Leis' conundrum: Homology of the clavus of the ocean sunfishes. 2. Ontogeny of the median fins and axial skeleton of Ranzania laevis (Teleostei, Tetraodontiformes, Molidae)

Abstract

One of the most conspicuous characters of the ocean sunfishes, family Molidae, is the punctuation of the body by a deep, abbreviated, caudal fin-like structure extending vertically between the posterior ends of the dorsal and anal fins, termed the clavus by Fraser Brunner. Homology of the clavus has been a matter of debate since the first studies on molid anatomy in the early 1800s. Two hypotheses have been proposed: 1) It is a highly modified caudal fin; 2) It is formed by highly modified elements of the dorsal and anal fins. To resolve this homology issue, we studied the ontogeny of the molid vertebral column and median fins and compared it to that of a less morphologically derived gymnodont (see Part 1 of this study), a member of the family Tetraodontidae. We show that in molids the chorda never flexes during development, that the claval rays form from the posterior ends of the dorsal and anal fins toward the middle, thus closing the gap inward, and that elements of the molid clavus have an identical development and composition as the proximal-middle and distal radials of the regular dorsal and anal fins. We thus conclude that the molid clavus is unequivocally formed by modified elements of the dorsal and anal fin and that the caudal fin is lost in molids.

Occipito‐vertebral fusion in ocean sunfishes (Teleostei: Tetraodontiformes: Molidae) and its phylogenetic implications

“…Although the earliest anatomical descriptions date from the 1800s (Meckel, 1828;Wellenbergh, 1840;Goodsir, 1841;Cleland, 1862;Steenstrup and Lü tken, 1898) and several monographs appeared in the 20th century (Gregory and Raven, 1934;Raven, 1939a,b;Winterbottom, 1974;Tyler, 1980), much of the unusual, highly derived anatomical structure of molids is still poorly understood. During our study of the development and homology of the molid clavus or pseudocaudal fin Johnson and Britz, 2004), we discovered a previously overlooked and highly unusual character of the vertebral column that characterizes the Molidae: ontogenetic fusion of the first vertebra with the occiput. Such fusion is rare in teleosts and occurs elsewhere among tetraodontiformes only in the boxfishes, family Ostraciidae (Tyler, 1963).…”

Leis' conundrum: Homology of the clavus of the ocean sunfishes. 1. Ontogeny of the median fins and axial skeleton of Monotrete leiurus (Teleostei, Tetraodontiformes, Tetraodontidae)

“…A large comprehensive ontogenetic series from a captive breeding of the Asian freshwater puffer Monotrete leiurus forms the basis for a detailed comparison with the development of the axial skeleton and median fins in the ocean sunfish, Ranzania laevis, in the second part of our study (Johnson and Britz, 2004).…”

Taxonomy and systematics of tetraodontiform fishes: a review focusing primarily on progress in the period from 1980 to 2014

“…Ostraciidae). Britz and Johnson (2005) and Johnson and Britz (2005a) found the fusion of anterior vertebral centra in the occipital region and a thick band of cartilage on the side of the pterygiophores of the vertical fins in an ostraciid and molids suggestive of a close relationship between the Ostraciidae and Molidae. However, because they studied only one species of Ostraciidae and two species of Molidae, it seems premature to hypothesize a close relationship of the two families until further support for it may be forthcoming.…”