Planktonic Feeding and Evolutionary Significance of the Lobate Body Plan Within the Ctenophora.

Abstract

Ctenophores are gelatinous marine invertebrates that prey upon zooplankton. The two main ctenophoran orders that affect planktonic communities are the Cydippida and the Lobata. The Cydippida possess two elongate tentacles. In the Lobata, two large lobes surround comparatively reduced tentacles, and water is drawn through the inter-lobe space by four flap-like ciliated auricles. Both groups are successful predators and are widespread in the world’s oceans (1). In coastal regions, members of the genera Bolinopsis (Lobata) and Pleurobrachia (Cydippida) may co-occur and often reach high densities simultaneously. The co-occurrence of Bolinopsis and Pleurobrachia suggests the possibility of dietary overlap and feeding competition between these genera, because both are commonly believed to consume copepods as their primary prey (2, 3). To examine this hypothesis, we recorded the gut contents of individuals from two species, Bolinopsis infundibulum (Lobata) and Pleurobrachia pileus (Cydippida); the animals were collected simultaneously on six dates between March and May 1998 from the Woods Hole Oceanographic Institution pier in Woods Hole, Massachusetts. Bolinopsis infundibulum preserved poorly (total disintegration within 24 h in 0.5% formalin solution), did not tolerate vessel containment well, and often regurgitated prey when held in a container for more than 30 min. As a result, ctenophore gut contents were observed microscopically an’d recorded at the sample site within 30 min of hand collection. Gut contents were easily observed in intact animals due to transparency of the body wall. On each sample data, 20-30 individual ctenophores of each species were sampled. Although the day-to-day selection by the ctenophore predators was subject to the availability of specific prey types, a pattern of prey partitioning between the ctenophore orders was evident (Fig. 1). The cydippid Pleurobrachia pileus consistently consumed larger, more strongly swimming prey such as gammarid amphipods, crab zoea, calanoid copepods, and barnacle cyprid larvae. In contrast, the lobate Bolinopsis infundibulum selected smaller, more weakly swimming prey such as copepod nauplii, gastropod veligers, rotifers, and tintinnids. Differences in patterns of prey selection reflected the mechanical bases of prey capture by each species. P. pileus sits motionless while its tentacles are extended and set in a wide net that ensnares highly mobile prey. B. infundibulum uses the ciliary lining of its auricles to create a feeding current that entrains low-speed and weakly swimming prey that are entrapped on a network of fine tentillae located near the oral region (4). The two mechanisms favor capture of a different fraction of the