# Ontogenesis from Embryo to Juvenile in Threadsail Filefish, Stephanolepis cirrhifer

## TL;DR

This study describes the early development stages of the threadsail filefish to help develop methods for its artificial breeding.

## Contribution

The study provides detailed morphological data on ontogenesis from embryo to juvenile in a threatened marine species.

## Key findings

- Embryonic development is divided into eight distinct phases from cleavage to hatching.
- Two inflection points in growth curves correlate with metamorphosis and feeding habit changes.
- Morphological markers like fin differentiation and pigmentation were identified at specific developmental stages.

## Abstract

This study details and evaluates the morphological characteristics of the early embryonic development and ontogenesis of Stephanolepis cirrhifer, commonly known as the threadsail filefish. This species has experienced a considerable reduction in numbers in recent decades to the point of being listed as a threatened species in 2017, yet techniques for developing artificial propagation of the species have remained elusive. Our findings probably define the morphological characteristics of ontogenesis from embryo to juvenile. The embryonic development was observed and divided into eight phases, which were cleavage, blastocyst, gastrula, neurula, organogenesis, muscular contraction, heart pulsation, and hatching. Additionally, we found that the weaning period occurred from 40 dph, and two inflexion points occurring in the growth curves of larvae and juveniles were found to be associated with metamorphosis and transitions in feeding habits.

The threadsail filefish, Stephanolepis cirrhifer, is an economically important marine species. However, wild catches have sharply decreased over the past 20 years, causing S. cirrhifer to be added to the IUCN Red List of Threatened Species. Accordingly, this study seeks to promote technological development for artificial breeding and early life-stage farming by defining the morphological characteristics of ontogenesis. The fertilized eggs, with a diameter of 0.62 ± 0.01 mm, were spherical and sticky and contained multiple oil globules of varying sizes. The embryonic development was observed and divided into eight phases, which were cleavage, blastocyst, gastrula, neurula, organogenesis, muscular contraction, heart pulsation, and hatching. At 3 days post-hatching (dph), the yolk sac was completely absorbed. The eye developed rapidly, and the mouth fissure and anus initially formed. Some larvae were fed on S-rotifers (Brachionus plicatilis). At 6–8 dph, the upper and lower jaws of larvae were gradually covered by leathery skin, and the head-to-body proportion increased. At 14–16 dph, the fin differentiation occurred in the dorsal, anal, and pectoral fins, with widespread distribution of yellow and melanin on the body surface. Swim bladder was clear. The swimming ability of larva was enhanced, resulting in an obvious clustering phenomenon. At 22–25 dph, the end of the notochord continued to tilt upwards, forming a tail fin. The trunk was evenly distributed with protruding circular punctate scales. The snout was covered with leathery epidermis, and the mouth began to round. At 40–45 dph, the juvenile completed metamorphosis, with horizontal dark stripes appearing on the trunk. Pigmented spots appeared on the tail fins. The counts of dorsal and anal fin spines were 34–36 and 32–34 dph, respectively. During the development of larvae and juveniles, the growth parameters, such as total length, standard length, body height, and body weight, were made as growth curves. The slopes of growth curves were calculated. We found two inflexion points occurring in the growth curves, which may be associated with metamorphosis and transitions in feeding habits. These results enrich the biological understanding of filefish species while providing guidance for artificial propagation and fry production in S. cirrhifer.