G.G. Foster, A.N. Hodgson
Abstract:
Turbo sarmaticus, a large macroalgal herbivorous gastropod (up to 115 mm shell length) common to rocky shores of South Africa, is exploited for food and bait. Over-exploitation of shellfish, particularly limpets, on some South African shores has resulted in a decrease in macroalgal species diversity and the establishment of coralline turfs only. Such changes in the algal community could influence the growth rate and reproductive fitness of macroalgal grazers such as T. sarmaticus. The aim of this study was to determine whether diet would affect growth and reproduction in this gastropod.
Experiments in which juvenile T. sarmaticus (42 mm shell length) were fed one of four algal diets (ad lib.) for 12 months, showed that the best growth rate (up to 13.8 mm shell length increase per annum) was achieved in those animals fed Gelidium pristoides, Ulva rigida or a mixed diet (G. pristoides, U. rigida and Corallina spp.). Negligible growth (<3 mm increase in shell length) occurred in those juveniles fed coralline algae only. At the end of the experiment all animals, except those fed coralline algae, had a high gonad index (>29%) and tissue glycogen levels (>3.6 mg/100 mg foot tissue). Those juveniles raised on coralline algae had a gonad index of <1% and tissue glycogen levels of <3.5%.
Adult animals (>80 mm shell length) fed for six months (July to December) on G. pristoides, U. rigida or the mixed diet had a large gonad index (24-27%) which was significantly greater (p<0.0001) than that of field animals (13.5%). By contrast, animals fed coralline algae only, had a gonad index (4.4%) which was significantly smaller (p<0.0001) than both the other experimental, and the field animals.
Results from these experiments indicate that any changes to the macroalgal community structure of rocky shores could affect both the growth and reproductive fitness of T. sarmaticus as well as other macroalgal grazers.
(Department of Zoology & Entomology, Rhodes University, Grahamstown 6140, South Africa)