Phylogenetic Status of Paralichthodes algoensis (Pleuronectiformes: Paralichthodidae)

Abstract

The phylogenetic position and classification of the flatfish, Paralichthodes algoensis, have been subject to several changes since its description by Gilchrist (1902). Regan (1910) classified this South African species within the Samarinae, along with Brachypleura and Samaris, based on the absence of a distinct caudal peduncle, the forward extension of the dorsal fin nearly to the end of the snout, and the asymmetrically placed pelvic fins. Regan (1920) suggested that, although P algoensis is a pleuronectoid based on the absence of spinous fin-rays, the forward extension of the dorsal fin on the head, the emarginate urohyal and the absence of supramaxillary and palatine teeth, it differs from pleuronectids in the arrangement of the olfactory laminae. As a result, Regan (1920) placed P algoensis in a separate family, Paralichthodidae. In a subsequent revision, Regan (1929) recognized P algoensis as a subfamily within Pleuronectidae, based on its dextral eye position, nerve of the left eye always dorsal, a terminal mouth, and prominent lower jaw. This later classification was retained by Norman (1934) and Hubbs (1945). However, polarity for eye position cannot be assessed (Chapleau, 1993), the distribution of optic nerve morphology is not well understood (Hensley and Ahlstrom, 1984), and a terminal mouth and prominent lower jaw are plesiomorphic for the order (Chapleau, 1993). Nelson (1984) classified P algoensis within the Samarinae, based on the advanced position of the dorsal fin in front of the eyes and symmetrical pelvic fins; however the former is commonly observed in other flatfish groups, whereas the latter is plesiomorphic for the order. Sakamoto (1984) examined and illustrated the osteology of P algoensis as part of a phenetic study of pleuronectid intrarelationships. Paralichthodes was placed in Pleuronectinae based on overall osteological similarity. The similarities between P algoensis and other pleuronectins were deemed plesiomorphic by Chapleau (1993). Chapleau (1993) did not include P algoensis in his analysis of pleuronectiform relationships, and he suggested that, because P algoensis lacks the bothoid-type caudal skeleton observed in Hensley and Ahlstrom (1984), identifying the phylogenetic position for P algoensis would require a comparative analysis of groups such as the Rhombosoleinae, Poecilopsettinae, and Samarinae. Nelson (1994) reclassified P algoensis into a monotypic subfamily (Paralichthodinae) within Pleuronectidae, indicating that its relationship within the family was uncertain. This study establishes the phylogenetic position for P algoensis based on uniquely derived traits through a comparison with the most recent phylogenetic analysis of Pleuronectiformes (Chapleau, 1993).