Early ontogeny of the Atlantic halibut Hippoglossus hippoglossus head

What is missing from the sturgeon jaw: Developmental morphology of the upper jaw in Acipenser

Sturgeons belong to the family Acipenseridae, the most species-rich extant family of Acipenseriformes, a basal actinopterygian group of key importance in assessing the early radiations of the actinopterygians. At the same time, acipenseriforms display unique specializations in the morphology of the snout and jaws which make them a valuable model for studying evolutionary novelties. However, despite a long history of research, the homologies of the snout and the mandibular arch of acipenseriforms remain uncertain preventing further studies on the evolutionary origin of their unique snout and jaw structure, and in particular, of the upper jaw symphysis, the key apomorphy of the group and the preoral snout. In the present study, a detailed description of the upper jaw morphology and development in sturgeons is provided in order to address its composition in terms of the common actinopterygian archetype. Based on the obtained results, the upper jaw of acipenseriforms is assumed to have lost the autopalatine portion, which most likely is represented by the separate cartilages supporting the tentacles. Also, the conventional interpretation of the sturgeon's maxilla as dermopalatine is rejected on the grounds of this bone structure and development. Paedomorphosis is proposed to be the most likely mechanism explaining the evolutionary origin of the upper jaw symphysis and supposed modifications of the snout in sturgeons.

Skeletal development of the chondrocranium in the tongue sole Cynoglossus semilaevis (Pleuronectiformes: Cynoglossidae)

This study provides a comprehensive description of chondrocranial development before, during and after larval metamorphosis in the tongue sole Cynoglossus semilaevis, a commercially valuable flatfish in China. Samples were collected at regular intervals ranging from 1 to 23 days post hatching (dph). Based on observations of cleared and double-stained specimens and images from sections stained with safranin O-fast green, major morphological events during early development were described. No cartilaginous structure was visible at hatching. From 2 dph onwards, cartilaginous structures such as the trabecular bar and some elements of the mandibular, hyoid and branchial arches appeared. At this time also, cartilaginous structures of the neurocranium started to form. Hypertrophic chondrocytes could be observed in many splanchnocranium elements at 5 dph. The start of ossification was indicated by alizarin red stain visible at 14 dph. At 17 dph, most of the cartilaginous skeleton was ossified. Soon after, the right eye started to migrate and pass through a slit beneath the dorsal-fin base and above the skull. Metamorphosis was complete at 20 dph, at which time the dorsal-fin base cartilage extended onto the anterior region of the head. Meanwhile, extremities of the hyoid and branchial arch elements remained cartilaginous. At 23 dph, endochondral ossification of the splanchnocranium was nearly complete. Unlike previous observations of other Pleuronectiformes, our study indicates that endochondral ossification of C. semilaevis skull cartilage occurs before metamorphosis.

Unravelling the ontogeny of a Devonian early gnathostome, the "acanthodian" Triazeugacanthus affinis (eastern Canada)

The study of vertebrate ontogenies has the potential to inform us of shared developmental patterns and processes among organisms. However, fossilised ontogenies of early vertebrates are extremely rare during the Palaeozoic Era. A growth series of the Late Devonian “acanthodian” Triazeugacanthus affinis, from the Miguasha Fossil-Fish Lagerstätte, is identified as one of the best known early vertebrate fossilised ontogenies given the exceptional preservation, the large size range, and the abundance of specimens. Morphological, morphometric, histological and chemical data are gathered on a growth series of Triazeugacanthus ranging from 4 to 52 mm in total length. The developmental trajectory of this Devonian “acanthodian” is characteristic of fishes showing a direct development with alternating steps and thresholds. Larvae show no squamation but a progressive appearance of cartilaginous neurocranial and vertebral elements, and appendicular elements, whereas juveniles progress in terms of ossification and squamation. The presence of cartilaginous and bony tissues, discriminated on histological and chemical signatures, shows a progressive mineralisation of neurocranial and vertebral elements. Comparison among different body proportions for larvae, juveniles and adults suggest allometric growth in juveniles. Because of the phylogenetic position of “acanthodians”, Triazeugacanthus ontogeny informs us about deep time developmental conditions in gnathostomes.

High dietary arachidonic acid levels affect the process of eye migration and head shape in pseudoalbino Senegalese sole Solea senegalensis early juveniles

The effect of high dietary levels of arachidonic acid (ARA) on the eye migration and cranial bone remodelling processes in Senegalese sole Solea senegalensis early juveniles (age: 50 days post hatch) was evaluated by means of geometric morphometric analysis and alizarin red staining of cranial skeletal elements. The incidence of normally pigmented fish fed the control diet was 99·1 ± 0·3% (mean ± s.e.), whereas it was only 18·7 ± 7·5% for those fed high levels of ARA (ARA-H). The frequency of cranial deformities was significantly higher in fish fed ARA-H (95·1 ± 1·5%) than in those fed the control diet (1·9 ± 1·9%). Cranial deformities were significantly and negatively correlated with the incidence of normally pigmented animals (r² = -0·88, P < 0·001, n = 16). Thus, fish displaying pigmentary disorders differed in the position of their eyes with regard to the vertebral column and mouth axes, and by the interocular distance and head height, which were shorter than in fish not displaying pigmentary disorders. In addition to changes in the positioning of both eyes, pseudoalbino fish showed some ARA-induced osteological differences for some of the skeletal elements from the splanchnocranium (e.g. right premaxillary, dentary, angular, lacrimal, ceratohyal and branchiostegal rays) and neurocranium (e.g. sphenotic, left lateral ethmoid and left frontal) by comparison to normally pigmented specimens. Pseudoalbino fish also had teeth in both lower and upper jaws. This is the first study in Pleuronectiformes that describes impaired metamorphic relocation of the ocular side eye, the right eye in the case of S. senegalensis, whereas the left eye migrated into the ocular side almost normally.

Early development of the postcranial skeleton of the pikeperch Sander lucioperca (Teleostei: Percidae) relating to developmental stages and growth

The early development of the postcranial skeleton (pectoral girdle, pelvic girdle, vertebral column and fins) in pikeperch (Sander lucioperca (L.)) was studied from hatching to days 47 and 43 post fertilization (dpf) at two different rearing temperatures, 15.5 and 18.0°C. Four embryonic and six larval stages were described, ranging from 3.4 ± 0.3 mm to 21.8 ± 2.1 mm in total length. The crucial point in larval development is swimbladder inflation, which enables larvae to swim energy efficiently. Until this time point, only the most essential skeletal elements to enable swimming movements have developed. As the larvae become neutrally buoyant, they grow and differentiate postcranial elements rapidly. Concurrently, swimming performance and foraging success seems to improve. A specific size is correlated with a distinct developmental stage defined by a set of traits that includes the skeletal elements. The developmental sequence of skeletal structures is temperature independent, although growth is slower and the individual developmental stages are reached later at 15.5°C than at 18.0°C.