found in The Deep Sea by Robison and Connor (1999). However, a larger part of
deep-sea biomass is made up of small crustaceans: copepods, euphausiids,
amphipods, ostracods, and mysids. There are numerous and remarkable jellyfish and
some specialized chaetognaths. Copepods in the deep sea are diverse relative to those
in the epipelagic. Common at the tiny end of the size scale are numerous species of
Oncaea, which crawl about on bits of mucus and submarine “snow”, eating attached
particles. The larger mesopelagic copepods are diverse calanoid genera, with many
species over 5 mm and Bathycalanus sverdrupi that grows to 16 mm. Some genera
(Bathycalanus, Megacalanus, Lophothrix, Scottocalanus) are robust-bodied detritus-
feeders that mostly eat fecal pellets sinking through their vicinity from above (Nishida
& Ohtsuka 1991). These copepods and some others of less-certain feeding habit
(Gaetanus, Gaidus, and others) are mostly bright-red, opaque, and heavily muscled.
In captivity, they alternate zooming about their aquarium with spells of stillness and
fairly rapid sinking.
(^) Lurk-and-grab predators are another large group of copepod genera: Augaptilus,
Euaugaptilus, Haloptilus, Disseta, Paraeuchaeta, Arietellus. Except for
Paraeuchaeta, these mostly have very thin muscles and elaborate sprays of long setae
to inhibit sinking. In aquaria they hang still in the water, tail down with antennules
extended, scarcely sinking or rising. Paraeuchaeta behaves similarly in the field, but
tends to rest on the bottom in containers. Unlike the red detritivorous copepods (and
large red shrimp, for that matter), these predatory copepods have elaborate color
variations. Euaugaptilus species come in brilliant yellow, lavender, orange, bright
green, and other colors. Some have pale tints over the body as a whole, through which
is visible a gut brightly colored with a completely different part of the spectrum.
Species of Disseta come in pale orange and bright white. Paraeuchaeta have
chromatophores that look like a decorative pattern of tiny brittle-star pictures just
under the exoskeleton, and they come in colors and patterns that are species-specific.
While dead specimens can be distinguished by experts from very subtle shape
differences (following Park 1993), live specimens wear their identities as distinctively
colored uniforms. Females of the genus Euchirella tow their purple, green, or black
eggs along behind them in long zig-zag rows. The adaptive value of this riot of fancy
dress in the near dark, with only blue photons to show it off, is hard to imagine. If
Annie Dillard is right and we are meant to be the witnesses of creation, then perhaps
the show is for us.
(^) Jellyfish and siphonophores are also common in the mesopelagic. They presumably
function as tentacular predators, much like their epipelagic cousins. Color is common
for them as well; in several groups it is a dark reddish purple, likely to hide
bioluminescence from ingested prey. The coronate scyphozoan Periphylla has this
pigment only in the lower tissue layer beneath a tall conical “lens” of clear mesoglea,
an effect that glassblowers have lately been imitating.