Pipefishes, like their seahorse relatives, leave most of the parenting duties to the male, which provides all of the postzygotic care for its offspring, supplying them with nutrients and oxygen through a
placenta-like connection. It broods the offspring either on distinct region of its body or in a brood pouch. Brood pouches vary significantly among different species of pipefish, but all contain a small opening through which female eggs can be deposited. The location of the brood pouch can be along the entire underside of the pipefish or just at the base of the tail, as with seahorses. Pipefish in the genus
Syngnathus have a brood pouch with a ventral seam that can completely cover all of their eggs when sealed. In males without these pouches, eggs adhere to a strip of soft skin on the ventral surface of their bodies that does not contain any exterior covering. The evolution of male brooding in pipefish is thought to be a result of the
reproductive advantage granted to pipefish ancestors that learned to deposit their eggs onto the males, who could escape predation and protect them. Furthermore, the ability to transfer immune information from both the mother (in the egg) and the father (in the pouch), unlike other chordates in which only the mother can transfer immune information, is believed to have an additive beneficial effect on offspring immunity. Furthermore, in
Corythoichthys haematopterus, similar ritualized mating dances were hypothesized to aid in reproductive synchronization, by allowing the female to assess male willingness to spawn so her eggs are not wasted. During pipefish copulation, which signifies the termination of the courtship dance, the female transfers her eggs through a small
ovipositor into the male brood pouch or onto the special patch of skin on the male's ventral body surface. While the eggs are being transferred, the mating pair rises through the water until implantation is complete. At this point, the male assumes an S-shaped posture and fertilizes the eggs, all the while descending back down the water column. Males possessing brood pouches release their sperm directly into them; the pouches are then vigorously shaken. The ventral seams are not opened until weeks later when the male pipefish give birth. Because the pipefish embryos develop within the male, feeding on nutrients supplied by him, male pipefish invest more energy than females in each
zygote. Additionally, they invest more energy per unit time than females throughout each breeding season. As a result, some males may consume their embryos rather than continuing to rear them under situations to regain energy in which their bodies are exhausted of resources. Pregnant male pipefish can absorb nutrients from their broods, in a manner very similar to
filial cannibalism found in many other families of fish. The smallest eggs in a brood of various egg sizes usually have lower survival rates than larger ones due to the larger eggs having a longer-lasting food source (absent contributions from the father), hence they more likely to develop into mature adults. In other instances, some pipefishes may consume the embryos of mates that seem less fit or desirable, as each male generally copulates with more than one female. Polyandrous species are also more likely to have females with complex sexual signals such as ornaments. For example, the polyandrous Gulf pipefish (
Syngnathus scovelli) displays considerable sexual dimorphic characteristics such as larger ornament area and number, and body size. ==Genera==