Life cycle of the pea crab Pinnotheres halingi sp nov., an obligate symbiont of the sea cucumber Holothuria scabra Jaeger

A new species of pea crab, Pinnotheres halingi sp. nov. (Pinnotheridae), fo und encysted in the right respiratory tree of the sea cucumber; Holothuria scabra (Holothuridea), from Solomon Islands, is described. The reproduction , infestation and pairing behaviour of the crabs were investigated through field observations and experiments. Infestation frequency in 8 monthly samp lings of 25-30 holothurians was 98.6 +/- 2.6% in Kogu Halingi bay and 0% in two nearby sites. Of 403 pea crabs, 91.4% were found in pairs of opposite sex, 7.9% were single females and <1% were single males. The embryos develo ped on the female pleopods over ca. 30 days from fertilisation to the relea se of first zoeae and subsequently went through five pelagic zoeal stages. Infestation occurred at the megalops stage after about 59 days of developme nt. A single pea;:rab (male or female) per host was found three months afte r larval infestation. Young males appeared to be strongly attracted to host s that sheltered a single female, suggesting that pairing occurred as a mal e <6 mm carapace width joined a female. Larger crabs could not enter the ho st. Copulation was observed within the female cyst, preceding or overlappin g oviposition. The male then progressed away from the female and from the a nus, forming its own cyst along the way. Both larvae and small sub-adults i nvaded H. scabra with a minimum length of 80 mm, exclusively. P. halingi in duced atrophy of the right respiratory tree of its host.