The Whitney Laboratory for Marine Bioscience
Whitney’s Drs. Barbara Battelle, Joseph Ryan and Karen Kempler along with Dr. Megan Porter from the University of Hawaii, Manoa headed a research team, which included members of the Limulus (American horseshoe crab) Genome Consortium to publish the Limulus genome and a comprehensive analysis of the expression of visual pigments in horseshoe crabs. This work appeared in the June 3, 2016, issue of Genome Biology and Evolution.
Horseshoe crabs occupy a particularly interesting branch of the arthropod family tree. They are chelicerates, most closely related to scorpions and spiders. Chelicerates, in turn, are considered a sister group to all other arthropods (insects, crustaceans and myriapods, which include millipedes and centipedes). Although horseshoe crabs have acquired new characteristics during evolution, they also have retained many primitive morphological features and are thus considered invaluable for reconstructing the earliest chelicerate ancestor. Because of the position of horseshoe crabs in the arthropod family tree, the high quality genome assembly presented by Battelle and her collaborators is an invaluable resource for understanding the evolution of arthropods in general.
For the past 30 years, the Battelle lab has been a leader in researching the visual system of the American horseshoe crab and has provided insights into the evolution of visual systems, how visual information is processed in the nervous system and the function of photoreceptors. One key focus has been to characterize horseshoe crab visual pigments, molecules expressed in photoreceptors that are responsible for converting light energy into an electrical signal. Over the years, Battelle and her colleagues have characterized eleven of these molecules. Analyses of the horseshoe crab genome surprisingly revealed seven more visual pigment genes and permitted the team to compare the structure of these genes with those of diverse species and thus provide new insights into the evolution of these genes which are critical for vision.
The team also analyzed where in the horseshoe crab’s nervous system these visual pigments are expressed, and they found that many are expressed not only in eyes, but throughout the nervous system. This suggests that photosensitive cells are distributed throughout the nervous system of horseshoe crabs. There is a growing recognition of the importance of extraocular photoreception in diverse species. In horseshoe crabs it may be particularly important for the survival of newly hatched and juvenile animals which are largely transparent.