Rimicaris exoculata

A most surprising discovery

Although Rimicaris exoculata translates as “eyeless shrimp”, Rimicaris does indeed posses an eye but it is so unusual that it took over 3 years for researchers to recognise that it was an eye at all!

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Fig. 12 – The first scientific sketch of R. exoculata showing the lack of a stalked eye © Williams & Rona, 1986

When first describing Rimicaris, Rona and Williams, 1986 could find no stalked eye or cornea (fig. 12). However, the presence of a pair of large, reflective “organs” on the back of Rimicaris peaked the interest of vent scientist Cindy Van Dover. In 1989, Van Dover et al published a paper speculating that these dorsal patches (fig. 13) could be highly modified eyes. Van Dover based her incredible statement on the unusual neural morphology of the organs (which resembled the optic nerve) and the finding that the organs contained a visual pigment similar to that of rhodopsin (a retinal protein). Although the organs (termed novel eyes) contained no lens, the eyes were so modified that they could not possibly be evolutionary relics, thus presenting the question what light could there be at the bottom of the ocean to see by!

Fig. 2 - The large, reflective dorsal organs of  R. exoculata inferred to be eyes by Cindy Van Dover

Fig. 13 – The large, reflective dorsal organs of Rimicaris inferred to be eyes by Van Dover

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Fig. 14 – Photographs of the light emitted at hydrothermal vents © White et al, 2002

Van Dover et al, 1989 postulated that the hydrothermal vents themselves may emit extremely faint light due to a process known as thermal black body radiation. Studies have since proven this to be the case (Van Dover et al, 1996; White et al, 2002) and even have photographs to prove it (fig. 14)! Back in the late 80’s, other authors quickly established that the eye of Rimicaris was sensitive enough to detect the dim light given off by vents (Pelli & Chamberlain, 1989) and more in-depth research has confirmed the eye’s sensitivity to exceptionally low light levels (O’neill et al, 1995).

With the light source now established, the question became what advantage could the eyes be? Van Dover’s initially suggested that the eye could be a mechanism by which Rimicaris remained close enough to the vent so its chemosynthetic bacteria had access to vent fluid but not so close as to be cooked by water hot enough to melt lead! Although no-one has yet observed Rimicaris demonstrating any behavioural responses towards light (e.g. from submersibles), this theory is supported by circumstantial evidence. The dorsal position of the eye would enable the shrimp to “see” the vents whilst hidden within the swarm and the morphology of the eye suggests (to ophthalmologists) that the eye is optimised for light sensitivity rather than resolution (Chamberlain, 2000). Until Rimicaris can be shown performing behavioural responses towards light, this theory will remain unsupported however the supporting circumstantial evidence and lack of a rival theory makes Van Dover’s initial suggestion extremely plausible.

An unexpected consequence of scientific studies at hydrothermal vents can be read about here however readers are now directed towards the final section of this blog which discusses the swarming behaviour of Rimicaris.

 

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