ORIGINS OF FILTER FEEDING IN LAMNIFORM SHARKS

Elasmobranchii is a subclass of cartilaginous fish in the class Chondrichthyes. Elasmobranchii consists of sharks, rays, and skates. Of approximately 1,100 species of extant elasmobranchs, only 13 (1.2%) are filter feeders. Two such filter feeding species are the basking shark (Cetorhinus maximus) and the most recently discovered megamouth shark (Megachasma pelagios), both belonging to the order Lamniformes.

The discovery of the megamouth shark in 1976 has led to two main conflicting hypotheses regarding the origins of filter feeding in lamniform sharks. Maisey (1985) hypothesized that there is a single origin of filter feeding within Lamniformes. Conversely, Compagno (1990) hypothesized that the filter feeding adaptions have been developed independently within the two lineages due to different ancestral conditions. Morphological phylogenetics are the basis of both studies. In addition to morphological studies, DNA sequencing studies of the cytochrome b gene (Martin and Naylor, 1997) and the dihydronicotinamide adenine dinucleotide (NADH-2) gene (Naylor et al., 1997) have been performed on lamniform sharks to determine if any genetic evidence supports or refutes either of these hypotheses.

To fully answer the question of how filter feeding originated in lamniform sharks, the information obtained from these previous studies was used in conjunction with analysis of the fossil record as well as geometric morphometric analysis of the teeth and gill rakers of extant megamouth and basking sharks. The preexisting molecular and morphological phylogenetic information combined with additional analysis of the fossil record and geometric morphometric analysis provides further support for the hypothesis that filter feeding adaptations evolved independently within the order Lamniformes.