Regional morphological variations in the compound eyes of certain mesopelagic shrimps in relation to their habitat

Abstract

The anatomy of the eyes of several species of mesopelagic decapods (family Oplophoridae), obtained from the eastern north Atlantic, is described and related to the unique light environment of the deep seas. The oplophorid eyes are of the reflecting superposition type, but they show a number of regional morphological variations. The main rhabdom, formed by retinula cells Rl to R7, comes in a variety of shapes, from fusiform rhabdoms in the dorsal region of the eyes of Oplophorus spinosus to multi-lobed interdigitating rhabdoms in deepwater species. The distal rhabdom, contributed to each ommatidium by retinula cell R8, gradually increases in size towards the ventral part of the eye in Systellaspis debilis and O. spinosus. Histological examination of the tapetum shows that it is incomplete dorsally in some species from the upper mesopelagic zone (S. debilis, O. spinosus), and that the amount of reflecting pigment in the tapetal cells increases in the ventral part of the eye. The tapetum is complete in some deep-water species (Systellaspis cristata, Acanthephyra kingsleyi, A. pelagica). These adaptations of the rhabdoms and tapeta are thought to be concerned with increased sensitivity to the dim up-welling irradiance and to bioluminescence. A dorsal accessory compound eye consisting of a small group of apparently functional apposition-type ommatidia is described.

Ultrastructural Changes of Rhabdoms of the Eyes of Ocypode Species in Relation to Different Regimes of Light and Dark Adaptation

“…Both apposition Schiff et al, 1986;Cronin et al, 1994) and superposition eyes (Elofsson and Hallberg, 1977;Hiller-Adams and Case, 1988;Gaten et al, 1992;Richter, 1999) probably as an evolutionary consequence of environmental pressures, may display further structural and functional modifications. Forms, for instance, that flourish in extreme environments (mesopelagic and deep-sea crustaceans, and species adapted to a life underground or in caves) frequently exhibit morphologies that differ from the basic design (Meyer-Rochow and Nilsson, 1999).…”

“…1981a; Gaten et al, 1992). What is more, new findings have shown that regional differences are present in the eyes of many crustaceans and not just those adapted to extreme environments (Odselius and Nilsson, 1983;Tokarski and Hafner, 1984;Cronin et al, 1992;Zeil and Zanker, 1997), highlighting the need for additional research into how the various regions of a compound eye behave (and perhaps influence each other) and how, more generally, post-embryonic eye differentiation occurs (cf., Meyer-Rochow et Ziedins and MeyerRochow, 1990;Tokarski, 1998, 2001).…”

Contrast enhancement through structural variations in the rhabdoms of oplophorid shrimps

“…They have reflecting superposition eyes with large rhabdoms, characteristics that lead to high sensitivity. Several anatomical studies have described interspecific and intraspecific variations in eye morphology that are related to the depths at which the species are normally found (Welsh & Chace, 1937, 1938Gaten et al, 1992Gaten et al, , 2003. In particular, considerable variations in the relative sizes of the distal and proximal rhabdoms have been described (Gaten et al, 2003), although the reason for these differences has not been fully explained.…”

Laboratory-Based Observations of Behaviour in Northern Krill (Meganyctiphanes norvegica Sars)

“…In addition, it is of paramount importance in designing experiments in aquaria to consider the light environment carefully. Light is a key determinant of behaviour of mesopelagic organisms and they tend to be specifically adapted to cope with a particular light intensity, in addition to defined spectral, spatial and temporal distributions of light (Gaten et al, 1990(Gaten et al, , 1992Johnson et al, 2000aJohnson et al, , 2000bJohnson et al, , 2002. Failure to consider these important factors is likely to lead to anomalous behaviour, physiological damage and erroneous conclusions.…”