Re:Are harlequin shrimps safe?

Dear Judy:

Harlequin shrimp belong to a group of colorful, very fancy shrimp in the genus Hymenocera whose diet consists solely of starfish. They are commonly known as Elegant shrimp, Harlequin shrimp, Painted shrimp, Clown shrimp, and Dancing shrimp and are quite beautiful, having pinkish or purplish polka dots and saddles against a white background. They are 1-2 inches long with large, flattened claws that resemble flower petals, which they use to display to one another. To me, they always look like animated orchids. They are shy, gentle, and completely harmless (except to starfish) so they should certainly do well with seahorses. They occur as mated pairs and are one of my favorite shrimp.

Two species are available — Hymenocera elegans, which tends to have purplish or violet polka dots, and Hymenocera picta, which tends to have pinkish or pink-and-tan polka dots. I have kept both species and found them fascinating. They are rather pricey for shrimp but well worth the cost as long as you can provide them with adequate water quality (low nitrates and no copper!).

I have kept Harlequin shrimp in seahorse tanks with Hippocampus erectus and H. reidi in the past and never had any trouble whatsoever. But these are larger seahorse species and my stallions were 5-8 inches in length, so it’s possible that the small size of your two-inch seahorses makes a difference. Because of the specialized diet of the Harlequin shrimp, I wouldn’t imagine that the aggressive Harlequin shrimp is taking a culinary interest in your ponies, but it’s possible the Harlequin shrimp haven’t eaten for a long time and could be desperate (they often go unfed in pet stores and holding tanks because all they eat is live starfish).

I would suggest obtaining a cheap starfish and offering it to the Harlequins as soon as possible. I always fed my Harlequins with the common, ordinary, nondescript sea stars from my LFS that were always inexpensive and readily available. They are just the common brownish to tannish or orangeish seastars that come in to any pet store that keeps marine fish in shipments from Florida or the Caribbean or wherever, often as unwanted add-ins or throw ins. Look for a regular starfish of average size, say with an arm span 3-4 inches across, and they should fill the bill nicely. (Avoid the serpent starfish or brittle stars — you want a common sea star with tubefeet.) I suspect that if they have a starfish — their natural prey — to feed on, your Harlequins will quickly lose all interest in your seahorses.

However, having said that, I would keep a close eye on the Harlequin shrimp and be prepared to relocate either the shrimp or the seahorses if necessary, Judy. When discussing compatible tankmates for seahorses, it’s important to remember that one can only speak in generalities. There are no unbreakable rules, no sure things, no absolute guarantees. For instance, most hobbyists will tell you that small scooter blennies make great tankmates for seahorses and 9 times out of 10 they’re right. But every once in a while, you will hear horror stories from hobbyists about how their scooter blenny coexisted peacefully with their seahorses for several months and then suddenly went "rouge" overnight for no apparent reason and turned on the seahorses, inflicting serious damage before it could be captured and removed.

Does that mean that we should cross scooter blennies off our list of compatible tankmates for seahorses? Nope — it just means that we must be aware that individuals within a species sometimes vary in their behavior and respond differently than you would expect, so there are exceptions to every rule. It’s fair to say that scooter blennies generally make wonderful companions for seahorses, but there’s always a small chance you might get Satan reincarnated in the form of a scooter blenny. There’s no guarantee that adorable scooter you picked out at your LFS because of his amusing antics and puppy-dog personality won’t turn out to be the blenny from hell once you release him in your seahorse setup.

Likewise, micro-hermit crabs are generally entertaining additions to an aquarium that do a great job as scavengers and get along great with seahorses, but over the years, I’ve had a few seahorses that were confirmed crab killers. These particular ponies were persistent hermit crab predators that specialized in plucking the hermits out of their shells and attacking their soft, unprotected abdomens, and they honed their skullduggery to a fine art. They were experts at extricating the crabs and would eat only their fleshy abdomens and discard the rest. Mind you, that was only a few individuals out of a great many Hippocampines, but I could never keep hermit crabs in the same tank with those specific seahorses.

It’s the smallest hermit crabs that are at greatest risk, of course, but this behavior sometimes becomes habitual. So if my experience is any guide, crab killing could become a bad habit for the seahorse that is doing the stalking and you’ll have to watch that particular pony around hermit crabs from now on. Once they have discovered how to go about it, a seahorse may develop a taste for hermit hinders and consider them to be a regular part of its menu henceforth.

On the other hand, sometimes it’s the micro-hermits that are the troublemakers. Most of the time, they coexist perfectly well with their fellow janitors in the cleanup crew. But I’ve had more than a few tiny hermits with a taste for escargot that persecuted snails mercilessly. These cold-blooded little assassins would kill the snails in order to appropriate their shells. Once they had dined on the former occupant, they would take up residence in their victim’s cleaned-out shell! It soon became clear that these killer crabs were driven not by hunger, but by the need for a new domicile. Once I realized they were house-hunting, I found I could curb their depredations but providing an assortment of small, empty seashells for the hermits to use. Colorful Nerite shells are ideal for this.

The same thing can sometimes happen with the decorative shrimp we introduce to our seahorse tanks. Most of the time they do great with seahorses, but sometimes the cleaner shrimp cause problems at feeding time and, on other occasions, the seahorses may give the colorful shrimp a hard time.

For instance, Peppermint Shrimp (Lysmata wurdemanni) and/or Scarlet Cleaner Shrimp or Skunk Cleaner Shrimp (Lysmata amboinensis) are popular additions to seahorse tanks to augment the cleanup crew and add a touch of color and activity to the tank. Peppermint shrimp (Lysmata wurdemanni) are a favorite with seahorse keepers because they eat Aiptasia rock anemones, and both the peppermints and Scarlet cleaner shrimp (Lysmata amboinensis) will perform another useful service by grooming the seahorses and cleaning them of ectoparasites. As an added bonus, they reproduce regularly in the aquarium, producing swarms of larval nauplii that the seahorses love to eat.

Yet once established in the aquarium, those beautiful red shrimp species are much more active feeders than seahorses. They’ll come flying across the tank the moment that enticing scent of frozen mysids hits the water, raiding the feeding station and snatching Mysis right out of the ‘horse’s snouts. Does that mean they’re incompatible with seahorses? Heck no, you just shoo the pesky shrimp out of the way at dinnertime and target feed the seahorses, making sure each of them gets its fill.

Other seahorse keepers caution against cleaner shrimp not because the shrimp could outcompete the ponies for frozen Mysis, but because their seahorses don’t discriminate between feeder shrimp and decorative shrimp, and may be inclined to add the expensive cleaner shrimp to their dinner menu. When introducing decorative shrimp in a seahorse setup, it is important to select good-sized cleaner shrimp for this very reason. Live shrimp is the favorite food of all seahorses and, up to a certain point, they will not hesitate to attack shrimp that are too large to be eaten in one bite.

This often happens when feeding seahorses live ghost shrimp or grass shrimp, many of which are too big to be eaten intact. Seahorses are so fond of these shrimp that they often attack specimens that are far too big to swallow. In that case, they will attempt to break the back of the shrimp by snapping repeatedly at the carapace. If they are successful in severing the abdomen from the cephalothorax, the seahorses will then slurp up the tail section and head half of the shrimp separately. Or should their victim be so large it cannot even be swallowed in sections, they will snick out mouthfuls of the soft tissue exposed inside the abdomen or thorax.

At times, several seahorses will gang up on one big shrimp this way, like a pack of lions teaming up to bring down a water buffalo that’s too big for any one of them to tackle alone. Under the right circumstances, a sort of slow-motion feeding frenzy may then ensue, with the seahorses playing tug-of-war over the pieces of their prize.

Grass shrimp that are too large to be overcome by such tactics may survive to become long-term residents of the seahorse tank, coexisting with their reluctant tankmates in a sort of uneasy truce. Such die-hard shrimp provide a useful service as scavengers from then on.

We tend to think of our seahorses as gentle, nonaggressive creatures that wouldn’t harm a fly, but in reality they are surprisingly fierce predators in their own right. To small crustaceans, seahorses are the tigers of the grassblade jungle, striking without warning from ambush and devouring anything of the right size that moves.

When introduced to a seahorse setup, small cleaner shrimp face the same risks as large ghost shrimp and grass shrimp (a hungry ‘horse doesn’t distinguish between decorative shrimp that are intended as tankmates and eating shrimp that are intended as dinner). It is therefore important to select the largest possible cleaner shrimp for your seahorse tank(s). Seahorses will actively hunt small cleaner shrimp and they are quite capable of killing shrimp that are far too big to swallow whole, so the cleaners need to be large enough that they are not regarded as potential prey.

Just remember that crabs and shrimp are natural prey items that are on the menu of all large seahorses. Kealan Doyle conducted a study on seahorses in the wild in Portugal in which he did a stomach analysis of wild caught individuals, and was quite astonished to see parts of quite large crabs and shrimp in their stomach contents (Neil Garrick-Maidment, pers. com.). Of course, it’s good that Ocean Rider seahorses are such aggressive feeders, but it is inconvenient at times when they take a culinary interest in one’s hermit crab sanitation engineers.

Arrow crabs are another good example of the type specimens for which you might find conflicting advice with regard to their suitability as suitable tankmates for seahorses. I would characterize arrow crabs (Stenorhynchus seticornis) as opportunistic omnivores. I have kept them in a number of my aquaria over the years, including a few seahorse tanks, without any problems. They never bothered my Hippocampus erectus at all, but they can be hard on sessile invertebrates in general and I certainly wouldn’t trust them with dwarf seahorses. Nor would I trust them with a small goby.

I kept a couple of large arrow crabs in my Monster Bin with a 14-inch African lionfish (Pterois volitans) and a couple of overgrown ribbon eels, and the arrow crabs proved to be fairly proficient at capturing the live minnows I fed to the lionfish, particularly after the minnows had been weakened by the saltwater. If the opportunity presents itself, they are quite capable of capturing small bottom-dwelling fishes like your goby.

Arrow crabs will happily devour any bristleworms they can catch but they won’t eradicate them from your aquarium. Too many of the bristleworms always remain inaccessible to them within the rockwork and sand for that, but a small to medium-sized arrow crab or two can help control the bristleworm population. A fairly effective way to reduce their numbers is to regularly trap large bristleworms after lights out along with keeping a young arrow crab to thin out smaller worms (providing there are no sessile invertebrates in the tank the crabs could harm).

In my experience, small to medium-sized arrow crabs are safe with large seahorses and can be used to help limit the number of bristleworms in your tank. But if you want to try this, you don’t want to pick out the biggest, baddest, bruiser of an arrow crab to do the job! Go with a smaller specimen, keep a close eye on it, and be prepared to replace it with a smaller individual after it molts once or twice. They grow fast and can nearly double in size after each molt.

Remember there are always exceptions to every rule, and large crustaceans are never completely trustworthy. Even the most harmless and seemingly inoffensive crabs can cause trouble under certain circumstances. For example, not long ago I heard from a hobbyist that had been keeping a decorator crab in his seahorse tank. All went well at first and there were no problems of any kind for months until, for no apparent reason, the crab suddenly began to quite deliberately amputate portions of the seahorses’ tails. It was not attacking the seahorses as prey or attempting to eat its mutilated victims, it was merely methodically harvesting portions of their anatomy with which to adorn itself! It was simply doing what all decorator crabs do — snipping off and gathering bits and pieces of its immediate environment to attach to itself as a form of natural camouflage. It just goes to show, with crustaceans you can never be entirely sure how things are going to work out…

In short, Judy, try feeding your Harlequin shrimp a starfish and see if that solves the problem. If not, if one or both of the Harlequin shrimp continues to act aggressively toward the seahorses, then you should relocate either the shrimp or the ponies to be on the safe side.

Best wishes with all your fishes, Judy!

Respectfully,
Pete Giwojna