Pelagic Tunicates: Why Gelatinous?

Abstract

Neutrally buoyant zooplankton are virtually free from the gravitational forces acting on terrestrial organisms, which allows for their adoption of large, watery bodies (Harbison 1992). Although this morphological trait is present in several phyla and is nearly ubiquitous in marine systems, the reasons for its great success remain an evolutionary mystery. From a top-down perspective, a transparent, gelatinous body may offer effective concealment from predators (Johnsen 2000) and may deter predation because of its low nutritive value (Verity and Smetacek 1996). However, the light-deprived depths of the deep sea teem with gelatinous life, many gelatinous animals exhibit bioluminescence, and some are not transparent (see Harbison 1992), which casts serious doubts on the transparency hypothesis. Moreover, there are predators specialized in gelatinous plankton (see Harbison 1998 for a comprehensive account of salp predators). Last, from a bottom-up perspective, a gelatinous body may allow survival at the low food concentrations prevailing in the open ocean (see Harbison 1992 for this and other hypotheses related to gelatinous bodies). Salps, appendicularians, pyrosomas, and doliolids, collectively known as pelagic tunicates, are extremely watery animals that filter from very small colloids to large phytoplankton chains. They serve as prey for a host of marine animals, including fish, and under benign food conditions, they exhibit population growth rates that rank at the top among the metazoans (Bone 1998b). This allows them to