Infestation of shore crab gills by a free-living mussel species

Parasitic and commensal species can impact the structure and function of ecological communities and are typically highly specialized to overcome host defences. Here, we report multiple instances of a normally free-living species, the blue mussel Mytilus edulis Linnaeus, 1758, inhabiting the branchial chamber of the shore crab Carcinus maenas (Linnaeus, 1758) collected from widely separated geographical locations. A total of 127 C. maenas were examined from four locations in the English Channel, one location in the Irish Sea and two locations at the entrance of the Baltic Sea. The branchial chambers of three crabs (one from the English Channel and two from Gullmar Fjord, Sweden) were infested with mussels resembling the genus Mytilus. Sequencing at the Me15/16 locus on the polyphenolic adhesive protein gene confirmed the identity as M. edulis. Bivalve infestation always occurred in larger red male individuals. Up to 16 mussels, ranging from 2 to 11 mm in shell length, were found in each individual, either wedged between gill lamellae or attached to the branchial chamber inner wall. This is one of the first reports of a bivalve inhabiting crustacean gills and is an intriguing case of a normally free-living prey species infesting its predato

Digenean trematodes infecting the tropical abalone Haliotis asinina have species-specific cercarial emergence patterns that follow daily or semilunar spawning cycles

Approximately 1-2% of the tropical abalone Haliotis asinina inhabiting Heron Island Reef are infected with opecoelid digeneans. These largely inhabit the haemocoel surrounding the cerebral ganglia and digestive gland-gonad complex, and infected abalone typically have significantly reduced or ablated gonads. Observations of infected abalone reveal two distinct cercarial emergence patterns, one which correlates tightly with the abalone's highly regular and synchronous fortnightly spawning cycle, and the other which occurs in a circadian pattern. The former appears to be a novel emergence strategy not previously observed in digeneans. While the cercariae in all abalone are morphologically indistinguishable, comparison of sequences from the internal transcribed spacer 2 (ITS 2) region of the ribosomal DNA reveals a 5.7% difference between cercariae displaying different emergence patterns, indicating these are two distinct species that probably belong to the same genus. The ITS 2 sequences of the species with the daily emergence pattern are identical to that of an undescribed adult opecoelid from the gut of the barramundi cod, Cromileptes altivelis. Combined molecular, morphological and emergence data suggest that while these opecoelid cercariae use the same first intermediate host and are closely related species-members of the genus Allopodocotyle-they fill different ecological niches that are likely to include different definitive hosts