On our regular trips to Longboat Key, Florida, we encounter a spectacular diversity of wildlife, including dolphins, manatees, stingrays, ghost crabs and a wide variety of coastal birds. But yesterday afternoon, while sitting on a seawall along Sarasota Bay, I observed a flotilla of comb jellies for the first time. Drifting in the calm bay waters, they looked like miniature Goodyear Blimps, complete with neon messages, flashing along their transparent hides.
Members of the phyllum Ctenophora, these marine invertebrates consist of an ovoid mass of gelatinous tissue, enveloped in a thin ectoderm and pierced from front to back by a primitive gastrointestinal tract. On its outer surface are eight lines of cilia, which beat in a coordinated fashion to propel the jelly through the seawater; nevertheless, most of their movement is subject to currents and tides. Carnivores, comb jellies are devoid of stinging tentacles but are ravenous in their consumption of microscopic plankton. Like many other marine invertebrates, these jellyfish are hermaphrodites, producing both eggs and sperm; fertilization usually occurs in the open water but, in some species, takes place within the gut and the offspring are not released until their early development has begun.
My research turned up some disagreement on the source of the iridescent green and orange that flashed along their otherwise transparent bodies. Some attribute these bright colors to sunlight refraction by the beating cilia while others mention the presence of bioluminescence, originating in gastrovascular tubes beneath the lines of cilia. Based on my personal observations, I would favor the latter since the linear displays occurred in shaded waters, adjacent to the seawall. Whatever the source of their colorful light shows, the chance observation of these fascinating creatures was a unique and rewarding experience.
