Reproduction and population dynamics of the solitary entoproct Loxosomella plakorticola inhabiting a demosponge, Plakortis sp.

摘要

Solitary entoprocts are sessile aquatic invertebrates inhabiting various non-living or living substrata. They proliferate through both sexual and asexual reproduction. However, fecundity and population dynamics of entoprocts are poorly understood, probably because the small size of the individuals (0.2 to 5 mm length) makes field studies difficult. To clarify the seasonal fluctuation of fecundity via sexual and asexual reproduction, we sampled the solitary entoproct Loxosomella plakorticola Iseto & Sugiyama, 2008 (Entoprocta: Loxosomatidae) monthly over 2 yr. L. plakorticola often forms dense aggregates on a coral-reef demosponge, Plakortis sp. Budding occurred all year round and was most active from spring to early summer. There was a significant negative relationship between bud production and water temperature 2 mo previously. In contrast, sexual reproduction rarely occurred during our monitoring. Monthly monitoring of 4 sponges for 13 to 23 mo revealed that the density and distribution pattern of L. plakorticola on the sponges fluctuated highly regardless of season. Large populations sometimes disappeared from the host sponge within short periods, while, in other cases, a small, sparse population grew to a large, dense population within a few months. DNA sequencing analysis demonstrated that individuals on the same sponge did not belong to a single clone. Thus, the entoprocts probably disperse from one sponge to others. Populations on sponges were mainly formed and maintained by budding; sexual reproduction may contribute to longdistance dispersal and maintenance of genetic heterogeneity.