Variations in electrosensory system morphology in teleost and elasmobranch fishes

Abstract

The sense of electroreception is the ability to detect weak electric fields in the surrounding environment. This sense has evolved independently several times in vertebrates, and, in elasmobranchs is associated with the ampullae of Lorenzini, and in catfishes (Siluridae) with teleost ampullary organs. This thesis investigates how the lifestyle, diet, and environment of an elasmobranch impact the distribution and morphology of their electrosensory system, as well as whether the ampullary organs of silurids could be phenotypically plastic. I first studied three species of sympatric benthic rays, Neotrygon trigonoides, Maculabatis toshi¸ and Hemitrygon fluviorum. Despite all three species possessing markedly different diets the structure of their electrosensory system is nearly identical; however, I describe previously unreported features of their ampullae of Lorenzini. Some of the ventral ampullary canals were of a peculiar quasi-sinusoidal shape, and the supportive cells of the sensory epithelium extended out heavily into the ampullary lumen and were apically nucleated. I then investigated the electrosensory system of a bentho-pelagic eagle ray, Aetobatus ocellatus, the ultrastructure of which is identical to the previously studied benthic rays. However, the distribution of their electrosensory system was quite peculiar for a batoid, with a complete absence of ampullary pores on their pectoral fins, and only few of them distributed over their body. This species exhibits a high concentration of ampullary pores on the snout, which I hypothesize is used to locate prey. Quasi-sinusoidal ampullary canals were also observed on both the ventral and dorsal surface of the body of this ray, suggesting that the function of this peculiar shape, if any, is unlikely to be related to prey location. The third chapter compares the ampullary organs of two species of benthic sharks, Chiloscyllium punctatum and Hemiscyllium ocellatum. The sensory epithelium of both species appears to be relatively flattened compared to other elasmobranchs but is otherwise similar in structure to those of previously studied sharks. Interestingly, despite their phylogenetic proximity, similar environments, and diets, some of the ampullary canals of H. ocellatum were quasi-sinusoidal, similar to those observed in batoids, yet the canals of C. punctatum are all linear. The ten species of galean sharks studied, Prionace glauca, Carcharhinus cautus, Carcharhinus limbatus, Carcharhinus tilstoni, Carcharhinus longimanus, Carcharhinus falciformis, Galeocerdo cuvier, Hemigaleus australiensis, Carcharhinus brevipinna, and Isurus oxyrinchus, include coastal, oceanic, bentho-pelagic, and pelagic species. While they differ in their diets they do have a general preference for teleosts, and are all more or less heavily reliant on non-electrical senses, such as vision in I. oxyrinchus, or olfaction in G. cuvier. The ultrastructure of their ampullae of Lorenzini differs little among these ten sharks. Larger species of shark and larger individuals within a species tend to possess more numerous sensory chambers, and larger ampullary pores. Thus, size of the animal is seemingly more influential than its environment, lifestyle, or diet. However, clear differences were observed in the distribution and quantity of their ampullary pores, with members of the genus Carcharhinus displaying very similar distribution patterns and counts, despite coming from different environments and lifestyles, whereas other species, such as I. oxyrinchus, exhibit a markedly different distribution of ampullary pores that seem to fit in well with its highly visual nature and high-speed foraging strategy. The teleost ampullary organs of the salmontail catfish, Neoarius graeffei, differ in morphology depending on the environment of origin of the fish. Freshwater ampullae tend to be very short and stay within the limit of the epidermis, with few receptor cells per ampulla, while marine animals possess much longer ampullae with more numerous receptor cells. I investigated the potential phenotypic plasticity of the electrosensory system of this species. I collected juvenile N. graeffei from the Brisbane river and raised them in different environments (freshwater, estuarine, and marine) for six months and then investigated the morphology of ampullary organs at the end of this period. There was no evidence of change in the animals kept for six months from the control animals, or variations between each treatment, suggesting that these sensory organs do not undergo phenotypic plasticity if juvenile catfish move between environments that differ in salinity.