Branchiobdellid size-crayfish size: a possible relationship

The aim of this paper was to verify a possible relationship between the size of the ectocommensal annelid Branchiobdella italica Canegallo, 1929 and that of its host, the freshwater crayfish Austropotamobius pallipes (Lereboullet 1858). 225 worms were collected from a crayfish population inhabiting a stream in central Liguria, north-western Italy. The sizes of B. italica ranged from 1 to 3.7 mm total length. The highest proportion of immature worms (41.7%) was collected from juvenile crayfish (size-range 11-18 mm carapace length), 15% from subadult (19-26 mm) and 3.4% from adult crayfish (≥27 mm). The size-frequency distributions of worms from these three crayfish age groups differed significantly, except those from subadult and adult crayfish. It is arguable that the scarce presence of immature branchiobdellids on subadult and adult crayfish is due to the strong competition for space and food that these small worms have to withstand from co-specific adults.