HISTRIOBDELLIDAE CLAUS AND MOQUIN-TANDON, 1884

Histriobdellidae (‘Charlie Chaplin worms’) are tiny members of Annelida that live as commensals on a variety of Crustacea. There are currently 13 nominal species placed into Histriobdella, Steineridrilus, and Stratiodrilus, with most of them known from rivers of South America. The name Histriobdella can be loosely translated as ‘clown animal’ and the common name used here refers to the amusing way in which they crawl, which is reminiscent of the walk of the famous comic actor. Living Histriobdellidae are mostly transparent, lacking in chaetae, but distinguished by dark chitinous jaws. Males of Steineridrilus cirolanae and all Stratiodrilus also show a dark chitinous penis in the mid-body and mature females may have a few whitish eggs. Adults range from 0.7 mm (e.g. Steineridrilus cirolanae) to around 1.5 mm in length (e.g. Stratiodrilus novaehollandiae) and have a fixed number of segments. This number appears to be nine in Histriobdella homari and may be a few more in the other taxa. Depending on the taxon, Histriobdellidae live among the branchial filaments, pleopods, or egg masses of their crustacean hosts.