hermit crabs

Dear Julie:

You’re very welcome to any and all of the information we can provide to help keep your seahorses happy and healthy! It sounds like you have been doing a fine job of handling your pet shop ponies without much guidance to go on, and I would be happy to help fill in the gaps in your knowledge when it comes to the care and keeping of seahorses.

With regard to your question about hermit crabs, no, there is no reason at all that you must include hermits in the cleanup crew for your seahorse tank. A fine assortment of snails will make an exemplary set of sanitation engineers for your aquarium, particularly if you include a number of nassarius snails, which will fill the role of the hermit crabs by cleaning up meatier leftovers, such as uneaten Mysis. So feel free to rid yourself of the pesky hermit crabs, Julie, and fill their place in the cleanup crew with a few more snails instead. A good snail assortment may include bumble bee snails, trocha snails, margaritas, Astrea and Cerith snails, red foot Moon snails, etc., but especially Nassarius snails.

Nassarius snails are terrific detritivores and amazingly active for snails. They’ll bury themselves until they detect the scent of something edible, and then erupt from the sand and charge out to clean it up. They do a good job with the meaty leftovers that often don’t interest other snails.

A varied assortment of snails is very desirable because different types of snails have different habits, seek out various microhabitats within the aquarium, and prefer to eat different things. Some are herbivores that feed on microalgae, and some of the herbivorous snails prefer to graze on it from the substrate, others like to to clean it from the rocks, and still others love to scrape algae off the aquarium glass. Furthermore, the different herbivorous snails tend to specialize on different types of microalgae and have definite preferences as to the types of algae they will eat, so it’s important to have a nice variety of snails that cover all the bases in that regard. It’s equally important to include some omnivorous snails in your assortment, which will go after meaty leftovers, along with the vegetarians. And you’ll want to have plenty of detritivores, too, which will feed on detritus and decaying organic matter in the aquarium

For best results, Astrea sp. snails should go in the tank as soon as the ammonia and nitrite levels are down to zero in order to keep nuisance algae from gaining a foothold in your tank. Introduced as soon as possible to a new aquarium, that has reached this cycling phase, Astrea snails effectively limit the development of all microalgae. In other words, they are good at eating diatoms, but will consume red slime and green algae as well.

But you must avoid predatory snails such as tulip snails, horse conchs, crown snails (Melanogena corona), and the venomous cone snails (Conus spp.), which can kill a human with a single sting from their harpoon like radula. Tulip snails, horse conchs, and crown conchs will hunt down and eat the other snails in your cleanup crew, whereas cone snails prey on small fishes in addition to presenting a deadly hazard to the aquarist.

So I would recommend replacing your hermit crabs with a few of new snails, including a couple of extra Nassarius, Julie.

I am surprised to hear that you have had trouble with your Scarlet Reef Hermit Crabs (Paguristes cadenati), however, because they are one of the micro-hermit crabs that normally do very well with seahorses. They stay small and are primarily herbivorous and their dining habits, performing a useful service in the aquarium because they include nuisance algae (e.g., slime algae and hair algae) in their diet. Reef keepers trust them with their delicate live corals and decorative clams, and I have had good success with them as sanitation engineers in my seahorse tanks. Will Wooten rates them as a 2 in his seahorse compatibility guide, meaning that they are safe to keep with seahorses, with the exception of the occasional "rowdy specimen."

I would agree with his assessment in that regard, but there are exceptions to every rule and your Scarlet reef hermits are not behaving like the perfect little gentleman that I ordinarily find them to be, so you certainly did the right thing by removing them from your seahorse tank, Julie. That’s just another example of why you can never completely trust even the most seemingly benign crustaceans with seahorses. Many times they do great at first when they are small and new to the aquarium, but become more aggressive once they have molted once or twice and grown in size (and attitude) accordingly.

This is what I normally advise hobbyists regarding compatible tankmates for their seahorses, Julie:

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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. I have seen a few cases in which individual hermits have developed a bad habit of harassing the seahorses. Like the rare seahorses that occasionally develop a taste for hermit hind-ends, there are microhermit crabs that on rare occasions will bother the ponies, often by latching onto their tails. This most often occurs when the seahorses and hermits are attracted to the feeding tray at mealtime and brought into close proximity, and it is usually just an isolated incident. Oftentimes this can be prevented simply by elevating the feeding station, which puts it out of reach of the pesky hermits. But just occasionally, a microhermit makes himself a serious persistent pest, and these unusually boisterous individuals should be weeded out of your cleanup crew immediately.

Likewise, most of the time, microhermit crabs 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. However, sometimes the bad behavior persists and the offending individual has to be relocated.

The same sort of 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 certain gobies.

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 crabs you can never be sure how things are going to work out…
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Since you are relatively new to seahorse keeping, Julie, I would also like to invite you to participate in Ocean Rider’s seahorse training program, which is designed to help hobbyists in your situation.

This basic training is very informal and completely free of charge. Ocean Rider provides the free training as a service to their customers and any other hobbyists who are interested in learning more about the care and keeping of seahorses. It’s a crash course on seahorse keeping consisting of 10 separate lessons covering the following subjects, and is conducted entirely via e-mail. There is no homework or examinations or anything of that nature — just a lot of good, solid information on seahorses for you to read through and absorb as best you can, at your own speed:

Aquarium care and requirements of seahorses;
Selecting a suitable aquarium for seahorses;
size (tank height and water volume)
filtration
aquarium test kits
Optimizing your aquarium for seahorses;
lighting
water movement and circulation
hitching posts (real and artificial)
aquascaping
substrate
Cycling a new marine aquarium;
The cleanup crew (aquarium janitors & sanitation engineers);
Water Chemistry
optimal parameters
water quality & water changes
aquarium maintenance schedule
Feeding seahorses;
Compatible tank mates for seahorses;
Courtship and breeding;
Rearing the young;
Disease prevention and control;
Hippocampus erectus
natural history
professional rearing protocols
Acclimating Ocean Rider seahorses.

If you are interested, Julie, I will be providing you with detailed information on these subjects and answering any questions you may have about the material I present. I will also be recommending seahorse-related articles for you to read and absorb online.

If you would like to give the training program a try, Julie, just send me a short e-mail off list ([email protected]) with your first and last name, which I need for my records, and I will get you started with the first lesson right away. I think the training course is the best way for you to fill in the gaps in your knowledge as an inexperienced seahorse keeper and help you to give your pet shop ponies the best possible care.

Best wishes with all your fishes, Julie!

Respectfully,
Pete Giwojna