Ecology of Caribbean sponges: are top-down or bottom-up processes more important?

Abstract

Benthic-pelagic coupling and the role of bottom-up versus top-down processes are recognized as having a major impact on the structure of marine communities. While the roles of bottom-up processes are better appreciated they are still viewed as principally affecting the outcome of top-down processes. Sponges on coral reefs are important members of the benthic community and provide a critically important functional linkage between water-column productivity and the benthos. As active suspension feeders sponges utilize the abundant autotrophic and heterotrophic picoplankton in the water column. As a result sponges across the Caribbean basin exhibit a consistent and significant pattern of greater biomass, tube extension rate, and species numbers with increasing depth. Likewise, the abundance of their food supply also increases along a depth gradient. Using experimental manipulations it has recently been reported that predation is the primary determinant of sponge community structure. Here we provide data showing that the size and growth of the sponge Callyspongia vaginalis are significantly affected by food availability. Sponges increased in size and tube extension rate with increasing depth down to 46 m, while simultaneously exposed to the full range of potential spongivores at all depths. Additionally, we point out important flaws in the experimental design used to demonstrate the role of predation and suggest that a resolution of this important question will require well-controlled, multi-factorial experiments to examine the independent and interactive effects of predation and food abundance on the ecology of sponges.

Figure 1. Sponge densities and diversity on Carrie Bow Cay, Belize (m², mean ± SE).

Density of sponges, number of sponge species and density of Callyspongia vaginalis from different depths. Common superscripts indicate groups not significantly different from each other.

Figure 2. Tube length and growth for Callyspongia vaginalis on Carrie Bow Cay, Belize.

Tube length (cm, mean ± SE) (A). Growth over a one-year period (cm, mean ± SE) (B). Common superscripts indicate groups not significantly different from each other.

Figure 3. Picoplankton availability and feeding for Callyspongia vaginalis on Carrie Bow Cay, Belize.

Depth-dependent concentrations of total picoplankton (A). Instantaneous total cells consumed per individual at each depth (B). Instantaneous intake of total particulate carbon per individual (C). Daily intake of total energy (J) per individual (D). Common superscripts indicate groups not significantly different from each other.