Dive Dry with Dr. Bill

#430: Skeletons... But Not In the Closet

Recently I was out at the dive park to test my new underwater video rig. I hadn't suited up when Paul, our local (or is it loco?) L. A, County lifeguard walked up to me. He had been cleaning boat hulls and his wetsuit was covered with tiny crustaceans moving about. Having seen Paul in this state (and a host of others) over the years, I immediately knew the critters were caprellid amphipods. I pulled my camera out of its underwater housing and began to film them using Paul's wetsuit as the film "set."

Caprellids, also known as skeleton shrimp, are one of my favorite critters. Ones the size of those on Paul's wetsuit (1/4 to 3/4" although they may reach lengths of nearly 1 1/2") are too small to see the interesting body structure except under a microscope. However, if they were 30 feet tall and lived on land, we humans would be in deep doo doo! Under a microscope you can easily see that caprellids look similar to our land-based "praying" mantises, although "preying" might be more accurate for both. Given their huge (relative to body size) claws, we'd be the ones "praying" if some radioactive disaster suddenly mutated them into giants!

My most comprehensive guide to the invertebrates of our State lists just a single species, Caprella californica, although an unidentified species in the same genus is also discussed. A "long" spine on the head is one of the distinguishing features. The described species is said to be common on eelgrass, in the low intertidal and subtidal waters and in bays in central and southern California. We can now add Paul's wetsuit to the list of suitable habitats... at least until he returns topside and rinses it in freshwater, causing them all to fall off!

The caprellids evolved from the also abundant gammarid amphipods such as beachhoppers seen on our sandy beaches and on rocky shores. Their abdomen shrank and their thorax increased in length as this transition occurred. The legs on two of the thoracic (mid body) segments disappeared. Their "claws" are located on the forward segments of the thorax, while three pairs of "legs" are located towards the rear and are used to hold on to the substrate, or algae or... Paul's wetsuit. Given the location of these legs, the caprellid often seems to be standing up straight and will appear to bob up and down.

Many casual observers may have seen caprellids based on one of their methods of movement. They will use the rear "legs" and the forward "arms" in a manner that makes them look like inchworms moving along on your garden plants. If they lose their grip, skeleton shrimp can "swim" through the water column. They do so by drawing their forward and rear sections together and then straightening out the body. Back in the days when I was a competitive swimmer, I think this "stroke" would have been illegal in all events other than "freestyle." Oh, and I wouldn't bet on the caprellids in a long distance race... I think their range is pretty small.

No "Dive Dry with Dr. Bill" column would be complete without mention of the two fundamental behaviors of any critter in the kelp forest, "munching" and "mating." Our local caprellid species is considered an opportunistic omnivore, very much like the author of this column. Both of us will eat almost anything that gets in our path... except asparagus and peas for yours truly. The caprellid's claws are used in at least two ways. First, they may scrape against the substrate to remove edible algae (including diatoms), dinoflagellates, detritus and small critters. They will also scavenge on small, dead critters. The feeding mode that would be truly frightening if they were giants is the capture of small crustaceans including other amphipods that are unfortunate enough to swim within reach of their claws. They will grab them out of the water and tear them to pieces. I'm a bit more civilized when I feed... but not much more.

Fortunately for parents of young children reading this column, not much is known about their reproduction. The two sexes are separate, just like in humans and other non-hermaphrodites. The females carry their young in a brood pouch located on the under (ventral) side of the body. It is formed by two leaf-like projections that extend from segments of the thorax. How the young are actually produced is a subject well beyond my current pay grade... but I'll be glad to receive a multi-million dollar grant to study the capricious courtship of caprellids.

After filming the skeleton shrimp on Paul, I submerged to test video footage with my new rig. I didn't stick around to see him "evict" the caprellids from his wetsuit by dousing them in fresh water. Therefore, I had to wonder if Paul might have broken out in song once the critters were gone. If he did, would he be singing "a caprella?" Sorry, I couldn't resist! I think my musical friends can see why I got my only D ever in 8th grade music.

© 2011 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Caprellid amiphpods clinging to Paul's wetsuit and a close-up of one courtesy of Peter Bryant

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia