Hypersymbioses in the pinnotherid crabs (Decapoda: Brachyura: Pinnotheridae): a review

Members of the brachyuran family Pinnotheridae are nearly all symbionts of other invertebrates; some crabs are parasitic and others are commensal. Most live inside bivalve molluscs or in the tubes or burrows of polychaetes and other marine organisms. Animals living on or in pinnotherid crabs are considered to be hypersymbiotic and are reviewed here. Hypersymbionts are poorly represented within only 26 members (8.6%) of the Pinnotheridae (20 species in the subfamily Pinnotherinae and six species in the Pinnothereliinae). Parasitic hypersymbionts are as follows: three species of fungi; one cestode larva (undescribed procercoid of Trypanorhynchida); one trematode metacercaria (undescribed Microphallidae); nematode cysts (undescribed); three species of Nemertea (Carcinonemertidae, two undescribed); two species of rhizocephalan Cirripedia (Sacculinidae; plus five or more undescribed potential sacculinid species); epicaridean Isopoda [13 species of Bopyridae (one undescribed) and four species of entoniscids (Entoniscidae, three undescribed)]. Preliminary biological information on undescribed entoniscids is presented. Several mainly incidental hypersymbioses involving ectosymbionts are known primarily from Pinnixa chaetopterana, a symbiont of polychaete burrows. The ctenostome bryozoan Triticella elongata is the only known obligate ectosymbiont in P. chaetopterana, but it also infests five other species of pinnotherids. Some of the other ectosymbionts (all incidental) on P. chaetopterana are stalked ciliates, hydroids, juvenile polychaetes, bivalve molluscs, balanomorph barnacles, harpacticoid copepods and urochordates. Species of ectosymbionts including an ectoproct bryozoan have been reported from other pinnotherid species. Factors influencing our meagre knowledge of hypersymbioses in the Pinnotheridae are discussed, among them the inaccessibility of crab hosts and research emphasis on taxonomy within the family.