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Coelenterates (Phylum Coelenterata or Cnidaria)
include jellyfish, anemones, corals, and hydras. The phylum is characterized by
a gelatinous body, tentacles, and stinging cells called nemadocysts. Most species
are found in marine waters, but some occur in brackish or even fresh water. Coelenterates
may be either sessile or free swimming, depending on the species and/or stage
of the life-cycle. Standard methods of introduction include ship fouling or transportation
in ballast water of ocean-going vessels. Potential impacts include competition
with native species for suitable substrate or food, negative effects on organisms
to which they attach, utilizing native species as a food source, and "swarming",
where thousands of jellyfish mass in an area and can affect the local fauna. An
example of an introduced coelenterate is the Black Sea jellyfish, Maeotias
inexspectata. These small purple jellyfish were introduced via ballast water
from Russian ships into San Francisco Bay in 1993. They under went a population
explosion and infested tributaries of the bay. This outbreak caused considerable
public alarm and inspired news headlines like "Jellyfish Jam the Petaluma River".
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Cnidaria
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