Lipid content variation among broods in developing
Cancer oregonensis embryos: relationships to larval fitness
Rhonda Elliott, Stephen Sulkin
Western Washington University, Bellingham, WA
Shannon Point Marine Center
The developing embryos of brachyuran crabs depend entirely on energy reserves supplied by the parent up until hatching. The newly hatched, independent planktonic larval stage, also known as a zoea, must use what little energy reserves it hatches with to support metabolism until it obtains nutrition by feeding in the plankton. The goal of this study was to determine the variation among broods in the parental investment of energy reserves in the eggs of the crab Cancer oregonensis and the subsequent relationship to larval fitness. Lipids were extracted from the eggs of ovigerous females during embryonic development from an undifferentiated state until hatching of the zoeae to obtain a percent lipid value with respect to egg mass. The C:N ratio present in the eggs throughout development of the broods was used as an indicator of lipid mobilization. We detected variation among broods in both the initial lipid content of the eggs and of freshly hatched zoeae. The C:N ratio also varied among broods but a general decline during development suggested lipids were metabolized. We found significant variation among broods in the ability of the larvae to withstand starvation. However, because we found a poor relationship between percent lipid at hatching and the subsequent zoeal survival rates, other environmental factors may be more important in explaining the larval fitness of zoeae when hatching into an uncertain prey environment. Different ratios of essential long-chain polyunsaturated fatty acids, comprising the lipid fraction in the embryos, may play an important role in the ability of the larvae to withstand starvation.
- During development from an undifferentiated state until hatching, the embryos in the brood of brachyuran crabs rely entirely on parental yolk reserves (Anger 1996).
- The zoeae must use what little energy they hatch with to swim in the water column and feed on plankton (Sulkin 1975).
- Larval fitness, measured as the capacity to resist nutritional stress in a scarce prey environment, may be enhanced by high lipid reserves still present in the egg at hatching.
- The goal of this study was to investigate energy reserves during embryogenesis of the brachyuran crab Cancer oregonensis. We seek to determine the variation in lipid content of freshly hatched zoeae and the subsequent relationship to larval fitness. Understanding this relationship may be helpful in predicting the survival rates of zoeae when hatching into an uncertain prey environment.
- We collected ovigerous female C. oregonensis from Burrow's Channel and held them in seperate labeled bins with fresh running seawater.
- Eggs were collected from each of the broods during embryonic development from an undifferentiated state until the heartbeat stage.
- We used an elemental analyzer to calculate the C:N ratio present in the eggs among broods and to determine temporal variation in energy use
- Total lipids were extracted from freshly hatched zoeae of five broods to obtain a percent lipid with respect to dry egg mass.
- To evaluate the larval fitness, we obtained 36 zoeae from the same five broods and placed them into separate cell wells with 5-µm filtered seawater.
- We starved the zoeae and calculated mean day of death.
Embryonic Development of Cancer oregonensis
January 13 ---> February 5 ---> February 11 ---> February 18 ---> February 26 ---> March 2
Figure 1 shows that the C:N ratio declined during embryonic development from an undifferentiated state to the heartbeat stage.There was significant variation among broods in the initial C:N ratio and the rate at which the C:N ratio declined (F=60.81, p<0.05, df=58). The error bars represent the standard error of the mean C:N ratio among ten broods.
Figure 2 shows that percent survival of the starved zoeae varied among broods; the starved zoeae lived for an average of 7 days.There were significant differences in the percent survival of zoeae among the five broods
(F=36.58, p<0.05,df=54).
Figure 3 shows that there were significant differences in the percent lipid at hatching among broods
(F=10.843, p<0.05, df=14). We found a significant relationship between percent lipid at hatching and the mean day of death (ability to withstand starvation) (R2=0.045, p<0.05, df=179). Although it appears that zoeae die faster with higher percent lipid at hatching, a slope of -0.060 is a weak relationship. The error bars represent the standard error of the mean day of death of 36 zoeae from each brood.
The decline of the C:N ratio suggests lipid mobilization over time, a trend seen in another brachyuran crab species, Chasmagnathus granulatus (Bas et al. 2007) . The C:N ratio varied among broods; perhaps due to slight variation in embryonic development stage within the brooding season. Variability in percent survival among broods may be attributed to different concentrations of long-chain polyunsaturated fatty acids (PUFAs) in the reserves, which were not measured. Long-chain PUFAs, especially eicosapentaenoic acid, are essential for the development, survival, and growth of brachyuran crab larvae (Levine and Sulkin 1984). Variation in specific lipid constituents may explain the weak relationship found between total lipid content of freshly hatched zoeae and subsequent ability to withstand starvation. Perhaps larval fitness of brachyuran crabs is dependent not only on high lipid content, but high concentrations of long-chain PUFAs.
This work was funded by National Science Foundation Grant #OCEO741372 through the Multicultural Initiative in the Marine Sciences Undergraduate Participation program (MIMSUP Grant #OCE-0228618). We thank Horng-Yuh Lee for assistance in crab egg collection and elemental analyzer data collection. We also thank Lauren Chomiczewski for assisting in the lipid extraction methodology. Gene McKeen and Nate Schwarck helped with SCUBA to collect ovigers for the research.
- Anger, K. 1996. Physiological and biochemical changes during lecithotrophic larval development and early juvenile growth in the northern stone crab, Lithodes maja (Decapoda: Anomura). Marine Biology 126: 283-296.
- Bas, C, Spivak, E, Anger, K. 2007. Seasonal and interpopulation variability in fecundity, egg size, and elemental composition (CHN) of eggs and larvae in a grapsoid crab, Chasmagnathus granulatus. Helgoland Marine Research 61: 225-237.
- Levine, D and Sulkin, S. 1984. Nutritional significance of long-chained polyunsaturated fatty acids to the zoeal development of the brachyuran crab, Euryanopeus depressus. Journal of Experimental Marine Biology and Ecology 81: 211-223.
- Sulkin, S. 1975. The significance of diet in the growth and development of larvae of the blue crab, Callinectes sapidus, under laboratory condition. Journal of Experimental Marine Biology and Ecology 20: 119-135.
Please contact me by email
Rhonda Elliott
Elliotr4[at]students.wwu.edu
Curriculum Vitae