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Comb jellies belong to their own phylum, Ctenophora, which encompasses about 90 species of gelatinous, carnivorous zooplankton. Like their distant relatives, the cnidarians (the phylum to which the Chesapeake Bay's familiar sea nettles belong) they possess jelly-like, radially symmetrical bodies, but they have no stinging cells with which to stun their prey. They are however voracious filter feeders, ingesting a wide range of planktonic animals, including oyster larvae, which they are capable of consuming in great quantities. Two ctenophore species occur in pelagic Chesapeake Bay habitats-the sea walnut, Mnemiopsis leidyi, and the pink comb jelly, Beroe ovata. Both species have become important predators in Chesapeake Bay waters over the past 15 to 20 years. Chesapeake Bay monitoring programs during the 1990s indicated that there was an increased trend in ctenophore biomass during this period and a concomitant decreasing trend in populations of bay anchovy.
The comb jelly's most striking physical characteristic is its system of eight rows of combs, which consist of plates of densely organized cilia that are fused at the base and that beat synchronously or in a pattern to move the animal forward slowly and draw water (and prey organisms) into its oral canals. Inside, a rounded cyst-like organ contains a statolith, a granule that gravity pulls downward, thus stimulating the nerve tissue beneath it and regulating the beating of the combs. The comb jelly is the largest animal that uses cilia for locomotion. Each comb in a row contains hundreds of cilia, and each row may contain dozens of combs, thus individual ctenophores possess thousands of cilia. At night ctenophores luminesce, and it is possible to see the combs beating. Instead of stinging nematocysts, comb jellies have colloblasts, adhesive cells that ensnare prey organisms such as copepods, zooplankton eggs, juveniles and adults; and fish and bivalve larvae. The comb jelly's body is covered by a thin ectoderm that also lines the mouth (consisting of a small slit) and pharynx, which lead to the stomodaeum or stomach, lined with endodermal cells. Nutrients from partly digested food are dispersed throughout the body through a system of gastrovascular canals.
Sea walnuts and pink comb jellies, like most ctenophores, are hermaphroditic (contain both male and female reproductive organs) and release both eggs and sperm into the water, where fertilization and development take place. The sea walnut, however, holds its fertilized eggs internally until they develop into larvae. Individuals of both species mature quickly and may produce many generations of offspring during a single summer. The sea walnut in the Maryland portion of the Bay can grow to a mature length of 4 inches. According to some hypotheses, the Bay's lower salinities in recent years have led to poor habitat for jellyfish and a corresponding decline in predation on ctenophore populations (despite their consumption by finfish and turtles), which has led both native ctenophore species to increase and to put more pressure on their prey species, including the bay anchovy. Comb jellies occur throughout the world's oceans in both pelagic and nearshore habitats, in both tropical and Arctic waters. There is some speculation that the numbers of species and individuals have been greatly underestimated due to the difficulties in collecting samples of these delicate creatures using trawl nets. Most ctenophore species inhabit surface waters, to a depth of a few hundred meters, but some ctenophore species have been found at depths of nearly 3,000 meters.
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