The cypris larvae of the parasitic barnacle Heterosaccus lunatus (Crustacea, Cirripedia, Rhizocephala): some laboratory observations

Heterosaccus lunatus parasitizes the portunid crab, Charybdis callianassa i n Moreton Bay, Australia. With the host crabs maintained at 22.5 degreesC t his sacculinid rhizocephalan released larval broods every 6-7 days. During July-August 1996 and particularly August 1999 such broods showed the change -over from male only larvae in the early broods to females only in the late r broods. As the host crabs were maintained under similar aquarium conditio ns in both years it is concluded that the light/dark cycle is the principal cue triggering this larval sex reversal. Oogenesis in the parasite externa is somehow controlled to produce two different sized ova male larvae devel op from large ova and females from small ova. A working hypothesis outlinin g how sex is probably determined for the larvae of sacculinids is erected. H. lunatus is considered the ideal sacculinid for the further experimental work necessary to verify the proposed sex-determining mechanism and its con trol processes. Measurements of the maximum swimming speeds of H. lunatus m ale and female cyprids showed the larger males to be the faster in absolute terms (27.95 compared with 17.60 mm s(-1), respectively), however, the cal culated relative speeds were almost identical at approximate to 90 body len gths s(-1). Settlement experiments confirmed that female Ii. lunatus cyprid s settle only on the gills of C. callianassa; these cyprids needed to be at least 2 days old before they were able to settle. (C) 2000 Elsevier Scienc e B.V. All rights reserved.