Big-eyed transparent amphipods that ride, and eat, the salps and jellies they live on.
A laterally compressed crustacean, transparent with silvery or amber patches, seven pairs of legs, and eyes that are enormous in proportion to the head — in several families the eyes take up almost the whole of it. Most of those met on a blackwater dive are between 5 and 25 mm. Benthic amphipods, the gammarids, look broadly similar in outline but have small eyes and are not found hanging in open water; mysids and larval shrimp are the other common confusion, and both have a carapace and stalked eyes that a hyperiid does not. This entry is pitched at the suborder. A few kinds are recognisable in a photograph — Phronima in its barrel, which has its own entry, the long-snouted Streetsia, the large and almost invisible Cystisoma — but the characters that fix a genus and a species are on the mouthparts and the limb segments.
Nearly every hyperiid is associated with a gelatinous animal for at least part of its life: salps, doliolids, pyrosomes, siphonophores, medusae and ctenophores are all used. The relationship runs from simple riding through grazing on the host’s tissue to killing it outright, and females of many species deposit their young onto or into a host. The eyes are the other half of the same story — they are hunting transparent prey in very little light, and several genera have the eye divided into an upward-looking and a forward-looking part.
The practical consequence for a blackwater dive is that the passenger is usually the better subject. Any salp chain, comb jelly or medusa in the beam is worth a second look at the surface of the bell and the inside of the barrel, and a rider on a host makes a picture that neither animal makes alone.
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.
