Parasites and other symbionts of squat lobsters

Squat lobsters (members of the anomuran superfamilies Chirostyloidea and Galatheoidea) are known as hosts for a variety of symbiotic organisms, particularly parasites. One hundred and thirty-four of the approximately 930 described squat lobster species harbour 122 symbiont species from nine metazoan phyla, as well as protistan and bacterial taxa. The nonparasitic groups include at least 20 associate species, but further analyses are required to clarify many of the symbiotic relationships with squat lobsters for these taxa (Bacteria, Foraminifera, Mesomycetozoa, Ciliata, Cnidaria, Annelida, Mollusca and poecilasmatid barnacles). Records of nemerteans (or suspected nemerteans) found in association with ovigerous female squat lobsters are likely to be egg predators. Among the confirmed primary parasites of squat lobsters, 22 species of parasitic barnacles (Rhizocephala) and 71 species of parasitic isopods (Bopyroidea and Cryptoniscoidea) are known. A further 53 records of bopyrid isopods and 73 records of rhizocephalans are noted but not identified to species (or often even to genus) in the literature; most of these records were from squat lobsters not otherwise recorded as hosts for described parasites. Parasites in other phyla are few but include endoparasitic helminths (Nematoda, Nematomorpha and Platyhelminthes). In terms of trophic strategies, rhizocephalans, nematomorphans and platyhelminths are complete parasitic castrators and the parasitic isopods are partial parasitic castrators. Nematodes and trematodes are trophically transmitted parasites and use squat lobsters as intermediate hosts, but the importance of squat lobsters as intermediate hosts may be greatly underestimated due to insufficient worldwide sampling for parasites. In addition, many common parasitic taxa (e.g. coccidians, cestodes, acanthocephalans) are not currently known from, but likely occur in, squat lobsters. The best-studied squat lobster host species in terms of symbiont diversity are from European waters, the western Atlantic and the eastern Pacific, a pattern similar to that seen in other anomurans. The lower parasite diversity of the Indo-West Pacific region is presumably due to limited sampling, and many species are known only from single records in localities such as Indonesia and New Zealand. Species belonging to three genera of hyperparasites (a rhizocephalan and two cryptoniscoid isopods) have been found infecting bopyrid primary parasites of squat lobsters. The biology and life histories of most squat lobster parasites and symbionts, with the exception of rhizocephalans and some isopods, are poorly known. The information presented herein summarises the geographical distribution of squat lobsters as host species, prevalence of their parasites, shared evolutionary histories of hosts and parasites, and known host– parasite relationships.