Genus Holothuria Research Articles

Sediment patch selectivity in tropical sea cucumbers (Holothurioidea: Aspidochirotida) analysed with multiple choice experiments

To investigate patch selectivity in aspidochirotide holothurians, individuals of five species ( Holothuria ( Halodeima) atra Jäger, H. ( H.) edulis Lesson, H. ( Microthele) nobilis Selenka, Stichopus chloronotus Brandt and S. variegatus Semper) were subjected to multiple choice experiments. As a food source, sediments were pre-cultivated in petri dishes under different light and nutrient regimes. This resulted in four sediment treatments with different levels of microalgal biomass (measured as chlorophyll a and phaeophytin concentrations). Only two sediment treatments were used for experiments with H. nobilis and S. variegatus. The sediments were offered simultaneously to individual holothurians (six per experiment), and the weights of the sediment in each petri dish at the start and after 48 h were used to calculate a selection index together with confidence intervals for each food type. In experiments with H. atra and H. edulis, the animals exhibited no preference for any food type. In contrast, S. chloronotus significantly selected sediments with the highest contents of microalgae and avoided the sediment with the lowest pigment concentrations. These results were supported by field collections of sediments found directly underneath holothurians. Sediment underneath H. edulis did not differ from the average sediment of the habitat, while H. atra was found on sediments only slightly higher in chlorophyll a. Chlorophyll a concentrations underneath S. chloronotus were distinctly higher than in the adjacent sediment and that underneath H. atra. H. nobilis showed only a weak preference for sediments with higher pigment concentrations in aquarium experiments, and no patch selectivity in this species was found in the field. Stichopus variegatus exhibited a very distinct patch selectivity towards sediments with more nutritional value in both aquaria experiments and field measurements. Thus members of the genus Holothuria had no, or only a weak, tendency to select their food source, whereas both Stichopus species appeared to carefully select the sediment patch to feed on.

Specificity of sperm chemotaxis among great barrier reef shallow-water Holothurians and Ophiuroids

Sperm chemotaxis and sperm motility activation by egg or ovarian extracts were demonstrated in 24 holothurians (six genera from three families) and 22 ophiuroids (eight genera from five families) from the Australian Great Barrier Reef. Specificity was observed mainly at the family level in holothurians, but a single case of specificity at the species level was found in the genus Bohadschia. No recognizable specificity was found between any of the currently recognized subgeneric groups within the genus Holothuria. A similar pattern of specificity was previously observed in a group of dendrochiridotid holothurians in temperate waters. In contrast to the holothurians, specificity in ophiuroids existed mainly at the genus or species level. The best case of specificity at the species level was among six species in the genus Macrophiothrix which were reciprocally tested and a further three species in the same genus which were partially tested. Examples of partial specificity at the species level were also found in the general Ophiarthrum and Ophiocoma. The sperm chemotaxis assay reliably sorted to species a random collection of unidentified Macrophiothrix ophiuroids. These results suggest that sperm chemotaxis may play a role in gamete recognition prior to fertilization in one group of echinoderms. J. Exp. Zool. 279:189-200, 1997. © 1997 Wiley-Liss, Inc.