In the plankton, these advanced crustaceans are represented by several families of
“true” shrimp, which are distinguished by a carapace attached along the entire back
and covering the gills. At least some thoracic legs are modified as maxillipeds. Some
shrimp are holopelagic and qualify as zooplankton or small nekton. The most
widespread of these in the upper ocean belong to the genus Sergestes, which are “half-
red” shrimp. They have red pigment covering the food crop in the thorax, so that
luminescent meals don’t give away their position to other predators. The rest of the
body is transparent. At mesopelagic depths there are a number of dark-red shrimp
belonging to the caridean and penaeoid groups. Most of these are about 10 cm long,
but have antennal flagellae reaching as much as a meter away from the body. Thus the
shrimp’s vibration detectors extend through a very large volume, giving it a chance to
find prey where very few are passing through. In addition, most benthic shrimp and
crabs contribute larvae to the plankton.
Urochordata
(^) There are two important chordate groups in the oceanic plankton, the urochordates
and the vertebrates. Urochordates, familiar to tide-pool zoologists as tunicates or sea
squirts, are represented in marine plankton by the Thaliacea (salps, pyrosomes, and
doliolids) and Appendicularia.
Salpidae
Salps
(^) Salps (e.g. Salpa, Plate 6.12, and Thalia) are tube-shaped, gelatinous zooplankters
with flap valves at either end. Fully grown, they are a few centimeters to about 20 cm
long, typically with an aspect ratio of about three (length/width). The tube, or test, is a
stiff, gelatinous structure supported by cellulose fibers. Muscles around the anterior,
incurrent opening will close it, and then muscles in the test wall will compress the
cylinder and produce a propulsive jet from the excurrent opening. Re-expansion is
driven by the resilience of the tube, which fills through the incurrent opening. A cone
of mucus is spun inside the cylinder, which acts as a water filter for obtaining
particulate food. Particles are progressively wadded together along a ciliated band, the
hyperpharyngeal lamina, and moved along for digestion to the posterior gut mass near
the excurrent opening. Some selection is exercised; filtration can stop based on
olfactory signals, and sufficiently malodorous water or particles will cause the
expulsion of the mucous cone.
(^) Salps have a moderately elaborate alternation of generations. There is asexual
reproduction from an individual called a solitary, by pinching off of sections of a long