Proceedings of the 11th International Coral Reef Symposium, Ft. Lauderdale, Florida, 7 – 11th July 2008 Session number 11
The effects of temperature and light on the gametogenesis and spawning of four sea urchin and one sea cucumber species on coral reefs in Kenya
N.A. Muthiga1,2, J. Kawaka2 1) Wildlife Conservation Society, 2300 S Boulevard, Bronx, New York 10460, USA 2) Wildlife Conservation Society, 99470, Mombasa 80107, Kenya Abstract. Seasonality in reproduction in temperate marine invertebrates is well recognized, but reproduction at the equator where environmental variability is lower is typically assumed to be continuous. This paper reviews studies of the reproductive cycles of four species of sea urchins and one species of sea cucumber to test for seasonality and sexual synchrony along the Kenyan coast (10S - 50S). Gonad index measurements and macro and microscopic observations of gonads were used to evaluate monthly changes in gonad growth. The urchin Echinometra mathaei and sea cucumber Holothuria arenacava displayed a seasonal reproductive pattern with gametogenesis commencing in July during the southeast monsoons and spawning peaking between March-April during the northeast monsoons. Monthly temperature and light significantly correlated with gonad index in these species but light displayed a stronger response. The sympatric urchins Diadema savignyi and D. setosum had a monthly gametogenic cycle with D. savignyi spawning after full moon and D. setosum spawning in the last quarter. These species also showed seasonal gonad growth with D. savignyi following a pattern similar to E. mathaei. The life history strategies of these species included a high level of sexual synchrony and spawning at the time most favorable for larval growth. Key words: Reproductive seasonality, sea cucumbers, sea urchins. are suggested to exercise a strong selective pressure promoting reproductive synchrony and Introduction success. Reproduction in echinoids and holothuroids is The Kenyan coast occurs at the equator (10S – reported to display seasonal reproductive cycles 0 with temperate species generally having discrete 5 S) but is exposed to monsoonal seasonality spawning periods and tropical species with varying oceanographic conditions of reproducing for longer periods throughout the temperature, light, and productivity year (Orton 1920, Giese and Pearse 1974, (McClanahan 1988). This makes it suitable for Smiley et al. 1991). Reproductive activity was testing the effects of monsoonal seasonality on therefore hypothesized to be less synchronized breeding patterns of marine invertebrates at the and continuous throughout the year at the equator. This paper reviews studies of four equator since environmental variability is less species of sea urchins and one species of sea pronounced (Orton 1920, Giese and Pearse 1974, cucumber that were carried out on the Kenyan Pearse 1974). However, few studies have coast (Muthiga, 2003, 2005, Muthiga and compared the reproduction of several species of Jaccarini 2005, Muthiga 2006), to test the echinoderms at the same location at the equator hypothesis that reproduction will show a weak (Pearse 1974). seasonal pattern at the equator due to reduced Gametogenesis in echinoderms has been variability of the environmental factors that correlated with environmental factors such as control gametogenesis and spawning. water temperature and photoperiod, while spawning is suggested to be triggered by changes Materials and methods in water temperature, food availability, light The studies were undertaken at Kanamai an intensity, water turbulence, salinity and unprotected reef and Ras Iwatine within the phytoplankton blooms (Krishnaswamy and Mombasa marine reserve located between 30S Krishnan 1967, Himmelman 1980, Engstrom and 50S. Study sites are described in Muthiga 1980, Cameron and Fankboner 1986, Pearse et and Jaccarini (2005), and Muthiga (2005, 2006). al. 1986, Ramofafia et al. 2000, 2003). These At each site ten to twenty specimen were factors often vary spatially and temporally and collected each month for a period of one to three

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