Pete Giwojna

Dear Chris:

Regarding your compatibility question, I would be inclined to remove the black spiny urchin to be on the safe side. Many hobbyists caution against keeping the long-spine urchins (Diadema species) because of the possibility that the seahorses may accidentally impale themselves on their sharp spines. I suspect that risk is overstated, however, and is more of an imagined danger than an actual threat. For example, seahorses in the wild are known to associate with urchins and seem to regard them as part of the substrate, commonly even using them as hitching posts.

In fact, Tom Bowling has observed all ages of Hippocampus tristis on sea urchins, from newly settled juveniles to fully-grown adults, an unusual form of commensalism never before reported for seahorses (Bowling, pers. comm.). He believes they remain with the urchins throughout their lifetime (Tom Bowling, pers. comm.). Bowling reports the seahorses actually perch amidst the spines and ride around on the sea urchins, feeding on schools of larval shrimp and other small crustaceans. In their deep-water environment, the urchins apparently provide them with shelter and a microhabitat that attracts small crustaceans and tiny fish, creating a wide range of feeding options for the seahorses (Tom Bowling, pers. comm.).

So I wouldn’t worry too much about a seahorse running afoul of the prickly spines on a spiky seahorse, but I am concerned that the urchin could do damage to the sedentary seahorses as a predator under the right circumstances. Many times such urchins do well as long as you can meet their dietary requirements, which means a tank with lots of algae for them to dine upon. But they go through the available algae surprisingly quickly and then begin to look for alternate food sources. Urchins can and will become opportunistic predators in such a situation, Chris.

Pencil sea urchins and club sea urchins are the most problematic in that regard when it comes to seahorses. They are much more carnivorous than the other urchins, which tend to be primarily herbivorous. Pencil urchins will feed on sessile animal life, particularly once they’ve depleted all will be available algae, and I once saw a pencil urchin capture and consume a small horseshoe crab in an aquarium. It’s quite conceivable that they could also pin down the tail of a seahorse and do serious damage with their bony mouthparts.

Even the spiny urchins can become predatory toward seahorses when the opportunity presents itself, Chris. For example, this is what Monique (one of the members of this forum) reports in that regard:

<Open quote>
Also.. here is a horrific story for you.. and for anyone with these urchins.

I had a long spiney urchin and a crab that just came in on one of my live rocks that had red eyes and 2 white racing stripes down the center of his back.. kinda creepy looking actually.. but I was told the urchin was harmless.

however. I bought another seahorse for a companion for my girl when she healed (as i think they dont like to be alone) and when I woke up in the morning I went to the tank and noticed the crab had snatched the tail of the new seahorse where she was hitched on a rock and was pulling it in the hole! and the long spine urchin scurried over to her and started eating her back!!!!!!!!!!!!!!!!!!!!!!! I freaked out! that poor thing was looking at me like.. HELP ME!!… so I pulled the spiney off her and tried getting her tail out of the rock without hurting her and couldnt! the crab had ahold of her tight! so as i went to get a knife to wedge in there.. the stupid spiney tried getting her again! I finally got her free.. and unfortunately she passed over night 🙁 I was devastated.. I cried all morning.

Needless to say i got rid of the urchin which i assumed was also eating on my coral cuz there was nothing else to pick on them.. AND could not get the crab out so I just took the whole rock in too…. I do not like aggressive animals and they were as I was told they werent. so people beware.

<close quote>

In short, Chris, I would play it safe and relocate the black spiky urchin just in case.

When it comes to mixing different species in the same tank, not all different kinds of seahorses are compatible with each other, Chris. For example, you cannot mix temperate (cool water) seahorses with tropical (warm water) seahorses for obvious reasons. It is never a good idea to attempt to keep warm-water and cool-water species together. That unfortunate experiment has been tried many a time and has always proven to be disastrous for one or the other in the long run. Ideally it is always best to have separate tanks for tropical, subtropical, and temperate species. So you can’t consider keeping temperate seahorses like Ocean Rider Brumbry seahorses (Hippocampus abdominalis), often known as big belly seahorses or potbelly seahorses or simply Pots for short, with tropical seahorses such as H. reidi, H. barbouri, or H. kuda.

Nor is it a good idea to keep miniature species, such as Pixies or dwarf seahorses (Hippocampus zosterae) together with larger seahorses, such as Mustangs or Sunbursts (H. erectus), due to their incompatible feeding requirements.

However, there are a number of large, tropical seahorses that have very similar aquarium requirements and which therefore make good tankmates for one another. For instance, H. erectus, H. reidi, H. barbouri, H. comes, or H. kuda seahorses can all be kept together in an aquarium with a stable water temperature of 75°F. But, for best results, when you are considering keeping seahorses of different species together in the aquarium, it’s best to limit yourself to specimens provided by the same breeder or aquaculture facility.

Among the Ocean Rider seahorses, this means you could safely keep Mustangs, Sunbursts, Pintos, Fire Reds, Barbs, and Brazileros together in a tropical aquarium with the temperature of 75°, providing the aquarium is large enough to safely accommodate all of the seahorses you are interested in, Chris. The suggested stocking density for such seahorses is typically one pair per 10 gallons of aquarium water, meaning that a 40 gallon aquarium, for example, could hold up to 4 pairs or 8 individuals when it was fully stocked.

Best of luck with your new seahorse tank, Chris!

Happy Trails!
Pete Giwojna

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