Morphometric Relationships Between the Hermit Crab Isocheles sawayai (Forest & Saint Laurent) (Crustacea Anomura Diogenidae) and its Shell from Southern Brazil

A study on morphometric relationships between the hermit crab Isocheles sawayai and its shell in a population from Parana-Santa Catarina States coast was carried out. Hermit crabs from the by-catch of shrimp fishing (4-15m depth) were collected. Cephalothoracic shield length (CL) and width (CW) from hermit crabs and total length (SL), aperture length (AL) and weight (SW) from shells were taken. A total of 266 hermit crabs occupying eight species of shell - Buccinanops lamarckii, Dorsanum moniliferum, Oliva reticularis, Olivancillaria steeriae, O. urceus, Polinices hepaticus, Semicassis granulatum and Stramonita haemastoma - were obtained. Males (N = 189) that dominated strongly (1M: 0.4 F) occupied eight shell species, while females (N = 76) occupied only six. There was only one intersex individual that occupied a Stramonita shell. Contrarily to intertidal populations, O. urceus was the most frequently occupied shell by males (57.14%) and females (62.33%) in this studied area. Due to the smaller size of females (mean CL 5.19+/-0.84mm), these ones occupied shells of a smaller size than males (mean CL 6.49+/-1.14mm). The highest O. urceus shell frequency of occupation is connected to the hermit crabs life style: it is a filter feeding species and its shell remains totally buried in the sandy substratum having only its cephalothoracic appendages free. The heavy shell of this gastropod (highest SW/SL) favors the stability in strong current water conditions, as it functions as an anchorage for this semi-buried hermit crab.