Nutrient utilization, acquisition and distribution during embryonic, larval development and metamorphosis of the sea cucumber Australostichopus mollis (Holothuroidea: Aspidochirotida)

Abstract

The egg nutrient composition and utilization during embryonic and larval development of the holothuroid Australostichopus mollis are examined in the context of maternal investment of the Echinoderms. At the same time, this study assesses the influence of different microalgal diets on the nature and quantity of nutrients accumulated by the feeding auricularia larvae, leading to the identification of important nutrients for the successful completion of metamorphosis and juvenile formation. Nutrient composition of the egg, mainly represented by structural compounds (proteins and phospholipids), reflected well that of echinoderms with planktotrophic development, although A. mollis egg nutritional content was slightly lower than that of other echinoderms with similar egg size. Triacylglycerol (TAG) was the main energetic lipid provided by the mother in the egg to fuel the formation of the feeding larvae. More studies on the facultative feeding period (FFP) of A. mollis are required to establish if A. mollis have lower metabolic rates than other echinoderms, as appears to be the case of planktotrophic ophiuroids. After the onset of larval first feeding, the microalgal diet did not affect the type of lipids accumulated as a nutrient reserve. The microalgal diet did, however, affect the ability of the larvae to build energetic reserves. Dunaliella tertiolecta was found to be an unsuitable diet for A. mollis, while Chaetoceros muelleri led to increased larval lipid accumulation. In preparation for the perimetamorphic period, A. mollis accumulated free fatty acids (FFA) and the same maternally-derived energetic lipid, TAG. Feeding A. mollis auricularia larvae showed that the hyaline spheres (HS), unique holothuroid larval structures, played an important nutritional role during the A. mollis perimetamorphic period. HS were a good indicator of larval nutritional condition, and served as the main storage location for TAG accumulated from the diet. Lipids appeared to be transported from the digestive epithelium to the area of formation of the HS in a novel way, which involved the mobilization of lipids through the gel-filled blastocoel in specialized lipid transporting cells (LTC). TAG in the HS supported the formation of the fully functional juvenile. The information presented in this thesis, which shows the first nutritional data on eggs, larvae and juveniles of a planktotrophic holothurian, is not only valuable for studies of life-history theory, maternal investment and the field of larval biology, but has significant implications for sea cucumber aquaculture.