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- Oliver Lewin
The energy cost of reproduction and its effects on the parasite infestations in Scyllium Canicula and Scyliorhinus stellaris
Higher investment in reproduction decreases the immune defences, and conduces to the use of structural resources, which may render individuals more susceptible to some parasites. (Veiga,. Et al 1998) this is consistent with the idea that an increase in reproductive effort has a negative effect on the ability to counteract parasite infection. This is due to the overall amount of energy an animal can devote to reproduction being limited. The habitat in which an animal lives normally contains limited resources, the environmental conditions may hinder energy acquisition by reduced foraging activity and there are morphological and physiological constraints on the rate at which food can be obtained and processed. (THOMAS H,. et al, 2004) The total energy that an animal obtains must be allocated in a way that balances homeostasis, growth, reproduction and survival. It is within the survival category that its immune defences would be placed. Therefore if more energy is placed into one category the others are then left with less.
Because of wide variation in life history traits, reproductive biology, and mating strategies, the ways, in which energy is expended for reproduction differ, greatly among species and between the sexes. Because most vertebrate species breed seasonally, they do not typically produce gametes year-round and many species maintain no reproductive gonadal, tissues in a reduced state. Thus, the initial investment, of energy into reproduction actually begins prior to, mating when energy is expended for gonadal, recrudescence. (THOMAS H,. et al, 2004)InS. caniculatheir Spawning can take place almost all year round. (Ellis, J.R.,et. al, 1997) However, there are can be seasonal patterns in spawning activity as well. For example,S. caniculafemales that are located off the Mediterranean coast of France lay their eggs from March to June and in December. In the waters surrounding Great Britain, egg laying occurs in spring with a gap between August and October. In the Tunisian Coast, the sharks lay their eggs starting in spring, peaking in the summer and then slightly decreasing during autumn. (Capae, C.,et. Al 2008.)
Once hormones are produced, they, stimulate the development or enlargement of the, testes, ovaries, and oviducts or uterus. Once the, reproductive organs are ready, energy is expended to, produce gametes. For males, this includes spermatogenesis, and production of seminal fluids. For, female’s egg production starts with oogenesis but, goes beyond the simple production of sex cells. In S. caniculareproduction is oviparous as they deposit egg-cases that are protected by a horny capsule with long tendrils. The deposited egg-cases are mostly on macroalgae in shallow coastal waters. When the egg-cases are deposited farther from shore, they are placed on sessile erect invertebrates. These eggs usually measure 4cm by 2cm, without ever exceeding 6cm. (Ellis, J.R.,et. al 1997) These egg-cases can be found around the coasts of Europe. The embryos develop for 5–11 months, depending on the sea temperature and the young are born with a measurement of 9–10cm.
In addition to physiological investment in reproduction, a large amount of energy is expended for reproductive activities. Examples of reproductive activities include finding and defending a suitable territory or nest site, fighting or repelling competitors and finding, choosing, and, courting a mate. The act of copulation is a, reproductive activity, as is the laying of eggs and carrying eggs. Males of some species actively guard their mates prior to and/or after, copulation to ensure paternity. Males reach sexual maturity with a length of about 37.1-48.8cm. Females reach sexual maturity with a length of 36.4-46.7cm. (Ivory, P.et al 2005)
In some cases, it may be difficult to draw the, line as to whether a particular activity should be, classified as reproductive. Nonbreeding types of, activities that occur during the breeding season may, occur more frequently to support reproduction. For, example, some animals might need to forage and, feed more often prior to or during the breeding, season to support reproduction, and males might, forage to provide nuptial gifts to impress prospective, also reproductive females might need to, maintain higher than normal body temperatures and, basal metabolic rates (BMRs) to promote the optimal to increase the rate of, digestion, assimilation, and egg production. (THOMAS H,. et al, 2004)
The existence of trade-offs between life-history traits is, a central assumption in life-history theory, several studies have demonstrated, that an increase in reproductive effort compromises, offspring fitness, future reproduction or survival (Lessells 1991, Stearns 1992). Infectious diseases are crucial elements in establishing how variations in reproductive effort, affect allocation of energy among different life history traits ( Hamilton and Zuk 1988). In males of vertebrate species without, parental care, reproductive effort is, in general, directly associated with circulating testosterone levels because, testosterone can induce the production of secondary, sexual characters and can increase mobility, aggressiveness and sexual behaviour (Ketterson , Nolan 1992).
The energy costs associated with the increase of testosterone levels cannot, be compensated by additional food intake, as males with increased testosterone decrease their food intake, and gain less weight than untreated controls (Gentry and Wade , 1976, Marler and Moore 1988, Abelenda et al. 1992). It, has been proposed recently that testosterone production is traded against immune defence, as this hormone, directly deteriorates some white blood cell types or, because it detracts resources that otherwise would be, allocated to immune defence. Oestrogens on the other hand modulate immune function in females and may contribute to resistance against infection. Oestrogens affect both innate and acquired immune function. Oestrogens can enhance both cell-mediated and humoral immune responses; there are, however, reports of oestrogens suppressing some cell-mediated immune responses. The cellular and molecular mechanisms mediating oestrogenic effects on immune function have not been fully elucidated. Oestrogens also may enhance immune function in females by protecting immune cells against apoptosis (Hofmann-Lehmann R. , et al, 1998, Vegeto E,. Et al, 1999)
Level of maturity?
To find out the effects of parasite infestation on sexually mature specimens a standard level of maturity needs to be set. Rodriguez-Cabello,et results indicate that the length at sexual maturity of females is between 49.7 – 59.1 cm, when all their data was combined. This length is shorter than that found in the North Atlantic, and longer than that found in the Mediterranean Sea, in accordance with the hypothesis that the first- maturity length is determined by latitude. Although no data were collected on male sexual maturity, several studies conducted in different areas seem to agree that the length of first sexual maturity is the same for males and females. The size of the specimins gonads and reproductive organs could indicate the level at which they are sexually mature, ie the larger the mass of the reproductive organs the more mature they are, the mass could also indicate the level of hormones present, such as testosterone and oestrogen. Using these methods of measuring sexual maturity, various discoveries could be made. Such as with the use ofTestes/Ovaries (g)Claspers/Egg mass (g) as a level of sexual maturity and compare that to parasite numbers as a way of seeing if the level of sexual maturity has an effect on the parasite infestation, showing if energy consumption could hinder their immune response.
Other factors could be looked at and compared, such as the different levels of parasite infestation between mature males and females. Mature specimens could be compared with immature ones, Females that are carrying eggs could be interesting to look at and see if the production and carrying of eggs has an impact on the specimen’s ability to fend of parasite invasion. Hormone levels could also be an interesting area to investigate, due to the fact that hormone levels within the specimen could drastically affect its immune responses and therefore lead to an increase or decrease in parasite infestation.
General methods: 1500
Results – 1500
Dic/conc – 2000
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