Abundance and reproductive patterns of the excavating sponge Cliona vermifera: a threat to Pacific coral reefs?

Cliona vermifera is a common excavating sponge in coral reefs from the East Pacific. Abundance and reproductive patterns of the sponge in a Mexican Pacific coral reef over a 4-year period are herein described. Sponge abundance was estimated along three transects 50 m long which were randomly placed on the reef, and along each one, a piece of coral rubble and a branch of a live coral from the Pocillopora spp. coral colony closest to the transect were collected at random, approximately every 2 m, yielding 25 pieces of each category per transect (and 75 pieces total of each category). A 2-way ANOVA revealed that invasion was significantly higher in living coral colonies (34.8 %) than in rubble (13.7 %). It also indicated that the abundance in both coralline substrates showed a temporal variation without a clear pattern of increase over the years. It was estimated that 60–85 % of sponges in the population reproduced sexually every year. The sponge proved gonochoristic, with a sex ratio strongly departing from parity (1 male: 3 females). Over the 4-year study period, at least two cohorts of oocytes with densities of up to 3.5 oocytes per mm2 tissue were observed. Spermatogenesis lasted about a month, but often producing more than a pulse from July to November, coupled with peaks of oocyte maturation. Fertilization occurred internally to produce encapsulated zygotes that were released in one or more spawning events from July to November. In the following months (December to February), which were the periods of lowest temperature (~18.5–20 °C), no gametic activity occurred in the sponges. Because anomalous temperature rises that are detrimental to corals do not appear to negatively affect the reproduction and abundance of C. vermifera, it is likely that the excavating activity of this sponge may be compromising the health of those coral reefs that are recurrently affected by episodes of thermal stress.