to settle (Gimenez, 2004), their life span as a larva is finite and survival depends ultimately on successful settlement and recruitment. Delay can influence post metamorphic effects and ultimate success. Larvae that have spent too long in the water column may settle and recruit juveniles that have less chance of survival to adults. In this way the influence of the larval stage has bearing on the subsequent adult population.
Successful recruitment involves the selection of and often the attatchment to a suitable substrate and subsequent metamorphoses. There are many dangers at this stage. Larvae of shore dwelling species need to avoid becoming stranded by the tide and becoming desiccated. They must find a settlement site at an appropriate tidal height for the requirements of the adult phase and avoid competition. This is a limiting factor for sessile invertebrates space as the larvae need to find space on the habitat where they can settle as well as avoiding predation from adult filter feeders. Overcoming these problems depends on larval behaviours and responses to chemical cues and physical cues such as geo taxis and/or photo taxis. Different species have different triggers (Morse, 1991, Gebauer et al 2004). The interaction of physical processes and biological reactions to chemical cues. are particularly significant on small spacial scales. These interactions represent active selection of micro-sites and effect both the settlement processes and the abundance of settlement. Chemical cues can be from conspecific individuals (Crisp and Meadows, 1962, Kingsford et al 2002, Pawlick 1986) microbial films (Rodrigues et al.1992) and prey species. Many herbivorous species are induced to settle by presence of crustose algae on which they feed eg abalone (Morse1990 and limpets (Steneck,1982) Barnacle larvae at settlement are influenced by the speed of water flow, contours of the sub-stratum. and increases in light levels (Crisp, 1976).
It is thought that some recruitment may take the form of short and episodic pulses (Levin 2006). Recruitment windows, (Pineda,2007) where settlement events take place simultaneously in large numbers, sometimes over wide areas have been identified for some species such as corals, but mechanisms and interactions at work are not understood fully. In open systems like these, degradation of breeding stocks could result in a reduced recruitment to a wider areas.(Hughes et al 2000). Variation in dispersal and the processes and patterns of demographic connections work together to influence patterns of distribution and abundance.
Variation in recruitment can also effect the potential survival of the recruits to form adult populations, as numbers of recruits can potentially effect the extent of subsequent biological inter-actions such as predation.(Fairweather, 1988). In this way post-settlement effects can be influenced both by patterns of settlement and environmental factors.
Understanding the population dynamics of marine invertebrates requires the consideration of the interplay of all stages of the invertebrate life cycle with its environment not only those affecting the adult forms. Larval abundance, mortality, transport mechanisms and behavior before and during settlement are all significant variables that can effect adult populations. The role that multiphase life cycles and their complex inter-relationship with marine ecosystems play in determining population abundance and distribution is not clear. The larval stage of invertebrates has for many years been a largely unknown quantity, but knowledge about the role of larval behaviors is growing. In order to understand the processes by which larvae are dispersed in the water column and to assess recruitment potential, new chemical methods of identifying larval species with similar morphologies using an environmental sampling processor may make it easier to detect, identify and quantify different larval species in situ in the marine environment (Jones et al 2008) Where different behavioural patterns between species evade or take advantage of the general effects of physical oceanographic conditions such as currents or temperature variations, detection, analyses and quantification may further the understaning of the influence this may have on the complexities of settlement and recruitment and their subsequent effects on population abundances and distribution..