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.
Highlights
Sponges are ecologically and functionally important members of the benthic community on coral reefs [1,2,3,4,5,6]
It is increasingly recognized that sponges are crucial members of benthic food webs because of their ability to couple water column productivity with the secondary productivity of benthic communities [11,14,15,16]
We present a natural field experiment sensu Diamond [40] demonstrating the importance of food supply in the ecology of C. vaginalis, one of the species used in Pawlik et al [39], and compare this study with studies by Lesser [11], Trussell et al [15] and Pawlik et al [39] on C. vaginalis
Summary
Sponges are ecologically and functionally important members of the benthic community on coral reefs [1,2,3,4,5,6]. In the Caribbean over 80 species have been recorded on reefs in the Florida Keys [7] and close to 300 species have been identified on Bahamian reefs [8]. In addition to their efficient filtering of seawater during feeding [9,10,11], sponges provide essential reef ecosystem functions such as providing habitat for numerous reef species including fish, brittle stars, and shrimp [12,13]. Many mobile predators consume sponges (e.g., sea stars, fish, sea turtles), and spongivores can influence competitive interactions between sponges and corals [17]. While it has been shown that cryptic or mangrove sponge species may be limited to specific refugia by spongivores [18,23,24,25], sponges throughout the Caribbean show a repeatable pattern of increasing biomass and diversity with depth to 150 m [7,8,26,27,28,29,30,31]
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