Re:Stocking Density

Pete Giwojna

Dear Nigel:

The suggested stocking density for dwarf seahorses (Hippocampus zosterae) is two pairs per 1 gallon (4 L) of water, which is actually a very conservative figure, so a well-filtered 5-gallon aquarium could easily support up to 10 pairs or 20 individual adults.

I would say 4 dwarf seahorses is too few of these pigmy ponies to make a good herd for a five-gallon aquarium. I would suggest perhaps doubling that number. A colony of 8-10 dwarfs would be a good number to start out with in an aquarium of that size. A group that size would encourage breeding yet still allow plenty of room for your herd to grow, so you could raise the newborns right alongside the adults if you wish.

Unlike most other seahorses, H. zosterae is a colonial seahorse that lives in small groups rather than as isolated pairs or individuals. These miniature marvels are accustomed to being surrounded by others of their kind.

As a result, dwarves or Pixies breed best in large groups and are the most sociable of all the seahorses. What makes it extra fun is that these pint-size ponies are as prolific as they are promiscuous. Any time you have an adequate number of H. zosterae together — say several pairs — and conditions are to their liking, mating is a foregone conclusion. Once your dwarf seahorse herd includes 10-12 adults, you can be sure that one or more of the males will be pregnant during the breeding season at all times.

Heck, anytime you order several pairs of dwarves during the months of May to August, the height of their breeding season, you’re virtually guaranteed that some of the males will be pregnant when they arrive (Abbott 2003). In that case, expect your first dwarf babies to be born in the shipping bags en route or while you’re acclimating your new additions or immediately after you introduce them to the aquarium (Abbott 2003). Or all of the above. Happens all the time!

Far from inhibiting courtship, crowding seems to stimulate breeding in dwarf seahorses, almost as if they reach "critical mass" at a certain population density, triggering a chain reaction of mating attempts. Thus, provided water quality can be maintained, "the more the merrier" appears to be the rule with this species.

For instance, pet dealers must occasionally crowd large numbers of fish together in cramped quarters due to a lack of space, including dwarf seahorses. Robert Straughan was once forced to keep 300 H. zosterae in a 10-gallon tank in such a situation back in the old days, and was pleasantly surprised to find that over 100 of them managed to pair off and breed nonetheless. He reported that at any given moment, dozens of dwarves were actively engaged in courtship, so it was a common sight to witness several couples rising simultaneously to exchange eggs, and that one or more of the gravid males would be delivering young virtually around the clock (Straughan, pers. comm.)!

For all practical purposes, of course, it is always better to understock your aquarium than the push the envelope and test the limits of its carrying capacity. Straughan was dealing with an exceptional situation, but those unusual circumstances do provide a good illustration of a colonial nature of these pigmy ponies.

Best of luck with your dwarf seahorses, Nigel!

Happy Trails!
Pete Giwojna

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