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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:264594 |
| Date | January 1998 |
| Creators | Harling, Christine |
| Publisher | University of Sussex |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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