Swarms of tiny translucent shrimp over rubble, and food for much of what feeds above them.
Small shrimp-like crustaceans, mostly between 5 and 15 mm, translucent to faintly amber or red, and almost always in numbers. They are neither decapod shrimps nor amphipods. Two characters separate them from the shrimps they are taken for: the carapace covers the front of the thorax but leaves the rear segments free, and there is a statocyst — a pale, roughly spherical spot — in the inner branch of each side of the tail fan, which in a close photograph is the most reliable mark available. Females carry the developing young in a brood pouch slung beneath the thorax, which is where the older name opossum shrimp comes from.
This entry is pitched at the order. Idiomysis is one of the genera involved in the swarms over muck sites, along with Anisomysis, Siriella and Heteromysis, but separating mysids needs the telson, the uropods and the male pleopods under magnification, and no combination of characters visible in a photograph will take one reliably to genus, let alone to species. Logging a swarm as Mysida is honest; a species name on it is not.
The swarms hang in the water immediately above rubble, dead coral heads, wreck plating, jetty pilings and the mouths of overhangs, holding station against the current and shifting as a unit when disturbed. They feed on detritus, diatoms and small zooplankton, and some genera live down in the sediment or inside sponges and urchins rather than swarming at all. Their significance on a muck site is as food. Cardinalfish, seahorses, pipefish, dartfish, gobies, juvenile lionfish and small cuttlefish all take them, and a fish holding position in open water above a coral head is frequently feeding in a swarm. Numbers on one piece of structure rise and fall over weeks, so a head thick with them on one dive can be bare on the next.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.