The Role Of Early Life History Strategies On The Population Dynamics Of The Sea Urchin Echinometra Mathaei (de Blainville) On Reefs' In Kenya
Sea urchins are marine benthic invertebrates that are dominant grazers in a wide range of habitats in tropical and temperate environments. Their grazing activities have major biological and geological effects on coral reefs, sea grass beds, and kelp forests (Lawrence, 1975; Estes et al., 1978; Lawrence and Sammarco, 1982; Carpenter, 1986; Lessios, 1988; McClanahan and Muthiga, 1988; Watanabe and Harrold, 1991). The sea urchin, Echinometramathaei, the world's most abundant sea urchin, (Palurnbi and Metz, (991) is a common inhabitant of reef lagoons, reef flats and back-reef rocky shores and fringing reefs along the east African coast (Khamala, 1971; Herring, 1972; Ruwa, 1984; Muthiga and McClanahan, (987). Recent studies have shown that the population of E. mathaei is increasing on Kenya's fished reefs. The increase in E. mathaei is due to a reduction in the numbers of its predators as a result of overfishing (Muthiga and McClanahan, 1987; McClanahan and Shafir, 1990; McClanahan and Kurtis, 1991). Increased densities of E. mathaei result in increased bioerosion leading to reduced topographic complexity, reduced species diversity and decreased fisheries productivity. Understanding the factors that control the population density of E. mathaei is therefore crucial to the conservation and sustainable utilization of Kenyan coral-reefs. This study provides information on important aspects of the population biology 'OfE. mathaei including reproduction, recruitment and growth and the factors affecting these life history strategies. The reproductive pattern of E. mathaei on the Kenyan coast and the influence of seasonality on this pattern is detailed in Chapter 2. The east coast of Africa experiences strong seasonality due to monsoons. This monsoonal seasonality allows for the testing of the effects of seasonally changing environmental factors on the reproductive activity of a tropical marine invertebrate. Results showed that on Kenyan reefs, E. mathaei has a seasonal reproductive pattern with gametogenesis beginning in July - August and spawning activity peaking in March-April. This pattern was similar to the reproductive patterns of E. mathaei in Japan and the Gulf of Suez except that spawning occurred in the summer months in these areas (Pearse, 1969; Arakaki and Uehera, 1991). The reproductive activity of E. mathaei on the east African coast was found to be highly correlated with temperature and light which also followed a similar seasonal pattern. Temperature however was not considered to be the important cue for the onset of gametogenesis. This is because minimum temperatures on East African reefs are above the critical temperature for the onset of gametogenesis for E. mathaei elsewhere in its distribution. Light may have an influence on reproduction in E. mathaei but the mechanism by which light controls gametogenesis in E. mathaei was not explored in this study. Reproduction in E. mathaei also correlated significantly withchloropbyll a concentrations offset by one month. This suggests that spawning in E. mathaei is timed to occur sometime prior to the phytoplankton peak which would ensure adequate food availability for the feeding larvae. Spawning in sea urchins has previously been shown to coincide with the spring phytoplankton bloom in temperate environments (Himmelman, 1980). The results of this study indicate that although environmental parameters on the east coast of Africa may not vary with as great a range as in temperate environments, temperature, light and chlorophyll concentrations all peak near the same time of the year. The combined effects of these factors-would act as an ultimate cause of seasonal reproductive behavior in this region. The reproductive strategy of E. mathaei is further explored by a detailed study of the fecundity and egg sizes of different populations on Kenyan reef lagoons (Chapter 3). The results indicate that E. mathaei produced significantly more eggs at Vipingo, the reef with the highest food availability which is consistent witb the classic allocation model (Gadgil and Bossert, 1970). Fecundity however, fluctuated from year to year probably as a response to fluctuations in food availability. Maternal size was a good predictor of parental investment at Vipingo where larger urchins tended to have a higher fecundity but not at Diani or Kanamai. Contrary to expectations, E. mathaei at Diani allocated more resources to reproduction despite food limitation. This was achieved at the expense of body size - an appropriate adaptation to food limitation (Thompson, 1982). Egg size is another common parameter used to gauge parental investment. The diameter of E. mathaei eggs however did not show any relationship with fecundity which is contrary to life-history models that predict the production of large numbers of small eggs or small numbers of large eggs (Vance, 1972,1973; Roff, 1992). Moreover, egg diameter showed no relationship with maternal size further indicating that egg size was not a good predictor of maternal investment in E. mathaei. Similar studies on other echinoderm species have shown no relationship between maternal and egg size (McEdward and Carson 1987; Lessios 1987) although George (1994) showed the opposite in a brooding starfish. E. mathaei like most marine benthic invertebrates has a planktonic stage and a sessile benthic stage. A study of its recruitment patterns and the factors that control recruitment are key to understanding the processes that control the abundance and distribution of this widespread sea urchin. A detailed study of the recruitment of E. mathaei is described in Chapter 4. Results indicate that E. mathaei recruits to the reef on an annual basis at the end of the northeast monsoon period (April-May) on the Kenyan coast. Recruitment is closely linked to reproductive activity of the local population with the earliest recruits appearing on the subslrate 1 to 2 months after the peak spawning period.