Independent Origins of Filter-Feeding in Megamouth and Basking Sharks (Order Lamniformes) Inferred from Phylogenetic Analysis of Cytochrome b Gene Sequences

There are conflicting hypotheses with regard to the origins of filter-feeding in lamniform sharks. Maisey (1985) proposed that the megamouth (Megachasma pelagios) and the basking shark (Cetorhinus maximus) are sister taxa, implying a single origin of filter-feeding within the Lamniformes. By contrast, Compagno (1990) hypothesized that adaptations for filter-feeding evolved independently in the two lineages from different ancestral conditions. Phylogenetic analysis of cytochrome b gene sequences from representatives of all genera of lamnoid sharks refutes the single-origin of filter-feeding hypothesis, implying independent origins. Moreover, the data permit confident refutation of published phylogenetic hypotheses of the group. However, for many lineages there is little hierarchical signal in the molecular data, prohibiting promulgation of a single, "best" supported alternative hypothesis for the group. Lack of confident resolution of the phylogenetic relationships among species of lamnoid sharks most likely reflects divergence of ancient lineages in a brief period of time. Fossils indicate a Middle to Late Cretaceous origination for most extant genera of lamnoid sharks. Filter-feeding has evolved independently in a variety of marine and freshwater animals. In elasmobranchs filter-feeders are classified in four families representing three orders (Myliobatiformes, Mobulidae [manta rays]; Lamniformes, Cetorhinidae [basking shark] and Megachasmidae [megamouth]; and Orectolobiformes, Rhincodontidae [whale shark]). Three of these families are sharks, and all of the filter-feeding sharks resemble baleen whales in size and life style. Two of the three species (the basking shark [Cetorhinus maximus] and megamouth [Megachasma pelagios]) are thought to be close relatives in the order Lamniformes (Maisey, 1985). The other species, the whale shark (Rhincodon typus), is a distant relative ofthe lamnoid filter-feeders, being classified in the Orectolobiformes, an order of sharks consisting mostly of benthic species. As in the baleen whales, specialization by sharks on small, planktonic organisms is associated with modification of a suite of morphological and behavioral traits. The chondrocranium is modified, there is a loss of dental differentiation and simplification of tooth cusp and root morphology, and the gill rakers are greatly enlarged (Compagno, 1984; Maisey, 1985). Moreover, with few exceptions, vertebrate filter-feeders are strong swimmers and are constrained to live near the surface of nutrient-rich waters where plankton concentrations reach their zenith. For example, the basking shark, which is the largest of the lamniform sharks, attaining a total length of up to 30 feet, sports a rigid and streamlined body, firm muscles and connective tissue, features that aid feeding by straining plankton through highly modified gillrakers as it moves through the water column with its huge mouth agape (similar to some baleen whales). The whale shark is also most commonly encountered in surface waters, and like the basking shark, has a streamlined body, and firm muscles and connective tissue. Unlike 'Department of Biological University of Nevada-Las Vegas, 4505 Maryland Pkwy, Las Vega~. NV 89154-4004, USA 'Department of Biology, Yale University New Haven, CT 06520, USA