Vellela, this is topped by a sail. The hydroids on one float are male or female that
release very small male and female medusae with gonads (Larson 1980). Zygotes
develop into new hydroid colonies. Vellela often appears in massive wash-ups on west
coast beaches of North America.
(^) Studies in the 1990s of the plankton over Georges Bank offshore from New
England (Madin et al. 1996) rediscovered that the enclosed anticyclonic circulation
over and around the bank provides a spring–summer habitat in which the colonial
polyp phase of Clytia gracilis, commonly found attached to shallow surfaces and sea-
grasses, can be free-floating and reach high density by recurring colony expansion
and breaking. They can color the sea surface yellowish green. They do not, however,
have algal symbionts, and they feed by predation on zooplankton. Similar populations
have been reported from the northeast Atlantic.
(^) Siphonophora are holoplanktonic (Plate 6.3), hydrozoan cnidaria with complex
body forms termed “colonies”. “Individuals” in the colony (Plate 6.4) are specialized
for propulsion, feeding, and reproduction. Typically, there is a long, tubular stolon
emerging from a large swimming bell, carrying a curtain of tentacles extending from
feeding polyps arrayed along the stolon. Nematocysts on the tentacles sting and
capture prey, which are then moved to the polyp opening for ingestion. Siphonophore
structure can be dramatically complex. In one abundant group (Physonectidae) a
vertically oriented body mass of many swimming bells is topped by a terminal
individual that secretes a carbon monoxide bubble internally to regulate colony
buoyancy, sometimes with resulting vertical migrations. Feeding curtains of
Calycophoran siphonophores can be many meters in length, making them very
important predators of small zooplankton, including larval fish. A well-known
neustonic siphonophore, the Portuguese Man o’War, Physalia physalis, has a gas-
filled float as large as 40 cm. Its long beard of tentacles can produce very painful
stripes of welts on unwary swimmers.
Scyphozoa
(^) This is another group of medusae (Plate 6.4). Most of the common genera grow to
large body sizes; in some forms the bell can grow to 1 m diameter with several meters
of layered tissue folds hanging from the manubrium. Some genera (e.g. Chrysaora,
Plate 6.3, Aurelia, Nemopilema) have a small asexual, benthic polyp phase that is
never colonial like the benthic phase of the Hydrozoa; other genera (e.g. Pelagia) do
not. Larval medusae (scyphistomae) are produced from the polyps by repeated,
transverse constrictions just below the oral opening. Scyphozoan medusae lack a
velum, but like hydromedusae they are the sexually reproducing life-phase. In recent
years, massive population outbursts of scyphozoans have become a problem for trawl
fisheries along many coasts, the most prominent example being summer–fall