The sensory biology and feeding behaviour of sawfish

Abstract

The family of pristid sawfish face a global crisis, with all species listed as critically endangered on the IUCN Redlist. One characteristic that all species have in common may also be a major factor contributing to their decline; the elongated rostrum that bears lateral teeth (the ‘saw’). As the saw becomes easily entangled in fishing gear and is also a valuable trophy, sawfish are regularly taken as by-catch. However, the use of the saw has never been studied. My objective is to identify the importance of the saw during prey manipulation, and to assess the relative importance of three sensory modalities (vision, electroreception and mechanoreception) in the same context. The morphology of the rostrum of pristid sawfish was analysed and compared to rhinobatid shovelnose rays and the extinct sclerorhynchid sawfish, as these three families are monophyletic. Comparison of the internal structure of the rostrum in the three taxa indicates that in pristids the anterior portion of the rostrum lengthened, which results in a slimmer construction than that of the sclerorhynchid rostrum. Moreover, pristid rostral teeth grow continuously from the base, whereas sclerorhynchids possess functional rostral teeth and dormant replacement teeth. Characteristics of the pristid rostrum are interpreted as adaptations to reduce weight and therefore potentially allow fast lateral swipes of the rostrum during food manipulation. Contrary to other batoids, the atlanto-occipital joint of pristids allows free movement of the cranium in all directions. Feeding observations of juvenile freshwater sawfish Pristis microdon lead to the establishment of an ethogram in the context of prey manipulation. Results indicate that an average feeding event that leads to the successful ingestion of a fish is comprised of five behaviours, of which at least one includes the use of the saw. However, freshwater sawfish use their saw during prey manipulation in three distinctive behaviours. Comparison of the two dimensional sensory array of the electroreceptive ampullae of Lorenzini in rhinobatids and pristids reveals how the elongation of the rostrum may have expanded the food spectrum of pristids: the total number of ampullary pores distributed ventrally on the head and rostrum of the pristid Anoxypristis cuspidata (490.3 ± 68.7) equals that of the rhinobatids Glaucostegus typus (452.0 ± 162.8) and Aptychotrema rostrata (461.5 ± 17.7). However, A. cuspidata possesses more than seven times the number of pores on the dorsal surface of the rostrum (297.1 ± 56.6 pores) than the rhinobatids (A. rostrata 40.7 ± 2.3 pores, G. typus 24.9 ± 5.1 pores), indicating that pristids can potentially sense the electric field of prey in the water column. One retina of a juvenile freshwater sawfish P. microdon was analysed in a retinal wholemount. The density distribution of photoreceptors shows several peaks mediating acute vision, which is comparable to the retina of a white tip reef shark Triaenodon obseus. These peaks may help sawfish to maneouvre structurally complex habitats, such as mangrove roots in rivers in order to hide from predators. Peaks in the rod and cone density of the temporal region may be useful during prey manipulation with the rostrum. Detailed knowledge of sawfish biology is crucial for maintaining optimal health of these endangered species in captivity and alleviating future fishing pressure