Evolution and eye design in stomatopod crustaceans.
The diverse visual specialisations of stomatopods are an important
consideration in studies of their radiation and evolution. Most stomatopods
in the Superfamilies Gonodactyloidea and Lysiosquilloidea have regionally
specialised eyes. A central band composed of six rows of ommatidia
contains an array of photoreceptive pigments and filters that allow for finely
tuned colour and polarisation vision. In other stomatopods the mid-band is
reduced and unspecialised, or is absent. Previously, this has been
considered to be the plesiomorphic condition.
Phylogenetic analyses of the Stomatopoda show that the extant
stomatopod lineages evolved from a gonodactyloid-type ancestor.
Characters for phylogenetic analyses have been derived from external
morphology, details of eye daslqn and mitochondrial DNA sequences.
Although not wholly congruent, the results from these separate analyses
indicate that species with a simpler eye design are not more primitive but
have lost parts of the mid-band arrangement. This regressive evolutionary
event has occurred independently on a number of occasions.
Observations on the neuroanatomy of the eyes in the stomatopod
Neogonodactylus oerstedii have revealed the existence of an accessory lobe
located distally on the medulla externa and connecting with the six mid-band
rows. The lobe is involved in processing colour and polarisation information.
The discovery of the lobe in species that lack the retinal specialisations for
colour vision provides further evidence that they are descended from a more
advanced ancestor. Similarities in the arrangement of eye muscles between
species with a two or six row mid-band also give support for this conjecture.
The ancestors of the modern stomatopods are likely to have evolved in
shallow water and coral reef habitats. The development of colour vision was
advantageous for prey location and in interspecific encounters.
Stomatopods subsequently radiated into a diverse range of habitats. For
those in more spectrally limited surroundings the colour vision system has
largely been lost but vestiges are still present today in the form of a reduced
mid-band and medulla lobe.