end with a metamorphosis to adult-like forms termed copepodites. There are six
copepodite stages, the last being the sexually mature adults.
(^) Copepods are readily studied both preserved and alive, so much is known about
them, some of which will be reviewed further on.
Cirripedia
(^) Barnacle larvae are important in coastal plankton, and larvae of oceanic gooseneck
barnacles (Lepas) are found in all tropical seas. Two distinctive developmental stages
are planktonic: shield-shaped nauplii and the cypris stage, which has a bivalve shell
reminiscent of the ostracods. Both of these stages are particle-feeders.
Amphipoda
(^) The evolutionarily more advanced crustacea are the Malacostraca, and a number of
malacostracan groups contribute to the plankton: Amphipoda, Mysidacea,
Euphausiacea, and Decapoda. The peracaridan subset of the Malacostraca (those
carrying their eggs on plates attached to the thoracic legs (amphipods, isopods,
mysids, and six other orders in recent systematics) are more dominant among benthic
fauna. However, one family of amphipods, the Hyperiidae, has diverse representatives
in the marine plankton. They are not shrimp-like and have no fused carapace;
segmentation of the thorax retains its dorsal articulations. Body plans vary widely
from sleek, fast swimmers (Streetsia) to sprawling tangles of grasping limbs
(Phronima). Most have very large, compound eyes just behind and surrounding the
stubby antennules and antennae. Anterior thoracic legs usually bear pincers for
grappling with prey, while rear legs are tipped with hooks for attachment to surfaces.
Hyperiids are usually associated with gelatinous plankton, catching rides on salps or
medusae. Some associations seem to be obligate, others more general. The
relationships are rarely obvious in plankton samples, where the rider and ridden have
generally been knocked apart. Phronima, however, is often caught with its “host”, the
hollowed-out gelatinous (but resilient) barrel of a salp or doliolid. It resides inside the
tube, propelling it forward with a jet stream generated with its abdominal legs.
(^) Reproduction is sexual. Like marine benthic and freshwater peracarids (most of
which are benthic or epibenthic), the female hyperiids carry their eggs under the
thorax attached to oöstegites, plates forming a brood pouch at the base of the legs.
Development in the egg produces juveniles resembling the adult in body plan and
activity. Much remains to be learned about every aspect of the biology and ecology of
this diverse group.