The ecology, age and growth of two muricacean gastropods from shores of Kuwait

• Main Author: Alsayegh, Lujain J.
• Published: Bangor University 2015
• Subjects: 594
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665651

Two predatory muricid gastropods, Thalessa savignyi and Ergalatax junionae commonly occur on the rocky intertidal shores of Kuwait. Three shores, Ra’s Ajūzah, Ra’s Eqaila and Ra’s al Qulay’ah between the north and south of Kuwait covering a distance of ~90Km, were studied to determine the geographical and seasonal abundance of these two muricids. Thalessa savignyi was absent from Ra’s Ajūzah in the north, but increased in abundance towards the south of Kuwait. By contrast the smaller gastropod E. junionae was present on the three shores but decreased in abundance towards the southernmost site, Ra’s al Qulay’ah. Thalessa savignyi prefers rocky platforms and associated fissures which are present on the southern shore whilst small boulders and crevices on the shores in the north of Kuwait were favoured by E. junionae. Egg laying by both species occurred in the laboratory in late June and these egg masses hatched ~30days later in late July. Intertidal recruitment of juveniles (<10mm) appeared into the populations at Ra´s Ajūzah and Ra´s Eqaila between October and January whilst no recruitment was observed at Ra´s al Qulay´ah. Polymodal size frequency distributions in some of the populations were separated into their component size (age) classes using the method of Bhattacharya. Using these estimates of the modal size and ages, Von Bertalanffy growth (VBG) curves were constructed to compare the growth of both species. Thalessa savignyi grew faster than E. junionae and both grew faster at Ra´s al Qulay´ah compared with the other two sites. Growth rings on the operculum and in the statoliths were used to estimate the age of T. savingnyi and E. junionae. Age estimates using whole and polished sections of the operculae were problematic because rings could not be seen through the thick and opaque operculum of T. savignyi. The operculum of E. junionae was thin and transparent but the number of rings counted was almost double those found in the statoliths. Operculae sections did not reveal clear growth rings. A latitudinal variation in the definition and the number of weak growth rings in the statoliths was observed. Generally rings in statoliths of both muricids from the northern sites in Kuwait were clearly defined and they contained less weak rings than statoliths from muricids from the southern site. These statoliths contained a wide variety of strongly and weakly defined rings. It was established from seasonal collections that statolith growth rings are deposited annually between November and January. A larval ring, deposited during the period of life in the egg and a settlement ring, formed at the time of metamorphosis from the plankton, were validated in developing T. savignyi larvae in egg masses and from newly hatched larvae. The age of both species was determined successfully using statoliths and VBG population growth curves constructed. Thalessa savignyi was found to be a faster growing species with a shorter longevity (4 years) than the smaller, longer-lived (5 years) and slower growing E. junionae. In the laboratory both muricids were found to be predators of the tube worm Septibranchia krausii and the mussel Brachidontes variabilis. A comprehensive seasonal survey of the three intertidal shores documented the tidal distributions of the macrofauna and macroalgae. No correlation between the presence of potential prey species and the distribution of T. savignyi and E. junionae was found.