Meeting Abstract
Mudskippers are amphibious fish of the Gobiidae that occupy tidal environments characterized by a well-defined air-water horizon. According to the terrain hypothesis, mudskippers should have a pronounced, horizontally oriented high density zone of neural cells across the retina. Such a horizontal streak would allow the fish to monitor the horizon with high visual acuity. Previous data suggest the presence of horizontal streaks in mudskippers yet it is unknown whether this feature can be interpreted as an evolutionary adaptation to tidal habitats. We tested for the presence of horizontal streaks and their potential adaptive significance by studying retinal topography of the African mudskipper Periophthalmus barbarus and two related gobiids, the ornate rainbow goby Stiphodon ornatus and the flaming arrow goby Sicyopus zosterophorus . Retinal topography maps were generated on the basis of stereological cell counts of Nissl-stained wholemounts. The resulting maps of the density of neural cells in the retinal ganglion cell layer revealed that horizontal streaks are not restricted to mudskippers. Contrary to our prediction, the horizontal streak was most prominent in the ornate rainbow goby, extending from the temporal region along the entirety of the retinal meridian. The horizontal streak seen in the flaming arrow goby was somewhat less pronounced but still extended along the entirety of the meridian, while that of the mudskipper was confined to the temporal aspect of the eye. Our results suggest that the horizontal streak of mudskippers cannot be interpreted as an evolutionary adaptation specific to an intertidal habitat. We propose that both intertidal and shallow river habitats with clear water can favor the evolution of horizontal streaks.
