Re:seahorse fry (reidi?)

Pete Giwojna

Dear Karen:

Thanks for sharing your experience in breeding the Brazilian seahorse (Hippocampus reidi) with us on the forum!

I’m sorry to hear that you have had such limited success with the newborns thus far, Karen, but that is often the case with the challenging H. reidi fry. The good thing about this species is that, once you obtain a mated pair, the Brazilian breeding machine will continue to churn out brood after brood with clockwork regularity, providing the aquarist with plenty of opportunities to experiment and improve at rearing the young. The bad thing about H. reidi, of course, is that they produce such large numbers of tiny fry in each brood, and that the undersized newborns must then undergo a prolonged pelagic phase of development, which is a very high risk for the fry. That is why H. reidi is considered to be one of the most difficult species to raise, Karen, and the number one cause of mortality is the air bubbles that plague the unfortunate "floaters" when they ingest air while feeding at the surface.

I like the idea of trying a larger, deeper, 20-gallon aquarium for your nursery tank, and using a spray bar return to provide the kriesel effect and to help keep the newborns away from the surface. What you might want to concentrate on now is to produce just the right rate of circular flow and the proper degree of turbidity in the nursery tank, Karen. Jorge Gomezjurado finds that the optimal water flow for rearing reidi fry is 10 mm/sec, and he notes that feeding decreases at lower or higher flow rates (Bull and Mitchell, 2002, p51).

Jorge also advises that turbidity is an important factor in the fry’s rearing environment. He uses a technique similar to greenwater nurseries to maintain the proper level of turbidity by adding algae (Nannochloropsis and Isochrysis) at a concentration of about 150,000 cells per ml (Bull and Mitchell, 2002, p51). However, that’s difficult for the home aquarist to duplicate since it requires maintaining "green water" phytoplankton cultures, so you might consider using methylene blue to darken the aquarium instead. Just add enough of the methylene blue, carefully proceeding drop by drop, to turn the nursery tank a nice shade of blue. You don’t want to produce a dark blue that would make it difficult for the newborns to see and target their tiny prey; rather, the idea is to achieve a blue tint that is just dark enough to help keep the phototactic newborns away from the surface without interfering with their vision (or with the vision of the aquarist, for that matter). You need the water to be clear enough to be able to easily see the newborns and the amount of nauplii in the water so that you can maintain the optimal feeding density, but blue enough to discourage the fry from gathering at the surface in one spot.

As an added benefit, the methylene blue will also help prevent fungal problems and discourage protozoan parasites in the nursery tank.

Also, Karen, it will be helpful if you provide your broodstock with the best possible nutrition. Keeping the parents in tiptop condition will result in newborns that are larger than normal and which enjoy improved survivorship as a result: Adult H. reidi at the National Aquarium in Baltimore receive a staple diet of frozen mysid shrimp (Mysis relicta) coated with essential vitamins and amino acids, Astaxanthin Natu-Rose, and Canthaxanthin (Bull and Mitchell, 2002, p51). They receive 3 feedings of day of the enriched frozen Mysis relicta, and this highly nutritious diet is one of the keys to their successful breeding and rearing program for H. reidi (Bull and Mitchell, 2002, p51). Jorge Gomezjurado finds that providing his broodstock with a well rounded, nutritious diet increases the size of the fry they produce (Bull and Mitchell, 2002, p51). Consequently, his reidi fry are closer to 10mm in length than the usual 6-7mm (Bull and Mitchell, 2002, p51). The larger reidi are able to feed more efficiently and can ingest larger prey items, including Artemia franciscana Instar I nauplii, thus giving them a considerable advantage over the smaller fry (Bull and Mitchell, 2002, p51).

If you can manage it, providing your newborn H. reidi with rotifers or larval copepods for the first week of their life, is another factor that may help improve survivorship.

I found your account of how to of the larger male seahorses in the same tank have learned to prey on the newborn H. reidi fry as they are delivered to be particularly fascinating, Karen.

Cannibalism in seahorses is very uncommon. It does occasionally occur among seahorses with pelagic fry, but such episodes are normally rare exceptions. Hippocampus erectus is not typically known for this, but it does take place in very isolated incidents. I have never personally witnessed a case of cannibalism in H. erectus. Hippocampus erectus fry undergo an abbreviated pelagic phase and ever once in a while there are reports of cannibalism, but 99.99 % of the time erectus make model parents. And the same thing is generally true for Hippocampus reidi parents.

However, some species are more prone to this aberrant behavior than others. For example, unlike most seahorses, captive-bred Hippocampus abdominalis are confirmed cannibals under certain circumstances. This unusual behavior does not appear to be a consequence of dietary deficiencies or overcrowding. Rather, it seems to be due to the simple fact that cultured Pots are accustomed to being fed; rather than hunting for live prey along the bottom, they expect their prey to be introduced from the surface and come down to them like manna from heaven. They are thus conditioned to take anything above them in the water column within the right size range that drifts past, and unfortunately this includes surface-hugging pelagic fry.

As a case in point, David Warland experienced cannibalism of fry by young Potbelly adults 18-24 months old maintained at a stocking density of 30 seahorses per 800 liters (211 gallons). Since that’s one seahorse for every 26.6 liters (7 gallons), he does not believe over stocking was a factor in the cannibalism, nor did hunger play a role since the H. abdominalis were fed to capacity 4-5 times daily during daylight hours (sunup to sundown).

It was also interesting that it was the biggest males that began predating the newborns in your case, Karen. Surprisingly, when cannibalism does occur, it is often the female that’s the culprit. That is the case with H. comes, one of the few seahorses species that is known to cannibalize its young on occasion (Neil Garrick-Maidment, pers. comm.). When this happens with Tigertails, it is typically the accompanying female who begins predating the fry as her mate gives birth (Lesley Holland, pers. comm.). The male may then follow suit as well and the entire brood can quickly be destroyed (Lesley Holland, pers. comm.). Once a pair of Tigertails turns cannibal, this unnatural behavior is likely to become habitual, putting their future offspring at risk.

Cannibalism is really quite rare in seahorses, so the hobbyist need not be overly concerned by this possibility, although there are a few species such as H. abdominalis and H. comes that seem to be a bit more predisposed to this bizarre behavior. Seahorse keepers, and Potbelly and Tigertail breeders in particular, should simply be aware that cannibalism could occur and be on guard lest it becomes a problem.

On those rare occasions when cannibalism does become an issue with Hippocampus, it is appropriate to transfer the gravid male to a paternity tank for the safety of the newborns, Karen, so that was good thinking on your behalf. And, should you transfer the male to the nursery tank to deliver his young in safety, it is always best to transfer his mate along with him so that the two are never separated for any significant period of time. Very often, a stallion will remate scant hours after delivering his latest brood, so you do not want to interfere with their breeding cycle by separating the male and female. So it sounds like you handled the situation very well, Karen.

Here’s hoping that your diligence is soon rewarded with increasing survivorship among the young, Karen!

Happy Trails!
Pete Giwojna

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