A neotenous dwarf-form ofCapulus ungaricus(L.) (Gastropoda, prosobranchia) commensalistic onTurritella communisrisso

Abstract

Capulus ungaricus is, in its typical form, a semi-commensalistic, protandric, brood-protecting prosobranch with a 15–40 mm long, cap-shaped shell. It is usually located near the shell margin of certain large lamellibranchs of the suspension feeding type, exploiting its host partially by profiting from the water current the latter creates, partially by introducing its long, muscular proboscis into the mantle cavity of the lamellibranch thus »stealing« its food. While the typical form of Capulus occurs infrequently in inner Danish waters, a dwarf-form of up to 11 mm length of shell and regularly sitting on shells of living Turritella communis proves to be common locally in the northern Kattegat in mud bottoms inhabited by an Amphiura-community. Since Turritella as a host can only offer conditions less favourable and strikingly different from those which Capulus may usually obtain when located on a large lamellibranch, an analysis was made to find out why it prefers Turritella on this bottom and of the ecological consequences for Capulus of inhabiting this unusual host - all in relation to its dwarfish appearance. Since Capulus is molded to the narrow last whorl of the Turritella so as to fit its curvature, at least the older specimens must be bound to that site for life, thus automatically giving them very restricted possibilities for growth. The dwarf-form has a strongly divergent shape of shell, and the long proboscis, so important for the feeding activity of adults of the typical form, is at best rudimentary and often absent. In spite of having retained such juvenile characters, 22 out of the 70 dwarf-form specimens examined were found brooding their egg-sacks under their foot, thus being sexually mature. This fact, together with the juvenile anatomy, designates the dwarf-form as neotenous. Since the dwarf population is mainly recruited by echinospira larvae of the typical form drifting in with the current from the Skagerrak and the North Sea, this neoteny must be ecologically determined. In many marine invertebrates growth will stop at the same time as the production of eggs sets in. Here, the reciprocal process seems to take place; growth is stopped by the location on the narrow shell of the host, and the nutritive surplus usually used for enlarging the shell and the body can be used for an early production of eggs and sperm instead. Among the 70 dwarf-specimens examined, those with male-phase characters were far less numerous than female-phase ones. In four cases one male-phase and one female-phase animal were found together on the same Turritella, but all the rest were isolated specimens. A functional male of the Capulus dwarf-form isolated on its Turritella seems only to have a fairly small chance of copulating with another specimen. So, most female-phase specimens seem largely to be dependent on self-fertilization, probably by spermatozoa stored in their own receptaculum seminis during an early male phase in which a copulatory organ mayor may not be developed. The sparse and scattered occurrence of specimens of the typical form in Danish seas means that this must rely still more upon self-fertilization than the dwarf-form. Rough sketches of the larvae are given and their occurrence is discussed.