#hyperiidae

It’s a hyperiid amphipod- a member of a group related to sandhoppers and scuds, but all members are fully pelagic and marine.

Many hyperiids have giant, translucent eyes with a sort of multifaceted retina inside. This guys’ dome- shaped head with a darker patch visible within is all eye.

Most species feed on salps or jellyfish- you might recognize the deep- sea genus Phronima from an episode of bbc’s Blue Planet.

(not my photo)

A more obscure but very large and bizarre- looking deep sea hyperiid, Cystisoma, takes both translucency and giant eyes to the extreme. The red spot is the aforementioned retina, which shows you just how much of this thing’s head is devoted to the eyes.

(also not my photo)

The one I found (1st pic) appears to be a member of the genus Themisto, which are both very abundant in temperate waters and somewhat unusual among huperiids in that they prey on smaller crustaceans such as copepods rather than gelatinous animals. You can’t see it very clearly in my picture, but their longer pairs of legs are mantis- like raptorial limbs, lined with spikes suited for snatching copepods out of the water.

Phronima sp.

by Alexander Semenov

THE STRANGE AND WONDERFUL EYES OF PARAPHRONIMA GRACILIS

If I lived in the deep, dark sea, in the open water column of the mesopelagic habitat, I would want to have eyeballs that took up nearly 50% of my body. Okay, maybe not. It might be hard for me to swim. Then again, the hyperiid amphipod, Paraphronima gracilis, apparently thrives with giant eyes that dominate nearly half its body. And it’s not just that the eyes are gigantic, they are weird, too. This animal has a configuration of compound eyes that has never been described before. Today in Current Biology, Karen Osborn, Sonke Johnsen, and I describe the strange eyes ofParaphronima gracilis and discuss the possible advantages of their novel design.

Paraphronima has a pair of bilobed apposition compound eyes. There is a large upward-facing portion and a smaller lateral-facing portion. Having four eyes isn’t unusual in the mesopelagic habitat, especially amongst the hyperiid amphipods. The unique aspect of the eyes is the multiple, discontinuous retinas. There are 12 retinal groups in each upward-facing portion and 5 retinal groups in the lateral portions, although there is one large, dominating group in the lateral portion. In most animals with compound eyes, the retina is one continuous sheet of photoreceptors, supporting cells and screening pigments.

We suspect the unique eyes of Paraphronima are acting to maximize light capture (and therefore, vision) in the mesopelagic habitat. The mesopelagic habitat could be considered an extreme habitat. In the deep water column, away from surface, shore, or ocean bottom, there are no physical objects to hide amongst, except other animals. Light is dim and blue, extinguished to pitch black after 1000 meters. Bioluminescence flashes and search lights become the major sources of illumination. Animals here are challenged to see, without being seen. 

The eyes of Paraphronima are large and nearly transparent, with the exception of the small, orange retinas. The ommatidia of the eyes are arranged in rows perpendicular to the body axis. A single row of ommatidia may function together to boost the light gathering ability of the eye, using a process called spatial summation. In most other animals using spatial summation, a circular group of ommatidia may pool the light signal, boosting light sensitivity at the cost of spatial resolution. In Paraphronima,summation may occur in a linear, not circular, fashion. Spatial summation in one axis would allow the eye to gather more light, while reducing the cost to spatial resolution in one axis.  We suspect that Paraphronima is mainly concerned with detecting linear appearing targets, like small siphonophores which could easily be targeted in a single axis.....