(^) Mysids are the most shrimp-like of the peracaridan crustacea. Older stages have
stalked eyes, and the basic body form is close to that typical of shrimp and
euphausiids. However, the mysid “carapace” is only a posterior expansion of the first
thoracic segment, not a dorsal fusion of thoracic segments. Members of the family
Mysidae, typically 1.5 cm long as adults, are transparent but spotted with
chromatophores. They have large statocysts on the uropods (tail fan), a certain
identifier of shrimp-like swimmers as mysids. Mysidae (Mysis, Hemimysis, and
others, sometimes called opossum shrimp) are bentho-planktonic in nearshore
habitats, often abundant in swarms along the bottom in or near the surf zone. Mysids
are very important food for fish in this high-energy habitat, and swarms over rocky
reefs are the food resource allowing year-round residence of a small number of gray
whales off Oregon, USA (Newell & Cowles 2006). Some mysids related closely
enough to be called by the same species name are found in both marine and
freshwater habitats and all along the salinity gradients between them. There are
populations in many lakes, and they are a major part of plankton biomass in some
very large lakes. Mysis relicta, abundant in Lake Michigan, is also found in coastal
seas and estuaries. Other mysid families, for example Lophogastridae (Lophogaster)
and Gnathophausiidae (e.g. Gnathophausia ingens) are mesopelagic. They are larger,
some over 10 cm, and bright reddish-orange. They have reduced eyes, no statocysts,
very long antennular flagellae, and feed by scavenging and predation. Gnathophausia
is one of the few mesopelagic animals that has been maintained in captivity (reviewed
in Chapter 12).
Euphausiacea
(^) Commonly called krill, euphausiids are important as food for whales, commercially
important fish like hake and salmon, even for very large squid that would seem to
have no means of catching them. They are shrimp-like (Plate 6.11), with seven
abdominal segments and the dorsal exoskeleton of the thoracic segments smoothly
fused into a carapace that attaches to the body along its whole length (distinct from
mysids). Euphausiid gills are lateral branches from the thoracic legs that extend
outside the carapace, and euphausiids have no maxillipeds, meaning that none of the
anterior thoracic legs are modified as additional mouthparts. Shrimp have maxillipeds,
and their carapace covers their gills. The size of euphausiid juveniles and adults
ranges from about 1 to 10 cm. Some euphausiids are strictly carnivorous, feeding
mostly on other crustacea. Several genera (Nematocelis, Nematobrachion,
Thysanoessa, Stylocheiron) have one or two pairs of greatly elongated thoracic legs
equipped with a claw or bundle of spear tips for prey capture. Other genera are
predominantly herbivorous, particularly the more-abundant species in coastal habitats,
but all forms supplement their diet with animal prey, particularly copepods. Filter-