Occurrence of interspecific cross-fertilization among echinoderms and mollusks

Summary

Cross-fertilization experiments were conducted using two species of asteroids, one echinoid, one holothuroid and two species of mollusks to dertermine whether gametes from members of different classes, and even different phyla, are naturally able to fertilize one another. All but one of the crosses were successful, showing variable success, 58–99% lower than for intraspecific fertilization. Although most were not viable and showed anarchic cleavage, some resulting embryos could be reared to the blastula stage. Furthermore, these experiments led to the first case of intergeneric hybridization among asteroids, resulting in viable post-metamorphic sea stars. The hybrids of Leptasterias polaris (spermatozoa) and Asterias vulgaris (oocytes) showed a developmental kinetic, a survival rate and an external phenotype similar to those observed after normal intraspecific fertilization in A. vulgaris. In natural conditions a temporal barrier appears to be the key in preventing such hybridization, as these two sympatric species of sea stars, commonly found in the coastal water of eastern Canada, have distinct breeding seasons. However, other members of the echinoderms and mollusks among which cross-fertilization occurred are known to spawn during the same period, increasing possible wastage of gametes due to non-specific interactions. A potential chemical control of hybridization was present in a diffusible compound originating from the mature oocytes of A. vulgaris. This reduced the motility of heterospecific spermatozoa, especially those of Strongylocentrotus droebachiensis which were immobilized within 12 s in the vicinity of an oocyte.

Key words:

• Cross-fertilization
• hybridization
• gamete wastage
• echinoderms
• mollusks