Development of the Rostrum and Upper Jaws in Squirrelfishes and Soldierfishes (Beryciformes: Holocentridae): A Unique Ontogenetic Trajectory

Abstract

The Holocentridae are nocturnal fishes inhabiting shallow to deep water coral and rocky reefs in the tropical parts of the Indian, Pacific, and Atlantic oceans. They comprise two subfamilies, the squirrelfishes (Holocentrinae) and the soldierfishes (Myripristinae). Unlike the adults, holocentrid larvae are pelagic and are characterized by elaborate head spination, including a prominent rostrum and supraoccipital, preopercular, and opercular spines. The rostrum, formed by fused or separate nasal bones, can project far anterior to the mouth and is supported by a large, bulbous ethmoid cartilage. The spine-bearing bones, the lower jaw bones, and one pair of upper jaw bones (maxillae) are the first to ossify (at 1.2 mm NL). In several cleared-and-double-stained ontogenetic series of Holocentridae (1.2 mm NL–35.0 mm SL), we found that the other pair of upper jaw bones (premaxillae) exhibits a unique ontogenetic trajectory. The premaxillae develop late (between 5.9 mm and 6.6 mm), well after the other jaw bones and head spines are developed, and most of the remaining skeleton is ossified. Furthermore, the protuberant ethmoid cartilage precludes effective upper jaw protrusion until the larvae reach ca. 35 mm SL. In most marine fishes, all jaw bones ossify simultaneously very early in development along with the caudal fin and pectoral girdles, thus ensuring the ability to swim and feed immediately after hatching. We describe the jaw and rostrum development in holocentrids, compare it with that of tilefishes (Malacanthidae), the only other teleost in which the larvae have a rostrum comprising the nasal bones, and discuss the implications.