Wet Beast Wednesday: skeleton shrimp
Happy almost Halloween, everybody. This month I've been covering animals that fill horror tropes on this series and how better to end the month than with some spooky, scary skeletons! Skeleton shrimp may not have bones, but they're here for the monster mash anyway. Just try not to step on them while you're dancing, they're quite small.
(Image: a skeleton shrimp on a black background. It is a long and very slender crustacean that looks a bit like a stick bug. It has a long neck with a small head that has four antennae. Near the neck is a pair of limbs that end in large, hook-like claws. In the middle of the body are four paddle-shaped gills. At the back end are a ground of long, slender legs. It is a translucent white color. End ID)
Skeleton Shrimp, also called ghost shrimp, aren’t actually shrimp, they’re amphipods of the family Caprellidae. Amphipods are a group of crustaceans only distantly related to actual shrimp. Skeleton shrimp get their name from their long, slender bodies, which look a bit skeletal and make them look totally unlike most other amphipods. Amphipods tend to have compressed bodies and have swimming lifestyles while skeleton shrimp are skinny and adapted for a more sessile lifestyle. There are thousands of species of skeleton shrimp known to science. The largest species get to about 5 cm (2 in) in length. The males of most species are significantly longer than females. The number of known species is contentious, especially as it can be difficult to tell different species apart. There at least 108 genera recorded, most of which have multiple species, meaning the number of skeleton shrimp species could easily be over a thousand.
(Image: a reddish skeleton shrimp perched on the end of a bit of seaweed with its grasping arms extended. End ID)
Skeleton shrimp bodies are divided into 3 main body segments: the cephalon (head), pereon (thorax) and abdomen. The head is small and contains the mouthparts, eyes, and two pairs of antennae. The pereon takes up most of the body and is divided into seven segments called pereonites. Each pereonite has a pair of appendages called pereopods, though the pereopods on the 3rd and 4th segments are usually reduced or absent. Instead, the 3rd and 4th pereonites contain the gills and the brood pouches in females. The first pair of pereopods is enlarged and adapted into special raptorial (grasping) limbs called gnathopods that look somewhat similar to a praying mantis’s claws. The 5th-7th pereopds are smaller than the gnathopods and adapted for grabbing onto things. The abdomen is extremely small, reduced to the point it is almost invisible, and attached to the last pereonite.
(Image: A skeleton shrimp clinging to some red algae. End ID)
Skeleton shrimp are found in oceans worldwide. While some species live in estuaries, there are no freshwater species. The majority of species prefer shallow water, but there are deep-sea species as well. Skeleton shrimp use their 5th-7th pereopods to grasp onto underwater objects. They prefer living things like seaweed (algae), seagrass (plants), and hydroids and bryozoans (animals). Their slender bodies give the shrimp camouflage amongst fronts and bristles. Skeleton shrimp are sometimes called the mantises of the sea based both on appearance and lifestyle. The majority are omnivores that lean predatory. They hold very still until prey (often a worm, protist, or planktonic animal) comes close. The shrimp then lashes out and grabs the prey with its gnathopods, killing or immobilizing it for the shrimp to eat. Some species have venom glads in their gnathopods to further incapacitate prey. Other species are filter feeders that use feathery growths on their antennae to capture food particles. They prefer staying still, but can move around to escape from danger of find better feeding grounds. Skeleton shrimp are poor swimmers and prefer to crawl, which they do in a similar manner to inchworms.
(Image: a group of 5 skeleton shrimp clinging to the same yellow object, possibly a sponge. 4 of them have bulbous pouches on their bodies, indicating they are females with active brood pouches. End ID)
Female skeleton shrimp can only reproduce during a short period of time after each molt while her body is still soft. When a female is ready to molt, the nearby males with fight each other over the opportunity. A male's gnathopods can sever the body of a rival male and the venomous species will also use their venom in these fights. In some species, the female will kill the male after mating using her venom. Males will guard the female while she molts and may help her remove her old exoskeleton. After mating, the female stores her fertilized eggs in her brood pouch. The eggs will hatch and the juveniles will continue to develop in the brood pouch before eventually emerging as miniature adults. Many species will cling to their mothers after birth for a few days, while other species are independent right away. The natural lifespan for female skeleton shrimp is estimated to be about a year.
(Image: a skeleton shrimp mother with a brood of offspring clinging to her body. End ID)
The conservation needs of most species of skeleton shrimp are poorly understood or unstudied. Being arthropods, they are likely threatened by ocean acidification, which impedes the development of exoskeletons. Pollution and habitat loss are likely also threats. Skeleton shrimp are prey to a number of larger animals, including fish, shrimp, crabs, worms, and more.
(gif: shots of skeleton shrimp moving with lyrics at the bottom reading "skeleton shriiiiiimp! You have all the decorum of a pugilistic reprobate." and the logo for Monterey Bay Aquarium in the top right corner. End ID. Context)
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