I am pleased to announce that this issue (Volume 2: Issue 1) I have the cover image and a feature article 'The Sex Life of Seahorses' in 72 & Rising Magazine, which is a magazine focusing on marine and freshwater environments and their conservation. 

Check out the article here:

The Sex Life of Seahorses by Richard Smith (© Richard Smith 2012)

72 & Rising Magazine Cover Image

It's easy to list any number of unusual features about seahorses: a horse-like head ending in a puckered mouth, a monkey-like prehensile tail and eyes that move about independently.  Even stranger than all of this is their reproduction where extraordinarily devoted fathers and strict monogamy are unique in the animal kingdom.

The most well-known and fascinating aspect of seahorse reproduction is male pregnancy.  Male seahorses aren't the only fish that put a great deal of effort into raising their young, but they are the only ones that become truly pregnant and even get stretch marks.  Whilst male cardinalfish mouth brood and some male frogs carry tadpoles around, the male seahorse nurtures his young inside a fully enclosed pouch full of blood vessels, which bring nourishment and oxygen, whilst carrying away waste products.  Paternal care is found across the entire family Syngnathidae, whose members include pipefishes, pipehorses and of course seahorses.  Less advanced forms, such as the pipefishes, merely adhere eggs to a patch of skin on the belly whilst the more highly evolved pipehorses, as their name suggests, are an intermediate form and have a semi-enclosed brood area.

Over time the role of the brood pouch has grown beyond the protection of young and has driven the evolution of mating systems in the family.  The brood surface on the belly of pipefishes is open to the environment but also potential mates, meaning that males are able to accept eggs from several females at a time.  This has driven the females of some species to compete for the attentions of males and in a surprise twist it is the females rather than males that develop sexual ornamentation, much like the peacock's tail but in reverse.  This situation contrasts with seahorses, where the male's brood pouch is sealed immediately following the transfer of eggs.  In this case, a male can only accept eggs from one female for the entire gestation period.  Females on the other hand are prevented from mating with other males by the scarcity of mates and the cost of egg production.  The result is ubiquitous monogamy in seahorses for the duration of at least each distinct brood.  The advent of DNA testing highlighted the exceptionality of this as virtually all other species previously believed to be monogamous, including many birds and even humans, were actually discovered to have produced young by multiple partners.

The direct transfer of eggs into the male's brood pouch also affords him a certain confidence in his fatherly duties.  Unlike other animals where promiscuity is rife, the male seahorse can be one hundred per cent sure that each of the offspring he carries is his own, with no risk of cuckoldry.  This explains the extreme lengths that male seahorses are willing to go to in raising their young.  It is certainly in his best interests to raise as many young as possible, in order for his genes to be passed on to the next generation.

The reproductive cycle of a seahorse begins with locating a mate, which can be easier said than done as they live at very low densities.  If there is opportunity to be picky, seahorses will preferentially chose a partner of a similar size.  This means that neither brood pouch space nor eggs will be wasted through a partner being too large or small for the clutch of eggs.  Once an unpartnered male and female have found each other, the pair begin to dance.  Courtship rituals are very important for the bonding pair as it allows them to synchronise their reproductive cycles.  Pair bonding provides the female with the assurance needed to hydrate a clutch of eggs, a necessary step before they are ready to be transferred to her mate.  Egg hydration is an irreversible process once it has begun, so it is important for the female to be confident that her partner is willing and able to accept them.  If she does not transfer the clutch at the end of the few days it takes to hydrate them, they can go bad and damage her reproductive organs and she must expel them, which wastes precious energy reserves.  Synchronisation is beneficial as it increases the pair's overall productivity, making monogamy a more stable option and switching mates between broods more inefficient.

After egg hydration is complete, the pair carry out an extended ritualised courtship involving colour changes and specific body movements, which culminate in the pair raising up together into the water column.  A heart shape is formed from the two seahorse's bodies as the two fish grasp each other's tails and their snouts meet in the middle whilst eggs are passed between them.  The eggs are transferred directly into the pouch, which is immediately sealed to prevent the intrusion of harmful seawater.  The male's pregnancy then lasts between ten days and six weeks, depending on species and water temperature.  Daily courtship continues throughout the gestation, allowing the female to gauge when her mate will give birth and to allow her to hydrate eggs accordingly, thus maximising the number of young they are able to produce.  This synchronisation is such that I have seen pairs remate within thirty minutes of the male giving birth!

The birth itself is slightly less traumatic than our own but the male is clearly exerted by the contractions that expel the babies.  The number of young produced varies between half a dozen for the smallest species and over a thousand for the largest seahorse, the pot-bellied seahorse, which reaches 30 cm in length.  Once they are born, the tiny seahorses, which are miniature fully formed copies of their parents, exploit ocean currents in a planktonic phase to be swept away to settle in a new area.  The juvenile seahorses are then ready to find a partner of their own and begin the cycle again.

Seahorses have fascinated humans since the Ancient Greeks but only in the last few decades have the true mysteries of these enigmatic creatures been revealed.  Male pregnancy and strict monogamy evolved over millions of years; however, loss of habitat and the Traditional Chinese Medicine trade pose significant threats to seahorse populations around the world.

PYGMY SEAHORSES are a fascinating group of miniature seahorses, measuring 1.4 - 2.7 cm as adults, depending on the species.  Six of the seven described species were described in the first decade of the 21st Century and
as a result very little is known about them. Although they belong to the genus Hippocampus, along with all other seahorses, they have several adaptations for their small size.  Among these unique morphological attributes is the brooding of young within a pouch located inside the body cavity of the diminutive fish, rather than a pouch located on the tail, as is the case in other seahorses.  This is thought to be an adaptation for their extremely small size as a tail pouch would hinder their movement.  Among other aspects of their biology and conservation, pygmy seahorse reproductive behaviour comprised part of my PhD studies.

 

 

 

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