We distinguish four developmental periods in early syngnathid development composed of 11 stages (Figures 1, 2, 3, 4), characterised by the appearance (or disappearance, in the case of the yolk sac) of external morphological traits. Naturally, these stages are not specific time points, but rather reflect discrete periods that subdivide continuous development into a sequence of distinct units (cp. [25]). Our description starts with the activation of the egg and ends upon the release of free-living juveniles.
Early embryogenesis
During the first period we distinguish four stages: zygote, early cleavages, embryonic shield, and primitive streak (Figure 1). Individual cell cycles can be further partitioned for studies aimed at investigating early development (cp. [11, 13, 26]). The first two and the last two stages roughly coincide with blastulation and gastrulation, respectively.
The initial stages in the embryogenesis of syngnathid fishes follow the pattern common to all teleosts [27]. Upon activation of the egg (zygote stage; Figure 1A) the cytoplasm accumulates at the animal pole. The first few cleavages are synchronous – in Syngnathus acusimilis up to the 64-cell stage [13] – and meroblastic (incomplete); they produce blastomeres of roughly equal size [10, 11, 13] (but see [26]). Further divisions lead to the formation of the discoblastula [9], which is characterised by the blastoderm resting on top of the uncleaved yolk mass (Figure 1B). The blastoderm then flattens and moves towards the vegetal pole [13, 15]), marking the onset of epiboly (the enclosing of the yolk by the blastoderm). Around the time the blastoderm margin (germ ring) passes the equator, cells accumulate at a specific position along the germ ring to form the embryonic shield (Figure 1C). At this stage of development, the dorsoventral and anteroposterior axes of the developing embryo can be identified – the shield designates what will become the dorsal region of the embryo, while cells located towards the animal pole will develop into the head region (cp. [25]). At a more advanced stage of epiboly, the embryo emerges as a primitive streak on top of the yolk (Figure 1D).
Eye development
Eyes are the first conspicuous trait to develop in the primitive-streak embryo [17]; somites become faintly visible at roughly the same time [11, 16]. We distinguish three stages of eye development: optic vesicles, optic cups, and eye pigmentation (Figure 2).
Optic vesicles appear towards the end of epiboly as semi-spherical structures parallel to the forebrain (Figure 2A; cp. [15]). At around the time these structures develop into optic cups (Figure 2B), the tail bud develops. Optic cups are banana-shaped, two-layered eye structures, with the inner and outer layer forming the neural and the pigmented retina, respectively (cp. [25]). Retinal pigments first appear in the dorsal region of the eyes and then start spreading ventrally (Figure 2C) [16]; the first body pigments become visible at roughly the same time [9, 15, 16]. As the embryo elongates, the tail gradually detaches from the yolk sac [13, 14, 16, 22, 27]. Embryos of species with closed marsupia (e.g., Hippocampus, Syngnathus) hatch around this stage but are retained within the pouch, thus entering the larval phase (cp. [25]), while embryos of species with open brooding structures (e.g., Nerophis, Phyllopteryx) continue to develop inside the egg shell [9–11, 13, 15, 17]. At this stage of development both embryos and larvae are still feeding on their yolk reserves.
Snout formation
Syngnathids are suction-feeding fish characterised by tubular snouts composed of elongated, fused jaws [1]. During snout formation, the yolk sac gradually disappears [10, 11, 13, 15, 16]. Many species- or group-specific traits start developing during this period (e.g., body colouration, fleshy appendages in seadragons, and prehensile tails in seahorses). However, such traits are not appropriate as diagnostic features in a family-wide classification of developmental processes. For the purpose of this classification, we distinguish three stages of snout formation: ventral jaws, frontal jaws, and protruding snout (Figure 3).
Jaw formation starts before the eyes are fully pigmented. Jaws develop ventral to the eyes (Figure 3A) and first grow horizontally to the dorsoventral axis (e.g. [14, 16]); later they rise vertically. When the tip of the upper jaw reaches the middle of the eyes, the jaws are still in close contact with the forehead (Figure 3B; e.g. [13, 16, 17]). The snout then gradually elongates as a result of a lengthening of the ethmoid and the quadrate cartilages [11], approaching the protruding adult form (Figure 3C; cp. [10, 14–16]). Snout formation is completed prior to release [9, 13].
Juvenile
Data on post-release development is generally scarce [27] (but see [28]). Here we describe only the first stage (newborn; Figure 4) of the juvenile period, which is the last stage in our description of early syngnathid development.
At release the yolk sac is usually fully resorbed (but see [17]) and juveniles switch to external feeding [8–10]. However, under stressful conditions males may release premature juveniles still carrying yolk sac reserves [10, 16]. Many traits continue to develop post-release. Most notable among these are the fins, which differ markedly among species in their pattern of development (for a discussion of pre-release fin development in syngnathids, see Additional file 1), scales and body pigmentation, as well as fleshy appendages in seadragons (e.g. [8, 10, 17, 27]).