Single-cell morphometrics reveals ancestral principles of notochord development

Embryonic tissues are shaped by the dynamic behaviours of their constituent cells. To understand such cell behaviours and how they evolved, new approaches are needed to map out morphogenesis across different organisms. Here, we apply a quantitative approach to learn how the notochord forms during the development of amphioxus, a basally-branching chordate. Using a single-cell morphometrics pipeline, we quantify the geometries of thousands of amphioxus notochord cells, and project them into a common mathematical space, termed morphospace. In morphospace, notochord cells disperse into branching trajectories of cell shape change, revealing a dynamic interplay between cell shape change and growth that collectively contribute to tissue elongation. By spatially mapping these trajectories, we identify conspicuous regional variation, both in developmental timing and trajectory topology. Finally, we show experimentally that, unlike ascidians but like vertebrates, posterior cell division is required in amphioxus to generate full notochord length, thereby suggesting this might be an ancestral chordate trait secondarily lost in ascidians. Altogether, our novel approach reveals that an unexpectedly complex scheme of notochord morphogenesis might have been present in the first chordates.

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.

Ontogenetic development of Heterocharax macrolepisEigenmann (Ostariophysi: Characiformes: Characidae) with comments on the form of the yolk sac in the Heterocharacinae

Fishes in early developmental stages frequently have morphological features that differ from those of adult stages, and many characters found later in ontogeny are not available in initial stages. Hence, morphological descriptions of early stages are useful to provide information for the identification of eggs and larvae, a knowledge still restricted among Neotropical fishes. We studied the development of Heterocharax macrolepis, a heterocharacine whose adult specimens from the aquarium trade were kept and spawned at around 23-24ºC. A developmental series of 51 specimens was preserved, ranging from 3.2 mm notochord length to 18.6 mm standard length, covering approximately the first 73 days post-hatching. We described the development of main morphological features emphasizing those useful in the identification of H. macrolepislarvae (i.e., appearance of preopercle spine and development of the pseudotympanum). We also compared H. macrolepis with photographs taken of live larval specimens of Gnathocharax steindachneri, recently included in the Heterocharacinae. Both species have a yolk sac with a small rounded projection directed posteroventrally. Although this information is not yet available for all pertinent taxa, the different yolk sac shape in other representatives of the Characiformes may indicate that this peculiar yolk sac represents an additional synapomorphy of the Heterocharacini.

The origin and recruitment of bass, Dicentrarchus labrax, larvae to nursery areas

The distribution of bass, Dicentrarchus labrax (L.), eggs taken in ichthyoplankton surveys of the Bristol channel and eastern Celtic Sea from April 1989 to May 1990 indicates that bass spawned predominantly offshore during March and April. Seventy-two bass larvae were captured by ring net and 16 by high-speed tow net during 19–27 May 1989, and 11 were captured by ring net during April and May 1990. Larvae were most abundant inshore where the water column was unstratified and less than 50 m deep.Back-calculated egg fertilization dates were determined for 58 larvae captured in May 1989 by ring net, using estimates of temperature-dependent egg and larval development rates and counts of daily growth increments on sagittal otoliths. These dates ranged from 5 April to 10 May 1989, later than those for most young-of-the-year bass which first appeared in Welsh estuaries during June 1989. This implies that bass larvae would have been more abundant before the May sampling period, even though these catches are the largest reported from UK waters.Bass larvae of 5–11 mm live notochord length were captured close to estuarine nursery areas, and their mean growth rate was approximately 0·2 mm per day. However, the smallest bass first arriving in the nursery areas during June, July and August were always larger than these larvae, and predominantly 15–20 mm total length. It is suggested that bass larvae hatching offshore either perish or are transported to unstratified coastal waters where they feed and remain for at least 30 days prior to their recruitment to the nurseries.