HARLEQUIN SHRIMPS: Beautiful but Barbarous!

 

 "Oh what pretty tube-feet you have!" A pair of harlequin shrimp on a rubble patch off the coast of Sulawesi are having a Linkia seastar for dinner. 

The beautiful harlequin shrimps have often been considered as a difficult crustacean to care for. In actuality, they are very easy to  keep as long as you meet their specialized nutritional need – that is, provide them with sea star flesh! The harlequin shrimp are members of the family Hymenoceridae and the genus Hymenocera. Some crustacean experts think there is only one species in this genus, while others split the Indo-west Pacific and central and eastern Pacific forms (the former is recognized as Hymenocera elegans while the latter would be H. picta). Both “species” of harlequin shrimp are white to cream overall with large purplish blotches with blue margins (H. elegans) or maroon with yellow margins (H. picta).

Not only are their colors spectacular, the appendages on the thorax are very ornate. There are several pair of anterior appendages that have flattened expansions. For example, the chelipeds are large and shield-like. The maximum length is approximately 5 cm.

What is particularly interesting about the Hymenocera spp. is that they feed on sea stars. A pair of these beautiful beasties (the adults are almost always found in pairs) will drag sea stars to their lair or incapacitate them where they find them. These shrimps will vigorously stab their sea star prey with the spike-like end of  one pair of legs. It may be that the shrimp are puncturing the water vascular system of the sea star so that it cannot move off. In this way, they would be able to control some of the large asteroids that they typically feed on. That said, some sea stars do escape even after having an appendage removed.

When you add a sea star to the tank, your harlequin shrimp will typically begin waving their appendages from side to side and begin searching for their prey. I assume that they locate their prey by smell. It may take several minutes before they find the unfortunate echinoderm and when they do, they will leap on it and begin flipping the sea star over on its back.

 

 Harlequin shrimp are fairly site specific, living in the same small area of the reef or rubble for months - as long as there are enough echinoderms present.

Their feeding behavior is somewhat Hannibal Lector-like. When they consume sea stars, they begin feeding at the end of the arms, working their way toward the central disc. In this way, they can keep their “living buffet” fresh longer. In some cases, the shrimp will amputate an arm and let the rest of the sea star escape. Although it depends on the size of their echinoderm prey, the harlequin shrimp will take several days to several weeks to consume an entire sea star. They prefer sea stars in the genera Fromia, Linckia, Nardoa and Archaster, but will eat other species as well (including the crown-of-thorns sea star, Acanthaster planci). In the aquarium, they will occasionally feed on sea urchins and brittle sea stars. They will need a ready supply of sea stars if they are going to survive in the aquarium.  A medium-sized chocolate chip sea star should last a pair of harlequin shrimp about one week, but you only need to feed your shrimp a sea star once every three or four weeks. Make sure you remove the uneaten portions of the sea star so that it does not pollute the aquarium. These shrimps will also eat Asterina sea stars, which can get out of control in some reef systems. Some reef aquarists have even employed harlequin shrimp to decimate Asterina populations.

 

A captive harlequin shrimp begins the process of flipping over its spiny skinned prey. The shrimp pushes against the bottom with its chelae and pulls with its legs to lever the sea star onto its back.  

If well fed, they should molt about once per month. You will know when molting is eminent by the appearance of the integument. The color will not be as vibrant and look opaque. Molting does not always go perfectly and the shrimp may end-up losing an appendage during the process. These are often “repaired” during future molts. All crustaceans are more vulnerable just after molting and as result usually hide out until the exoskeleton has “hardened-up.”

Although it is possible to keep these shrimp with fishes that do not eat crustaceans, I think they are best kept in a nano-reef aquarium. If you decide to keep your harlequin shrimp into a community aquarium, do not keep them with crustaceans-eaters (of course) or overly aggressive fishes. Suitable tankmates would include seahorses, pipefishes, anthias, small gobies, dartfishes  and dragonets. Of course, in small tank you will have to make sure you promptly take out sea stars remains that are beginning to decompose to prevent pollution problems.

 

 One they flip over a sea star it is almost like the echinoderm goes into a trance, putting up very little resistance to the hungry crustacean.

While harlequin shrimp can be kept in heterosexual pairs, males will spar with one another and will eventually have to be separated. (If you have a large enough aquarium you may be able to keep two pairs together as long as there is enough room for them to avoid one another.) When they do combat they splay their chelae apart then begin pushing against each other. The sexes are easy to separate. Females have spots on the ventral surface under the abdomen, while there are not markings in the male. Also, females tend to be larger than males. 

 

 A beautiful example of the H. picta color form. Note the yellow margins around the maroon blotches. 

Mating occurs just after molting. The eggs are attached to the female’s pleopods (appendages under the abdomen). She will rapidly beat these structures to aerate the eggs and will clean them frequently. Depending on the water temperature, hatching occurs within 14 to 28 days (incubation period is less if water is warmer). The female facilitates dispersal of the pelagic larvae by rapidly beating the pleopods. Just after the eggs hatch, the female will molt. A female can produce from 200 to 5,000 eggs in a month’s time – she will produce more eggs if she is well fed. Fortunately, the larvae are photo-tactic and can be attracted to the surface with a torch beam and removed from the adult tank by dipping them out with a cup. Larvae are 1.4 mm long at hatching and for the first few days food is not required. However, after the first molt planktonic foods (Artemia nauplii, copepods, rotifers) will be required.

 

 This beautiful little shrimp, known as the bongo shrimp in the aquarium trade, is a predator of brittle sea stars.

There is another member of the family Hymenoceridae that shows-up in aquarium stores. This odd little crustacean is known in the ornamental invert trade as the Bongo shrimp or the spiny tiger shrimp (Phyllognathia ceratophthalma). It is rarely seen in the wild or in the aquarium trade – this is probably more of a function if its small size (it only get just over two centimeters long) and its reclusive habits than its being a rare species. I have only seen three of these shrimp during years of diving in locations where they are known to occur (the Western Pacific from Okinawa south to Queensland, Australia).

Little is really known about this species, but aquarium expert Kevin Kohen was able to fill in some gaps in our knowledge. Kevin has been keeping  a specimen of P. ceratophthalma for several months now and has observed some very interesting things about its natural history. Up until now, data on the feeding habits of this little crustacean was not available. It has been assumed that they are echinoderm-eaters only because of their relationship with Hymenocera, but I have not found any direct observations of this published in the literature. Thanks to Kevin we know that they eat asteroid sea stars (namely the little Asterina stars that often pop-up in our reef tanks) as well as ophuroids!

Beautiful, ornate and tiny, the bongo shrimp is easy to lose track of in a larger reef aquarium.  

That ends our brief examination of the shrimp in the family Hymenoceridae that you might encounter in the aquarium trade. If you are willing to meet their very specialized dietary requirements, these crustaceans can make fascinating additions, especially in the nano-reef aquarium. 

References:

Fosså, S. A. and A. J. Nilsen. 2000. The Modern Coral Reef Aquarium. Volume 3. Birgit Schmettkamp Verlag, 448 Pp.


©  Scott W. Michael- Reef Tectonics

 Reef Tectonics Aquarium Maintenance and Design - Lincoln, Omaha, Des Moines, Kansas City