Standardised classification of pre-release development in male-brooding pipefish, seahorses, and seadragons (Family Syngnathidae)

Abstract

BackgroundMembers of the family Syngnathidae share a unique reproductive mode termed male pregnancy. Males carry eggs in specialised brooding structures for several weeks and release free-swimming offspring. Here we describe a systematic investigation of pre-release development in syngnathid fishes, reviewing available data for 17 species distributed across the family. This work is complemented by in-depth examinations of the straight-nosed pipefish Nerophis ophidion, the black-striped pipefish Syngnathus abaster, and the potbellied seahorse Hippocampus abdominalis.ResultsWe propose a standardised classification of early syngnathid development that extends from the activation of the egg to the release of newborn. The classification consists of four developmental periods – early embryogenesis, eye development, snout formation, and juvenile – which are further divided into 11 stages. Stages are characterised by morphological traits that are easily visible in live and preserved specimens using incident-light microscopy.ConclusionsOur classification is derived from examinations of species representing the full range of brooding-structure complexity found in the Syngnathidae, including tail-brooding as well as trunk-brooding species, which represent independent evolutionary lineages. We chose conspicuous common traits as diagnostic features of stages to allow for rapid and consistent staging of embryos and larvae across the entire family. In view of the growing interest in the biology of the Syngnathidae, we believe that the classification proposed here will prove useful for a wide range of studies on the unique reproductive biology of these male-brooding fish.

An evolutionary insight into the hatching strategies of pipefish and seahorse embryos

“…We further examined the expression levels during the development of messmate pipefish embryo, whose developmental stages were used according to the Sommer et al (). As shown in Figure B, HCE genes were strongly expressed in embryonic shield to eye pigmentation stage embryos, and quite weakly in protruding snout stage and just before newborn embryos.…”

“…However, in seahorse embryos, only HCE gene expression was detected, supporting the above results that the LCE gene was pseudogenized and was not active anymore in seahorses. We further examined the expression levels during the development of messmate pipefish embryo, whose developmental stages were used according to the Sommer et al (2012). As shown in Figure 5B, HCE genes were strongly expressed in embryonic shield to eye pigmentation stage embryos, and quite weakly in protruding snout stage and just before newborn embryos.…”

Histological development of the long‐snouted seahorse Hippocampus guttulatus during ontogeny

“…Such embryogenesis is a long process where duration differs among species, particularly when comparing tropical v . temperate species (Novelli et al ., ; Silveira & Fontoura, ; Sommer et al ., ; Wetzel & Wourms, ).…”

“…Seahorse eggs hatch before embryogenesis is complete, with the embryos remaining inside the male's brood pouch until full yolk resorption (Wetzel & Wourms, ). Hatched embryos continue developing inside the male pouch for several days before they are released (Sommer et al ., ). Embryonic and larval (post‐hatch–pre‐release) periods overlap in seahorse species and free‐swimming offspring emerge later from the pouch (Kornienko, ; Pham et al ., ; Wetzel & Wourms, ).…”

Embryo oxygenation in pipefish brood pouches: novel insights

“…Body size is a key fitness correlate in S. typhle, both in adults and in juveniles. Because by parturition the yolk sacs have usually been fully resorbed (Sommer et al, 2012), juveniles switch to external feeding once released from the paternal brood pouch. A previous study has shown that larger juvenile size at birth is associated both with subsequent faster growth rates, presumably due to higher foraging success, and with higher survival, due to greater success at escaping predators (Ahnesjö, 1992a).…”