R.H. Emson ® D. Morritt ® E.B. Andrews ® C.M. Young Life on a hot dry beach: behavioural, physiological, and ultrastructural adaptations of the littorinid gastropod Cenchritis (Tectarius) muricatus Received: 15 June 2001 /Accepted: 9 October 2001 / Published online: 8 February 2002 Ó Springer-Verlag 2002 Abstract The distribution and abundance of the littori- nid gastropod Cenchritis (Tectarius) muricatus were de- termined on an exposed shore at Lee Stocking Island, Bahamas in July/August 1995 and June 1999. Activity patterns were observed on two occasions shortly before a hurricane passed near the island in 1995. C. muricatus extended from the low water mark to 3.6 m vertical height above it. Highest densities (20/m 2 ) were found just above the high tide mark but the animal remained relatively abundant (8/m 2 ) at the highest point. Small animals were only found at high shore levels. With the exception of animals very near to the water’s edge C. muricatus was inactive by day and on dry nights. Many individuals of the high shore populations became active at night during periods of high humidity and rainfall, moving distances of up to 4 m overnight. At lower levels, extensive pairing and copulation was associated with these periods of rainfall. Abundance changed dramatically after the passage of the hurricane. At the level of maximum abundance numbers were re- duced from 20/m 2 to 2.8/m 2 . Long-term experimental studies of water loss rate and haemolymph concentra- tion showed that both water loss rate and increase in haemolymph concentration are greatest in the early pe- riod of desiccation. The concentration of the blood after 20 weeks is 250% of the original and the water lost is of the order of 22.5%. After an initial increase there ap- pears to be evidence of regulation of haemolymph con- centration possibly by exchange with free water in the shell itself. The considerable reserve capacity in toler- ance is no doubt in part due to the structure and func- tioning of the kidney. Adaptive features of the excretory system identified from ultrastructural examination in- clude the replacement of podocyte-containing filtration chambers in the auricle by extracellular tubules perme- ating the walls of both auricle and ventricle, which suggests a reduction in the rate of primary urine for- mation. This is also indicated by a reduced surface area of the nephridial gland, which resorbs organic solutes from the urine. Excretory cells contain single large vacuoles in which arise large multi-layered concretions composed of phospholipids and calcium salts typical of prosobranchs in which excretory products may be stored for long periods. Introduction The littorinid snail Cenchritis muricatus is a familiar component of the littoral fauna in the Caribbean, Bahamas and southern Florida, often abundant from the waterline to well beyond the highest parts of the splash zone (Clench and Abbott 1942; Abbott 1954; Lewis 1960; Borkowski 1971, 1974; Bandel 1974; Bandel and Wedler 1987; Britton 1992; Lang et al. 1998). Many individuals near the upper limit of dis- tribution are, in dry periods, attached by a mucous film to maritime vegetation, often as much as 0.5 m above the adjacent rock surface where they are fully exposed to the harsh conditions characteristic of this environment. Such individuals appear totally inactive in fine weather both by day and night and previous authors (e.g. Bandel 1974) have assumed that they Marine Biology (2002) 140: 723–732 DOI 10.1007/s00227-001-0738-9 Communicated by J.P. Thorpe, Port Erin R.H. Emson (&) Division of Life Sciences, King’s College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NN, UK E-mail: roland.emson@kcl.ac.uk Fax: +44-020-78484500 D. Morritt ® E.B. Andrews School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK C.M. Young Department of Larval Ecology, Harbor Branch Oceanographic Institution, 5600 U.S. Highway 1 N., Fort Pierce, FL 34946, USA