On March 30, 2004 we observed the giant barrel sponge Xestospongia muta broadcast spawning at the barrier reef, south of Carrie Bow Cay, Belize. X. muta was observed on the inner reef slope (10–20 m depth) and on the fore reef slope (15 to >60 m), but only individuals on the fore reef slope below 20 m were observed spawning. Spawning of X. muta was synchronous, involving numerous male and female specimens of various sizes. Spawning had already started at 8:00 a.m. and continued until at least 9:00 a.m. Eggs were negatively buoyant being accumulated in the sponge atrium and scattered on the benthos around the sponge (Fig. 1a). Sperm was positively buoyant and left a “cloud” in the water column (Fig. 1b). We dove further south along the reef on March 28 and 31, 2004 and did not observe X. muta spawning.

Fig. 1
figure 1

Xestospongia muta spawning on the Belize outer reef. A Female, B Male

Full size image

Even though sponges are a major component of Caribbean coral reefs, little is known about the timing of their spawning (Fell 1993). In Curaçao the sponge Neofibularia nolitangere is known to synchronously spawn in the afternoon of October and November 3 days after the full moon (Hoppe and Reichert 1987). Further studies are necessary to determine what factors trigger/control spawning in X. muta.