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It looks like a jellyfish
but it isn't…Porpita, shown
here, isn't even a single organism, but a colony! The central
disc is reinforced with a chitin-like material and is chambered
in cross-section; filled with gas, it keeps Porpita
afloat, for these are pelagic creatures. The "tentacles" of
Porpita are in fact individual zooids, each of which
is specialized for a particular function, such as digestion, prey capture, or
reproduction. Porpita is a member of a small but very
widespread group of hydrozoan
cnidarians,
the Chondrophorina. There are only two accepted genera of chondrophorines,
Porpita, shown above; and Velella, which resembles Porpita
but has a keel-like "sail" on its aboral surface. Both
often wash up on beaches in huge flotillas of thousands
of organisms.
Chondrophorines were once classified with another unusual group of
hydrozoans, the siphonophores. The most famous siphonophore is
Physalia, the so-called "Portuguese man-o' war." Like Porpita,
siphonophores somewhat resemble
jellyfish
but are actually colonial organisms, with different individuals
specialized for different functions; most, but not all, have a gas-
filled float.
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