A scanning-electron microscopic study of Dickdellia labioflecta (Dell, 1990) (Gastropoda, Littorinoidea) on Colossendeis megalonyx megalonyx Fry and Hedgpeth, 1969 (Pycnogonida, Colossendeidae): a test for ectoparasitism

Abstract

A scanning-electron microscopic (SEM) study of a clutch of eggs and juveniles of Dickdellia labioflecta on Colossendeis megalonyx megalonyx revealed that the young snails remove the upper layers of the pycnogonid cuticle, most probably by rasping. This way holes are created in the cuticle that could serve as a potential source of food for the snail. Measurements and statistics show that the holes have the same density as the cuticular glands. These glands are spread all over the pycnogonid cuticle. Additionally, they are filled with cytoplasmic material having a fine structure and cuticular surrounding that is typical for those glands. Hence, it is suggested that the snails get access to the interior of the pycnogonid through the holes and glands. Holes in the cuticle are absent under smaller Dickdellia specimens, but they are formed consecutively as they become older and grow larger. It is suggested that Dickdellia snails are ectoparasites because this would allow them to remain on pycnogonids until they exceed the initial egg volume, which would be difficult to explain without additional food uptake from the pycnogonid host.