Reproductive traits in dicyemids

Several characters involved in the life cycle in dicyemids were examined to understand reproductive strategy and adaptations to cephalopod hosts. In most dicyemids distinctly small numbers of sperms are produced in a hermaphroditic gonad (infusorigen). The number of eggs and sperms are roughly equal (means of the number of sperm:egg=1:1.58). An inverse proportional relationship was found between the number of infusorigens and the gametes, suggesting a trade-off between them. Fecundity was positively correlated with the body size of adult stages (nematogens and rhombogens). Fecundity of a single dicyemid is not very high compared with that of the other endoparasite taxa, but a total reproductive capacity per community is high, because a great number of individuals multiply asexually in the renal sac. The size of mature infusoriform embryos (dispersal embryos) that develop from fertilized eggs was not correlated with their adult sizes, but the size of embryos was correlated with the maximum mantle length of the host octopus species. Although at present the process of infecting new hosts is still unknown, the size of the infusoriform embryo is likely determined by host-specific factors in this process. The size of vermiform embryos that are asexually formed from agametes was positively correlated with size of the adults; however, the number of vermiform embryos present in the axial cell of adults was not correlated with size of the adults. A correlation was not found between maximum mantle length of the host cephalopod species and length of the adult dicyemids. In dicyemid species the size of adults appears to be constrained by the renal habitat, including renal-pancreatic complex and branchial hearts of each host cephalopod species. Size thus may be determined by the volume of the renal sac, the diameter of the renal tubules, or the depth of folding in the surface of glandular renal appendages of cephalopods.