A histological survey of avian post-natal skeletal ontogeny

Bone histology of crown-group birds is a research topic of great interest, permitting insight into the evolution of remarkably high growth rates in this clade and variation across the altricial-precocial spectrum. In this study, we describe microanatomical characteristics of the humerus and femur in partial growth series from 14 crown group birds representing ten major clades (Struthioniformes, Galliformes, Apodiformes, Columbiformes, Charadriiformes, Accipitriformes, Strigiformes, Psittaciformes, Falconiformes, and Passeriformes). Our goals were to: (1) describe the microanatomy of each individual; (2) make inter-and intra-taxonomic comparisons; (3) assess patterns that correspond with developmental mode; and (4) to further parse out phylogenetic, developmental, and functional constraints on avian osteological development. Across taxa, the femoral and humeral tissue of neonates can be broadly characterized as highly-vascularized, disorganized woven bone with great variation in cortical thickness (inter-and intrataxonomically, within an individual specimen, and within a single section). The tissue of precocial chicks is relatively more mature at hatching than in altricial, but other categories along the developmental spectrum were less easy to distinguish, thus we were unable to identify a definitive histological proxy for developmental mode. We did not find evidence to support hypotheses that precocial chicks exclusively have thicker cortices and more mature bone in the femur than the humerus at time of hatching; instead, this is a characteristic of nearly all taxa (regardless of developmental mode), suggesting deep evolutionary origins and the effects of developmental channeling. Bone tissue in adults exhibited unexpected variation, corresponding to differences in body size. Large-bodied birds have cortices of fibrolamellar bone, but organization of tissue increases and vascularity decreases with diminishing body size. The outer circumferential layer (OCL) also appears at earlier growth stages in small-bodied taxa. Thus, while the OCL is indicative of a cessation of appositional growth it is not always indicative of cortical maturity (that is, maximum organization of bony tissue for a given taxon). Small size is achieved by truncating the period of fast growth; manipulation of the timing of offset of bone growth is therefore an important factor in changing growth trajectories to alter adult body size.

Postcranial skeletal development of Mugil cephalus (Teleostei: Mugiliformes): morphological and life-history implications for Mugiliformes

Within the fish taxon Mugiliformes, the larval development of Mugil cephalus has been studied most intensively, because it has the widest range of distribution among all mugilids and is of interest to aquaculture all over the world. Although numerous studies have dealt with larval rearing, growth and development, the osteological development of M. cephalus and mugiliforms in general has largely been neglected. Herein, we describe the skeletal development of mullets for the first time. Cleared and double-stained specimens of aquaculture-reared M. cephalus and wild-caught mugilid larvae were examined to describe the early development of the pectoral and pelvic girdle, the vertebral column and the caudal and median fins. The description of four embryonic and six larval developmental steps within the embryonic and larval period enables us to compare larval sizes of reared and wild-caught larvae. Ontogenetic fusions of ural centra 1 and 2 into a compound centrum, in addition to the fusion of two pterygiophores in the anal fin, have implications for the perception of the adult morphology. Moreover, comparison of mugilid development with that of other ovalentarian taxa shows that recent phylogenetic hypotheses need further morphological investigation.

Larval development and osteology of Callanthias platei (Teleostei: Callanthiidae) from Desventuradas Islands, South Pacific

Larvae of the Juan Fernández Splendid Perch, Callanthias platei Steindachner, 1898 (Teleostei: Callanthiidae), are described for the first time. The five available specimens, ranging from 3 mm notochord length (NL) to 6.1 mm standard length (SL), were collected with plankton nets near Desventuradas Islands, Southeast Pacific, during the austral spring of 2015 and 2016. The larvae are deep bodied with a narrow caudal peduncle, a large head and a triangular, coiled gut. The pigmentation is scarce in preflexion larvae but increases after flexion. All individuals have several groups of head spines: anterior and posterior preopercular, interopercular, subopercular, opercular and supracleithral. The formation of all fins occurs early in development, with the pectoral fins appearing first and the pelvic fins last. The osteological development of the skull, vertebral column, fins, and scales are described.

Desarrollo osteológico de la columna vertebral, complejo caudal y aletas medias del botete diana Sphoeroides annulatus (Tetraodontiformes: Tetraodontidae)

Osteological development of the vertebral column, caudal complex and median fins in larvae of the bullseye puffer fish, Sphoeroides annulatus, were studied using specimens cleared and double stained for bone and cartilage. The first cartilaginous structures appeared 10 days (4.2 mm NL) after hatching and by day 28 (14.9 mm TL) most of the bone elements had already formed. Sphoeroides annulatus exhibited fusion of bone elements, the same as other Tetradontiformes, one of the most evolved teleosts.