Growth, reproduction and feeding biology of Turbo sarmaticus (Mollusca : Vetigastropoda) along the coast of the Eastern Cape Province of South Africa

Investigations were carried out on aspects of the biology of the vetigastropod Turbo sarmaticus. Studies included: 1) the distribution and standing stock of this animal at four sites along the coast of the Eastern Cape Province of South Africa; 2) the growth rate of animals on a wave-cut platform; 3) the reproductive cycle of an intertidal population; and 4) aspects of the feeding biology examining the ability of this mollusc to consume and digest six macroalgae, the influence of algal diet on growth rate and reproductive fitness and the polysaccharolytic activity of the digestive enzymes. On eastern Cape shores, T. sarmaticus had a size related distribution, with smaller animals being found towards the upper mid-shore and larger animals being found in a downshore direction. The mean shore densities of T. sarmaticus at three sites where exploitation of animals was minimal, were very similar (1.2 - 1.7 individuals/m²). The largest animals (up to 110 mm shell length) were found on an offshore island. This may have been a result of animals not being exploited, as well as a possible increase in primary productivity and food availability. The lowest density (0.2 individuals/m²) and animal size (<70 mm shell length) was recorded at a site (Kelly's beach - Port Alfred) where exploitation was more intense. It is probable that intense overexploitation was threatening the populations at this site. The growth rate of T. sarmaticus was determined by means of the von Bertalanffy growth model and expressed by the equation L[subscript]t = 81.07(l-e⁻°·⁵⁴⁴[superscript](t)). The initial growth rate of T. sarmaticus (up to ≈ 80 mm shell length) was similar on shores with different geomorphologies (i.e. boulder shores and wave-cut platforms). Growth rates of individuals were variable, which means that individuals within a population reached exploitable size (3 - 6 years old) and sexual maturity (1.5 - 2 years old) at different ages. Seasonality of reproduction of T. sarmaticus was determined using gonad index, egg diameters and spermatozoa content within the gonad. Turbo sarmaticus was dioecious and had a sex ratio in favour of males (1.2: 1). Animals attained sexual maturity at a size of about 52.5 mm shell length. There was little variation in the reproductive cycle over time with gametogenesis occurring from March/April until August/September, whilst maturity (Gonad Index = 15%) was maintained until the spawning event from December to March. After spawning the gonad regressed. Field and laboratory observations of the feeding biology of T. sarmaticus confirmed that this mollusc was a generalist grazer capable of consuming and digesting algae from the Rhodophyta, Chlorophyta and Phaeophyta. The consumption rates (juveniles: 1.45 - 9.50% body weight/day, adults: 1.06 - 6.08%) and digestibility (9 - 75% apparent dry matter) of six macroalgae was found to vary. For most algae, juvenile T. sarmaticus had higher consumption rates (1.6 - 2.8 times higher) and digestibility values (12 - 24% higher) than adults. It is suggested that consumption rates were dependent on the digestibility of the algae. In addition, it is suggested that the consumption rates of the different algae were not related to the nutritional content, but rather the energetic content of the algae. In both juvenile and adult animals, temperature had a positive influence on consumption rates, resulting in an increase at higher temperatures. However, in both juvenile and adult T. sarmaticus, algal digestibility was not affected by temperature. Finally, it was proposed that Viva rigida, Codium extricatum, Ecklonia radiata and Gelidium pristoides would provide the best nutritional value for growth and reproductive fitness in T. sarmaticus, whilst Jyengaria stellata and Corallina spp. would provide the poorest. Experiments on the effects of four algal diets on the biology of T. sarmaticus showed that the best growth rate (up to 13.8 mm shell length increase per annum), reproductive fitness (Gonad Index up to 33%) and energy levels (up to 4.76% glycogen in the foot) were achieved when T. sarmaticus was fed G. pristoides, U. rigid a or a mixed diet. Turbo sarmaticus fed Corallina spp. showed reduced growth (2.4 mm shell length increase), reproductive fitness (Gonad Index up to 4.4%) and energy levels (up to 3.42% glycogen in the foot). A study of the polysaccharolytic enzyme activity of T. sarmaticus indicated that this mollusc possesses enzymes that can, at least partially, digest most of the storage and structural polysaccharides found in the Chlorophyta, Rhodophyta and Phaeophyta. This further supported the findings that T. sarmaticus was a generalist grazer. Two levels of activity were detected: 1) high levels of enzyme activity (up to 328.2 Ilglmglmllhr)occurred on the storage polysaccharides that occur in the Rhodophyta and Chlorophyta, and 2) lower levels of activity were detected on the storage polysaccharides (up to 44.8 μg/mg/ml/hr) of the Phaeophyta and on all the structural polysaccharides tested (