Low abundance and skewed population structure of the whelk Stramonita haemastoma along the Israeli Mediterranean coast

Stramonita (=Thais) haemastoma (Kool, 1987), a relatively large predatory snail found in rocky littoral ecosystems can attain densities of hundreds of individuals per m² in warm temperate coasts of the western Atlantic. It has also been reported as common along the Israeli Mediterranean coast; however our preliminary observations there suggested low abundances of this species. We measured densities of the whelk and those of its major potential prey by quadrat and belt transect (plots) methods, supported by underwater photography when applicable. Our study demonstrates that indeed densities of S. haemastoma along the Israeli Mediterranean coast are very low (<0.5 m^-2) in most habitats and locations, with the exception of some midlittoral habitats (density of ca 7 snails m^-2) where food and shelter are abundant. Food is probably not a limiting factor in most midlittoral habitats, and the low abundance of S. haemastoma is likely to be associated with scarcity of shelters and high risk of dislodgment or of desiccation. The population structure of the whelks is skewed to large snails. Small individuals were absent throughout the entire 5 yr of the study, while mean whelk size increased during this period. These results suggest impaired recruitment of whelks in recent years. Assuming that S. haemastoma was once abundant along the Israeli coast, it is possible that the following contemporary changes which occurred in the Levant basin may have affected whelk reproduction and/or juvenile survival rates and therefore may also explain the low recruitment rates: (1) diminished seasonal reduction in salinity due to the damming of the Nile; (2) unregulated use of tributyltin-based antifouling paints in Israel, which may impair reproduction through imposex; and (3) massive periodic appearance of a new plankton feeder along the Israeli coast, the Indo-pacific jellyfish Rhopilema nomadica, which may prey on the snail larvae.