The female and male of Liropus minusculus, a new species of skeleton shrimp discovered off California. The female symbol seems to have mysteriously been repulsed by the male symbol in this image. Creative Commons SINC. Click image for source.

This is the fifth post in the Wonderful Things series.

This creature is not an insect, nor something you need to worry about exploding from your chest. It's a crustacean called a skeleton shrimp, and it's only a few millimeters long.

This particular skeleton shrimp is Liropus minusculus, a new species of Caprellid Crustacean discovered in a small sea cave on Santa Catalina Island just offshore from Los Angeles. In spite of its terrifying appearance under magnification, the males measure about 3 mm and the females a mere 2 mm.

Here's another caprellid, Paracaprella pusilla, which was recently found to have hitched a ride on or in ships from the waters off South America to the Mediterranean Sea.

Invasive species of caprellid crustacean or alien autopsy? You decide. Paracaprella pusilla, From Ros et al. 2013. Click image for source.

If you squint, these guys have a faint, strange resemblance to the weedy sea dragon.

Here's a beautiful video created by Anna and Ned DeLoach of The BlennyWatcher Blog that shows skeleton shrimp in action -- and I do mean action:

As you saw in the video, skeleton shrimp often cling to bryozoans, hydroids, or eelgrass by appendages called pereopods. Here's a closer look at their interesting anatomy and their various pods and peds:

Creative Commons Obsidian Soul. Click image for license and source.

Caprellids are amphipods, small crustaceans that get their name from their variously shaped legs, in contrast to isopods like wood lice, whose legs are all alike. Unlike the caprellids, most of the other amphipods scavenge food or eat detritus. For comparison, here's a representative amphipod.

Creative Commons Hans Hillewaert. Click image for license and source.

As you can see, skeleton shrimp have taken the basic amphipod body plan and dialed all the features to "scary".

Skeleton shrimp live only in saltwater, and mostly near the coast. They eat whatever they can get, which may include protists, detritus, worms, diatoms, and other crustacean larvae. Although some filter feed or scrape food into their mouths with their antennae, most are predators. Allegedly, they freeze and wait for suitable prey to come along, which they then ambush. How they manage to hold still for more than 5 seconds without getting into it with their next-door neighbor is hard for me to imagine.

In spite of their fiery temperments, they seem to be good mothers. Not only do their brood their young inside pouches, they also seem to take care of them until they are big enough to pick their own fights.

Caprellids are probably wildly under-described, like almost everything on Earth; one of the authors of the study on L. minusculus has described 62 new species in eight new genera in the last 10 years alone.

I leave you with another chance to spend a little time observing skeleton shrimp. Watch for the moms with babies near the end, and enjoy a few minutes of relaxing nature-tation watching these Ultimate Fighting Shrimp.