Contributions to Zoology, 67 (4) 223-235 (1998) SPB Academic Publishing bv, Amsterdam Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea) Edward Gaten Department of Biology, University’ ofLeicester, Leicester LEI 7RH, U.K. E-mail: [email protected] Keywords: Compound eyes, superposition optics, adaptation, evolution, decapod crustaceans, phylogeny Abstract cannot normally be predicted by external exami- nation alone, and usually microscopic investiga- This addresses the of structure and in paper use eye optics the tion of properly fixed optical elements is required construction of and crustacean phylogenies presents an hypoth- for a complete diagnosis. This largely rules out esis for the evolution of in the superposition eyes Decapoda, the use of fossil material in the based the of in comparatively on distribution eye types extant decapod fami- few lies. It that arthropodan specimens where the are is suggested reflecting superposition optics are eyes symplesiomorphic for the Decapoda, having evolved only preserved (Glaessner, 1969), although the optics once, probably in the Devonian. loss of Subsequent reflecting of some species of trilobite have been described has superposition optics occurred following the adoption of a (Clarkson & Levi-Setti, 1975). Also the require- new habitat (e.g. Aristeidae,Aeglidae) or by progenetic paedo- ment for good fixation and the fact that complete morphosis (Paguroidea, Eubrachyura). examination invariably involves the destruction of the specimen means that museum collections Introduction rarely reveal enough information to define the optics unequivocally. Where the optics of the The is one of the compound eye most complex component parts of the eye are under investiga- and remarkable not on of its fixation organs, only account tion, specialised to preserve the refrac- but also for the optical precision, diversity of tive properties must be used (Oaten, 1994). optical arrangements that have evolved. This is it is the However, perhaps response of the eye the particularly true of Crustacea, which includes to in adaptive pressures, resulting convergence, of nine out of the ten described examples types of that limits its usefulness in systematics. Particu- compound eye (Nilsson, 1989). Such is the com- larly in those habitats where the effectiveness of plexity of the of most it is the is such in optics compound eyes, eye lessened, as deep-sea, cave- that likely once one had it would not be or evolved, dwelling burrowing crustaceans, the secondary another unless the latter bestowed a reduction replaced by or loss of the eyes that usually occurs is It significant optical is for this reason and often advantage. unhelpful misleading. This leaves us that Land (1981) that structure with those animals suggested eye bearing highly-evolved eyes should be considered a conservative character. that be useful may in discussing evolutionary re- Given these facts, it might be that the Where expected lationships. similar advanced optics are functional of the used different morphology compound eye by groups of animals, it is tempting would be considered interested in the by anyone to assume that they are synapomorphies unless classification and of the Crustacea. there phylogeny are compelling reasons to think otherwise. However, this is not the case for a number of This is not always true, and even at the phylum Most reasons. important from a practical point of similarities in level, eye structure may not always view is that the used optics by a compound eye be indicative of the phylogeny of the animals Downloaded from Brill.com10/11/2021 08:33:28AM via free access 224 E. Gaten - Evolution of superposition eyes in Decapoda under consideration. The in both crusta- The main feature of the is presence superposition eye ceans and insects of a compound eye containing that within the crystalline cone the light must be the basic same structure has been considered one redirected across the optical axis of the ommatid- of the most serious problems to anyone maintain- ium (Figs. 1C, E, G) to focus onto the target rhab- ing the separate origins of these groups (Tiegs & dom. The different types of superposition eye are Manton, 1958). If they are not monophyletic, it is defined by the way in which this redirection of to this The is direct reflec- necessary explain phenomenon as ex- light occurs. simplest way by detailed of the of the tion of incident the wall of the tremely convergence eyes rays at crystalline crustacean and insect/myriapod groups. cone (Fig. 1C); this is the mechanism used in re- In of such there be role spite problems may a flecting superposition eyes (Vogt, 1975; Land, for the study of physiological optics in the con- 1976). It has been shown (Vogt, 1977) that focus- struction of decapod phylogenies. Even though ing in this type of eye only works if the crystal- the of modem such use techniques, as molecular line cones are square in cross section. Reflection biology, numerical taxonomy, and cladistic analy- within such a “mirror box” redirects light across the sis has increased in recent years, functional mor- optical axis, as can be seen when the cone is phological descriptions remain the principal tool viewed from above (Fig. ID). A more complex of the systematist. mechanism for redirecting light is seen in the which refracting superposition eye (Exner, 1891) utilises a continuous gradient of refractive index Crustacean compound eyes within the cone (Figs. IE, F). The third mecha- nism, termed parabolic superposition (Nilsson, in all comeal Compound eyes are present crustacean 1988) uses a powerful lens, a cylindrical classes and moveable stalked lens within the and except Copepoda, crystalline cone, a parabolic eyes are present in the Malacostraca. In most reflecting surface at the cone wall (Figs. 1G, H). these of the in xanthid crabs achieve cases, eyes are apposition type Some the same effect using which the lens and with dioptric apparatus (corneal a crystalline cone a square cross-section, crystalline cone) are in contact with the light-sen- rather than a cylindrical lens. A complete descrip- sitive rhabdom (Fig. 1A). In this type of eye, light tion of the optical mechanisms involved can be remains within a single ommatidium, passing found in Nilsson (1989). from the corneal facet to the underlying rhabdom; The identification of the optics being used by this results in reasonably high resolution but low any specimen is rarely straightforward, with the In the sensitivity. the superposition eye (Fig. IB) the appearance of the comeal facetting only ex- and the rhabdom dioptric apparatus layer are ternal evidence of optical type that can usually be clear This The efficient of separated by an unpigmented zone. seen. most way packing cylindri- in the of from cal ommatidia into is permits, theory, superposition light a hemispherical eye by us- a number of corneal facets to 3000 in Ne- an in this the (up ing hexagonal array; way angular phrops norvegicus - Gaten, 1988) onto a single separation of the ommatidia is reduced to a mini- in the and the resolution of the maximised. rhabdom, although practice superposition mum eye focus covers several rhabdoms as a result of op- This is the situation used in apposition, refracting tical aberrations. Even though the more periph- superposition and parabolic superposition eyes. eral of these facets contribute less the Where is in light to a square crystalline cone used, as than the central there is still in- and image ones, an reflecting superposition eyes some parabolic crease in of between 10 and 1000 sensitivity superposition eyes (Nilsson, 1988), a square times over that of an apposition eye. This confers packing array will give optimum resolution. Plac- considerable in much reliance a advantage low-light habitats, ing too on the external appearance there is concomi- of the lead to For although not, necessarily, any cornea can problems. many in the of the it assumed tant reduction resolution which eye years was that all eubrachyuran crabs is capable (Land, 1984). used apposition optics as they possessed small. Downloaded from Brill.com10/11/2021 08:33:28AM via free access Contributions to Zoology, 67 (4) - 1998 225 of the of Fig. 1. Diagrammatic representation main types crustacean compound eye: A, apposition eye, showing isolation of the redirection of from the ommatidium; B, superposition eye, showing light many facets to the target rhabdom; C-H, light path through viewed from the side and from above dotted line marks the ommatidial crystalline cones of superposition eyes (the axis); C, D, reflecting superposition; E, F, refracting superposition; G, H, parabolic superposition. After Nilsson, 1990 (not to scale). This not corrected the when the is hexagonally-faceted eyes. was eyeshine disappears eye exposed until the of the to and also the direc- optics parabolic superposition eye light may vary according to were correctly described (Nilsson, 1988). tion of observation (Exner, 1891; Shelton et al., In the of live specimens presence an eyeshine 1992; Gaten, 1994). Further identification of op- caused tical that patch covering several facets, by a tapeta! type requires the eye be hemisected to reflection from within the confirms that show the clear zone in that eye, superposition eyes or superposition optics are in use (Kunze, 1979). the component parts of the eye are examined mi- Care must be taken when using the absence of croscopically. Some information can be obtained confirm the in from the of the eyeshine to optics as many species shape crystalline cone, but deter- Downloaded from Brill.com10/11/2021 08:33:28AM via free access 226 E. Gaten - Evolution of superposition eyes in Decapoda initiation index the of the refractive profile of Very little work has been done on the develop- crystalline cone using an interference microscope ment of superposition optics in the Decapoda.