The thoracic eyes of vent shrimps

 Vent shrimps are caridean decapod crustaceans of the family Bresiliidae, found in the region of hydrothermal vents. 

This image shows the cephalothorax of an adult Chorocaris fortunata obtained from a hydrothermal vent on the Mid-Atlantic Ridge. The optical system consists of a small anterior eye (visible beneath the tip of the rostrum) and an extensive dorsal eye (seen beneath the dorsal carapace). Both types of eye have a reflective white tapetum to increase the efficiency of photon capture.

 Hydrothermal vents are found along the mid-oceanic ridges which are formed wherever there are tectonic plates moving apart. Here, plumes of seawater, heated to 350 degrees C, with a pH of 4 and containing a high concentration of minerals, are vented at intervals along the ridges. The emerging plumes are known as 'black smokers' where the precipitation of the sulphide particles occurs when the vent fluid comes into contact with seawater at around 2 degrees C. This precipitation also results in the formation of these spectacular vent chimneys, up to 10m high.

In this hostile environment, several species have evolved to cope with the high temperatures and sulphide concentrations. Of the 400 or so species recorded at hydrothermal vents, a large proportion are found nowhere else. In this ecosystem primary production depends not on photosynthesis, but on chemosynthesis by bacteria deriving energy from the oxidation of sulphide from the vents.

  Each vent site has its own characteristic fauna; vent sites in the Pacific tend to be dominated by giant clams or vestimentiferan tube worms up to 2m long: those along the Mid Atlantic Ridge are dominated by vent shrimps (Rimicaris exoculata) which cover the slopes of the chimneys in large swarms.

These shrimps are adapted in many ways to life at the vents. The most interesting adaptation from my point of view is the pink or white thoracic organ that can be seen on the dorsal surface of the cephalothorax. When first described, around 20 years ago, the species was named Rimicaris exoculata as it appeared to have no eyes. However, more recent work has suggested that this dorsal organ may be a functional eye. It is not immediately apparent how this most unusual eye could have evolved from the compound eye of an ancestral deep-sea shrimp.