Source: Western Fisheries, Autumn 1998, 39–40

There are approximately thirty known species of seahorses, nearly all of which are included in the genus Hippocampus. During 1989, I carried out an investigation of the reproduction and spawning of the Western Australian seahorses (Hippocampus angustus) in captivity.

What follows is a summary of my research and the methods used to successfully raise a number of these fish.

During the course of the study, I found that adult seahorses would not only accept live food. For convenience, the seahorses were fed live ‘enriched’ adult artemia.

Artemia are tiny crustaceans that live in hyper-saline bodies of water and can be bought in cyst form from the USA. Artemia can be treated like a vitamin pill, by feeding them on freeze-dried algae, such as spirulina, or hufa enriched diets, before passing them on as a bio-encapsulated food to seahorses.

Contrary to previous reports, our adult seahorses would not accept adult artemia unless they were 2.5 mm or greater in length. They also showed no interest in small fish or in cannibalising juvenile seahorses.

Mating

One of the most amazing things about seahorses is that the males give birth, instead of the female —what is termed ‘paternal brooding’. Instead of the female’s eggs being fertilised inside her, these are deposited into a special abdominal pouch in the male and fertilised.

Although this fact has been widely reported, first hand information on the breeding of seahorses in captivity is limited. What I discovered was that a number of aspects of reproduction in the Western Australian seahorse differ significantly from those reported in other species of seahorses.

Before courtship, the male seahorse prepares his pouch for the receipt of eggs. The opening of the pouch that usually appears as a vertical 5 mm slit, is dilated to produce a round hole about twice this size.

Through contraction and extension of the upper torso, the male stretches the dark fold of skin over the abdomen into a comparatively large opaque sac-like pouch. The previously small slit develops into a 10 mm diameter opening that is almost horizontal.

Early courtship involves repeated passes by the male who undergoes rapid colour changes whenever he is close to his mate.

Male Western Australian seahorses repeat courtship behaviour from October onwards, expanding their pouch every three to four weeks to receive eggs. If a male is unsuccessful in obtaining eggs, his pouch recedes and is then prepared again, over a similar time span.

The egg transfer takes place with the male intertwining tails with the female, positioning himself face-to-face with his mate but slightly lower, so his pouch opening is directly below her oviduct.

In contrast to several reports I have read about seahorse breeding, the female West Australian seahorse does not possess an intromittent organ, termed an ovipositor by these authors, for egg transfer.

The female, positioned above the male’s inflated pouch, squirts a string of eggs through the water into it. An egg transfer takes approximately 30–60 seconds and may be repeated from one to three times. The eggs are soft and oval in shape, with an average diameter of 0.65 mm.

Immediately after transfer is complete, the pair separate and the male’s brood pouch deflates, compressing the eggs to his body. The newly pregnant male’s pouch then changes from a light opaque colour to dark brown, irrespective of his overall body coloration, and its opening reverts to a vertical slit in the ventral wall.

The entire process from courtship to separation takes around 85 minutes.

The pregnant male’s pouch swells in proportion to the number of young developing within. A male carrying one to two young is barely discernible from one without juveniles, except that the pouch is darker in colour.

However, a male carrying many young can be easily distinguished by a swollen pouch; immediately before birth a male pouch with 600-or-so young inside will strongly resemble a table tennis ball! The number of baby West Australian seahorses born from one mating ranged from one to 623.

Note that ideal conditions for mating are stable warm water (temperature ranging from 21–23°C), good daylight, adequate nutrition and ‘attachment sites’ for seahorses to hold onto (in my case, simulated by the use of artificial weed). Appropriate light levels and water temperature appear to be the triggers necessary for breeding in H. angustus.

Birth

Just before birth, the male shows signs of distress and respiration increases to 72 beats per minute. Juveniles are usually born between midnight and dawn, arriving all at once or in multiple batches 24 hours apart.

After giving birth, the male flushes out his pouch, which returns to normal proportions over a two- to three-day period. Fourteen days after flushing, the male can be observed courting his mate and expanding his pouch in preparation for receiving eggs again.

At birth, juvenile H. angustus average 10 mm in length and are completely independent of parental care. Immediately after birth, they head directly to the surface and within an aquarium environment tend to mass together, becoming interlocked if not dispersed by currents.

Initially, juveniles are semi-transparent, but over the 14 days following birth, the majority change in colour from an opaque pale brown-yellow to white or black. They are attracted to light for the first 14–21 days, much of this time being spent skimming the surface, apparently searching for food.

As juvenile West Australian seahorses have a well-developed yolk sac and after birth will eat ‘instar I’ artemia nauplii (juvenile artemia, less than 24 hours old), they are relatively easy to feed. Juvenile H. angustus possess a small tubular snout 0.8 mm in diameter and consume their prey whole, like adults.

Consequently, prey size is very important, as the seahorse juveniles will starve if offered food too large for them. For example, juvenile seahorses can consume instar I nauplii on the first day after birth, but are unable to cope with instar II nauplii (juvenile artemia, less than 48 hours old) until after seven to ten days.

This means that hatching and harvesting of artemia nauplii must be closely monitored to ensure the desired prey size.

A number of methods of rearing juveniles were trialed, the most successful being recirculating tank culture and green water culture.

Great care must be paid to adjusting flow rates so that aggregations of juveniles are dispersed. In recirculating systems, reverse flow filters are preferable, as it is not uncommon for juveniles to rest on the bottom.

Reverse flow systems take water from the top of the tank and push it up through the gravel at the bottom, providing the juveniles with a cushion of water. In contrast, conventional flow filters push water down from the top of the tank, potentially forcing delicate juveniles into the gravel bottom.

Unlike adults, juveniles do not use their tails to anchor themselves until they are 14–17 days old.

The major cause of mortality in juveniles is due to poor swim bladder inflation, owing to the oily layer that commonly forms at the top of ponds and tanks preventing access by juveniles to the surface air vital for initial inflation. This situation may be rectified by using a protein skimmer, which removes the oily layer.

Growth rates of juvenile seahorses ranged from 2–5 mm per week, with an average rate of 2.32 mm per week.