infestations of the 2 m diameter (and ½ ton!) medusae of Nemopilema nomurai in the
Japan Sea and beyond.
Cubozoa – Box Jellies
(^) The name was given to these jellyfish because the very transparent medusae of the
group are square in transverse section, with one to three tentacles depending from
each corner. The bell has a hydrozoan-like velum. Cubozoan planula larvae settle and
become polyps that eventually metamorphose into medusae, detach, and swim away, a
pattern different from those of both Hydrozoa and Scyphozoa. The medusae have a
sensory complex on the bell edge at each corner, including several types of eyes,
some with a cornea, lens, and retina, and a statocyst (the other groups with medusae
also have distinctive sensory systems). Data from this complex apparently direct
cubomedusans’ rapid (for medusae) swimming, to ∼20 cm s−1, around obstacles and
away from predators. The nematocysts are in very large batteries and the venoms are
more toxic than those of most other cnidarians, occasionally killing swimmers in
tropical or subtropical waters. One small species (1 cm bell) abundant in reef areas,
Carybdea sivickisi, is active in the water column at night but attaches with sticky pads
on the bell to the undersides of rocks during the day (Hartwick 1991).
Ctenophora
(^) This is a pelagic phylum, apart from a group of extremely derived benthic forms
found primarily in the tropics. Planktonic ctenophores are commonly called comb
jellies (indeed, they are gelatinous). While still retaining a structure of two tissue
layers with intervening mesoglea, they have added an anus, and thus have spatially
separated feeding from waste elimination. The simpler forms are spherical (e.g.
Pleurobrachia) with diameters to about 2 cm. Eight “comb rows”, series of plates or
ctenes of fused cilia, run from the upper (aboral) to the lower (oral) end. Ciliary
lineations on the ctenes act as diffraction gratings, casting moving rainbows as
propulsive waves sweep in sequence down the rows and across the light field. The
resulting locomotion is slow and smooth, usually with graceful turns. Internally, the
gut has two side pouches containing retractable tentacles that extend through tubes
adjacent to the mouth. The tentacles bear cells called colloblasts, which discharge
much like nematocysts, but that entangle prey with sticky pads rather than stinging
them. Tentacles arc through the water as the ctenes row the animal ahead. One
common genus (Beröe) has dispensed with tentacles altogether, and has a powerfully
extensile mouth. They feed entirely on smaller ctenophores, such as Pleurobrachia.
The body in some forms (e.g. Mnemiopsis, Cestum) stretches during late development
into a strongly bilateral pattern, forming large flaps which propagate a wave-like
motion that results in oscillatory swimming. Mnemiopsis in coastal and estuarine