Kubicek, K. and Conway, K. W. 2016. Developmental osteology of Sciaenops ocellatus and Cynoscion nebulosus (Teleostei: Sciaenidae), economically important sciaenids from the western Atlantic. -Acta Zoologica (Stockholm) 97: 267-301.The adult skeleton in members of the economically important Sciaenidae is well documented, but information on earlier developmental stages is sparse and often focused on a particular character complex. To generate information on skeletal development in sciaenid fishes, we investigated the ontogeny of the entire skeleton in the western Atlantic Sciaenops ocellatus (Red drum) and Cynoscion nebulosus (Spotted seatrout), which are the focus of successful captive rearing programmes within the southern United States. Development of the skeleton (excluding the basisphenoid and sclerotic bones) is complete in S. ocellatus and C. nebulosus at 14.4 mm SL and 13.5 mm SL, respectively. The basisphenoid did not appear until later in development (21.9 mm SL in S. ocellatus and 19.5 mm SL in C. nebulosus), while the sclerotic bones are not present in the material examined. No major differences are identified between the ossification sequences compiled for each species. Cynoscion nebulosus exhibited variation in the presence/absence of two elements, supraneural 1 and the coronomeckelian. Lastly, we compile and compare available information on skeletal development across members of the Sciaenidae and compare the sequence of ossification compiled for S. ocellatus to that available for Danio rerio and Salminus brasiliensis (entire skeleton), and Chanos chanos (cranium only).Acta Zoologica (Stockholm) 97: 267-301 (July 2016)Kubicek and Conway Skeletal ontogeny of Sciaenops and Cynoscion

Background: Species of Danionella rank among the smallest of all vertebrates and their miniature size is correlated with an extreme case of progenesis, resulting in tiny, transparent sexually mature individuals. Progenesis has affected the entire skeleton of Danionella, in which 60 skeletal elements are absent, including some of the skull roofing bones. This lack of a skull roof combined with the presence of a fully formed hearing and sound producing apparatus has led to Danionella being used as an important model for neurophysiological studies. Results: Using both rank based and PGi analyses we investigate sequence heterochrony in the development of the skeleton of Danionella dracula and close relatives. Extreme heterochronic shifts affect the appearance of bony elements in Danionella dracula. This includes a delay in the appearance of most chondral skull bones, and a reduction or loss of dermal bones that would otherwise form the skull roof. In contrast, formation of the anterior region of the vertebral column, including the functionally important elements of the Weberian apparatus, is greatly accelerated. Conclusions: Here we show that the anatomical conditions that favor Danionella for brain research are the result of extreme heterochronic shifts that have acted differentially across the skeleton.

The characteristic and morphologically variable pectoral‐fin spine of catfishes (order Siluriformes) has been well‐investigated based on later developmental stages (juveniles and adults) but information on the earliest life stages are lacking. Here, we document the ontogeny of pectoral‐fin spines in four siluroid (Ictalurus punctatus, Noturus gyrinus, Silurus glanis and Akysis vespa) and two loricarioid catfishes (Corydoras panda and Ancistrus sp.). To further our understanding of pectoral‐fin spine development, we also examined adult and juvenile specimens representing 41 of the currently 43 recognized families of catfishes. Development of the pectoral‐fin spine is similar in all catfishes and resembles the development of a typical soft fin ray. Fusion between hemitrichia of the anteriormost lepidotrichium occurs proximally first, forming the spine proper, with growth of the spine occurring through the subsequent fusion of developing distal hemitrichial segments that comprise the spurious ray. The variation of pectoral‐fin spine morphology observed is largely attributed to the presence/absence of five traits, which either develop as part of the hemitrichial segments that are added to the distal tip of the spine during growth (distal rami, anterior/posterior serrae) or develop independent of these segments (denticuli and odontodes).

Miniaturization, the evolution of extremely small adult body size, is widespread amongst animals and commonly associated with novel ecological, physiological, and morphological attributes. The phenotypes of miniaturized taxa are noteworthy because they combine reductions and structural simplifications with novel traits not developed in their larger relatives. Previous research on miniature cyprinid fishes (focused predominantly on South and South East Asian taxa of a single subfamily) has identified two distinct classes of miniature taxa: proportioned dwarves and developmentally truncated miniatures. Miniaturization has also occurred independently in the subfamily Cyprininae, particularly in African lineages. We investigate the skeletal anatomy of Barboides, a genus of miniature African cyprinids that includes Africa's smallest known species of vertebrates, to assess whether miniaturization has resulted in similar organismal outcomes in different lineages of the Cyprinidae. The skeleton of Barboides is characterized by the complete absence of a number of dermal and endochondral ossifications, and marked reduction in size and/or complexity of other skeletal elements, particularly those of the dermatocranium. Absent skeletal elements in Barboides include those which develop relatively late in the ossification sequence of the non-miniature African relative 'Barbus' holotaenia suggesting that their absence in Barboides can be explained by a simple scenario of developmental truncation. In contrast to this theme of loss and reduction, the os suspensorium of Barboides is enlarged and the outer arm distally trifid and associated with a novel bulbous muscle in males. An evaluation of the skeleton of Barboides provides further evidence for a link between developmental truncation and evolutionary morphological novelty in Cyprinidae. In the spectrum of miniature cyprinids ranging from proportioned dwarves with few bones missing to highly progenetic taxa with dozens of missing bones, the two species of Barboides range roughly in the middle showing that the extremes are connected by intermediate levels of truncatedness.

The skeleton of Siluriformes is characterized by several autapomorphies, including secondary absence, extreme modification, and purported fusion of several ossifications. Although well documented in adults, information on skeletal development in catfishes is relatively sparse and typically focused on particular regions of the skeleton (e.g., Weberian apparatus). To further our understanding of the siluriform skeleton, I document the development of the entire skeleton in two ictalurid species, Ictalurus punctatus (channel catfish) and Noturus gyrinus (tadpole madtom) from five days pre-hatch to adult. I reexamine the homologies of bones previously hypothesized to represent compound elements in catfishes as well as an additional element only known to occur in some ictalurids. Development of the skeleton is complete in I. punctatus at 22.4 mm SL and almost complete in N. gyrinus (except dorsal- and anal-fin distal radials) at 14.1 mm SL. No signs of ontogenetic fusion were observed in any of the purported compound elements. Previous hypotheses of the homology of these elements and of additional ossifications are reviewed in light of developmental information obtained herein. No dermal parietal component is present at any stage in the so-called parieto-supraoccipital. The bone is the supraoccipital which ossifies from two lateral centers of ossification which later fuse, rather than from a median center. The ‘posttemporo-supracleithrum’ originates from a single center of ossification and represents the supracleithrum. The posttemporal is present in ictalurids and many other catfishes as a canal-bearing bone between the supracleithrum and the pterotic, a bone sometimes identified as the extrascapular. The extrascapular is missing in catfishes. Ictalurids have an additional dermal bone above the posttemporal, which is either an independently ossifying fragment of the posttemporal or a neoformation restricted to some members of this family. The single chondral bone of the pectoral girdle originates from a single center of ossification that represents the coracoid. The scapula is missing in catfishes. Dorsal-fin distal radial 2 is absent in catfishes and the foramen of dorsal-fin spine 2 is formed from modifications to the base of the fin-ray itself. Unlike loricarioid catfishes, the urohyal of ictalurids originates solely as an ossification of the sternohyoideus tendons. The anteriormost infraorbital element ossifies from a single center of ossification around the infraorbital sensory canal and represents the lacrimal. The antorbital is missing in catfishes. Finally, skeletal development of I. punctatus is compared to that available for other otophysans, including the cypriniforms Danio rerio and Enteromius holotaenia and the characiform Salminus brasiliensis.