Hitchhiker ID…

Dear Carrie:

Your red flame Gracilaria sounds great and I bet your seahorses will love it!

It’s difficult to say exactly what you’re hitchhiker may be without at least a picture to go from, but it sounds like it could be a Caprellid amphipod, or skeleton shrimp, as the Caprellids are commonly known. Caprellids have antennae and spindly segmented bodies with distinct heads, large formidable-looking claws (gnathopods) and prehensile terminal hooks at the end of their segmented tails, with which they attach themselves to the substrate. Skeleton shrimp look menacing because of those lobster like claws, but they are perfectly harmless and, in fact, seahorses absolutely love to eat them. They are commonly found on vegetation, hydroid colonies, encrusting organisms and other sessile life, and their transparent bodies typically take on the same coloration as the material they are living on because it shows clearly through their bodies as they feed on it. So my best bet is that your hitchhiker may be a Caprellid amphipod or skeleton shrimp, Carrie. Here’s how I described them in my new book (Complete Guide to Greater Seahorses in the Aquarium, TFH Publications, unpublished):

<Open quote>
Large numbers of Caprellid skeleton shrimp colonize fouling growths and organisms such as sponges, tunicates, macroalgae, and especially large colonial hydroids such as Obelia (Rudloe, 1971). At some times of year, these sessile organisms will be alive with swarms of skeleton shrimp. The best way to collect them is thus to look for such fouling growths on man-made objects (docks, wharves, jetties, breakwaters, buoys, etc.) and harvest the sessile animals complete with all the Caprellids inhabiting them (Rudloe, 1977). (The skeleton shrimp attach themselves tightly to such growths with grasping hooks and they will cling tightly to the hydroid colony and come along for the ride when you carefully place it in your collecting bucket.)

Skeleton shrimp are amphipods like Gammarus, but the Caprellids are very different in habits and appearance from Gammarids (The Caprellid, 2004). Whereas Gammarus are flat-bodied and seek shelter beneath vegetation and coral rubble, Caprella amphipods are thin and wiry (i.e., skeletal) and display themselves openly (The Caprellid, 2004). They have a long, slender thorax and almost no abdomen (The Caprellid, 2004). The spindly brown skeleton shrimp (Caprella acutifrons) are in constant slow motion, bending, stretching, somersaulting, and flexing languidly as they forage throughout the large hydroid colony they inhabit, gleaning diatoms from the stems and polyps and snatching up zooplankton (Rudloe, 1971). Thanks to their transparent bodies one can easily see the food particles streaming down their gut (The Caprellid, 2004). They owe their agility and acrobatic antics to the incredible flexibility of their slender, wire-like bodies and the fact that they have terminal hooks at their tail end and large grasping claws (gnathopods) like a praying mantis at the other end (Rudloe, 1971). Like a mantis, they often assume a prayerful attitude, slowly and reverently bobbing, then bowing their heads piously while clasping their "hands" together at their chests (Rudloe, 1971). They have two pairs of antennae and can turn their heads from side to side. Solemnly, they sway side to side, nodding and bowing down with great dignity.

Periodically they will interrupt their penitent meditation to begin actively foraging, and then they move altogether differently, with a unique method of locomotion that seems totally out of place in such clumsy looking creatures. Displaying surprising agility, they bend forward into a loop in order to get a good grip with their front claws. Then they swing their entire body over their heads, tail first, until their terminal hooks can grab a new hold, allowing them to release their grip with their claws and repeat the entire procedure (Rudloe, 1971). They are accomplished acrobats, advancing themselves end-over-end in a series of cartwheels and somersaults in this unorthodox manner. With the nimbleness and flexibility of a contortionist, skeleton shrimp can actually swing from limb to limb in this fashion (Rudloe, 1971), and it’s a comical sight to see them moving through the stems and branches and polyps of a bushy hydroid like a troop of drunken spider monkeys!

Thousands upon thousands of these tiny shrimp many inhabit a large clump of hydroids, and at first glance the entire hydroid colony appears to be writhing and crawling and pulsing with an eerie, unnatural life of its own (Rudloe, 1977). It is the tantalizing movement of these multitudes that apparently makes skeleton shrimp so irresistible to many fishes, and Jack Rudloe has often described how tossing a hydroid colony swarming with Caprellids into a seahorse tank will trigger a feeding frenzy worthy of a school of bloodthirsty sharks:

"Fish love to eat caprellid amphipods. Often we would tear off a clump of hydroids, toss it in the aquarium, and see even the most finicky reluctant feeders go wild and gobble up the tiny crustaceans as fast as they could pick them out of the hydroids. Sea horses especially love to eat them" ((Rudloe 1977, p100). <Close quote>

In short, Carrie, if your hitchhiker is a Caprellid skeleton shrimp, then it’s nothing to be concerned about and it would be a good thing to have a bunch of them in your seahorse tank. If that’s the case, your seahorses will find your hitchhiker to be a tasty treat.

Best wishes with all your fishes, Carrie!

Happy Trails!
Pete Giwojna

Dear Carrie:

Yup, it sure could be a skeleton shrimp. There are many different types of Caprellid amphipods and they come in various colors depending on the species and what they have been eating. If it has a distinct head with antennae, large clawlike forelimbs (the characteristic gnathopods), and grasping appendages at the end of its segmented tail (terminal hooks) that it uses to attach itself to the substrate, chances are good that it’s some sort of skeleton shrimp.

Do a web search for "skeleton shrimp" or "Caprellid amphipods" or "Caprella" and you may find pictures or drawings that you can compare to your hitchhiker to help you confirm its identification.

If it is a skeleton shrimp, your H. erectus would have made short work of him no doubt, but you’re right — it would be far too big for your Pixies to make a meal of, so it’s just as well that you removed it from your Pixie tank. I don’t think I could have done them any harm, but there’s no sense taking any chances.

Best of luck with your seahorse setups, Carrie!

Happy Trails!
Pete Giwojna

Post edited by: Pete Giwojna, at: 2007/01/10 03:46

Post edited by: Pete Giwojna, at: 2007/01/10 17:24