Shrimp! So, here’s me, an invertebrate paleontologist, thinking I know, I’ll write a blog about shrimp, or more specifically a blog about animals we call shrimp but aren’t. You know, quickly explain what makes a shrimp a shrimp, throw in a quick gif from the movie “Forrest Gump,” and then ramble on about non-shrimps. But alas, turns out, just because I like eating shrimp doesn’t mean I actually know anything about them. “To the interwebs, Batman!”
Let’s first talk about what shrimps actually are before we get into what they are not. Shrimps are decapod crustaceans, meaning they have ten pods, or more technically, ten feet. The term “shrimp” generally refers to crustaceans with long antennae, slender legs, and a laterally compressed, muscular abdomen which is highly adapted for forward swimming and backward escape response (Bauer 2004). Seems simple enough…but wait there’s more, and it’s vexing. True shrimp belong to the crustacean suborder Pleocyemata, infraorder Caridea. These shrimps typically have two pairs of claws, lamellar gills, unevenly overlapping abdominal segments (segment 2 overlaps segments 1 and 3) and whose females brood fertilized eggs on their swimming legs (pleopods) until they hatch. Though there are over 3000 caridean species, in general, with the exception of Pandalus borealis, these are not the shrimp we eat.
The vast majority of “shrimp” we eat are what folks in the Commonwealth Nations (e.g. the United Kingdom, Australia, etc…) call prawns. These “shrimp” belong to the suborder Dendrobranchiata and are characterized as having branching gills, three pairs of small claws, abdominal segments that overlap in an escalator-like fashion, and release their eggs directly into the water, rather than brooding about them. Reality is, the terms shrimp and prawn are not scientific terms, and both could be considered shrimp.
That said there are shrimpboatloads (I may have just made up this compound word) of organisms called shrimp, but aren’t actually shrimp, because they’re not decapod crustaceans. For example, clam shrimp. Clam shrimp are neither clams nor shrimp. They’re branchiopods or gill-footed creatures. Imagine that, having your breathing apparatus (gills) on your feet! Having grown up with two brothers, they probably would have asphyxiated themselves if they were branchiopods. Clam shrimp fossils have been found as far back as the Devonian Period, and they were abundant in Triassic freshwater lakes, so we frequently find them as fossils in North Carolina.
Extant (currently living, not extinct) clam shrimp kind of look like bath beads. Other shrimpy branchiopods include tadpole shrimp, fairy shrimp and brine shrimp also known as sea monkeys. Most branchiopods are small and generally live in freshwater.
Another non-shrimp shrimp I find entertaining is the skeleton shrimp. Skeleton shrimps are amphipods, meaning “different foot,” and belong to the family Caprellidae. Skeleton shrimps are the Muppets of non-shrimp shrimp world. With their slender almost transparent bodies, they attach to seaweed, submerged debris, hydroids, and other substrates, collecting prey, brooding their young and dancing.
Skeleton shrimps are small, exclusively marine critters, and in some species the female skeleton shrimp will kill the male after mating by injecting venom through a claw in its gnathopod (a modified appendage used for feeding). Love hurts!
Bobbling right along we come to another fascinating non-shrimp shrimp, mantis shrimps. Neither mantid nor shrimp, though called such because they hold their claws in similar ways to praying mantids. Mantis shrimps are stomatopods, marine crustaceans that have gills on their abdominal appendages.
Personally, I like to think of mantis shrimp as having abominable appendages, because some species have specialized calcified “clubs,” for smashing, while others have sharp forelimbs for spearing. Mantis shrimp are sometimes called “thumb splitters” because their abominable appendages can cause serious damage if mishandled.
Unlike true shrimp, mantis shrimp have very complex eyes that allow them to see colors we lowly humans have never dreamed of.
A common mantis shrimp species found along the east coast of the United States is Squilla empusa. Though Homer’s Odyssey did not have Odysseus travelling along this particular coast, wouldn’t it have been more invertebrately entertaining if Odysseus had set out from Ithaca New York, and instead of passing between Scylla (a six headed monster) and Charybdis (a giant whirlpool), he got caught between a Squilla and Charybdis. Could’ve happened…or not.
Last, but certainly not least, we come to my favorite non-shrimp shrimp, ostracods or seed shrimp. Again, ostracods are neither seeds nor shrimp, though they are tiny and because they have a shell, can sort of look like seeds. Ostracods are also sometimes called mussel shrimp, again because of their bivalved carapace (two-part shell). Ostracods are generally tiny, ranging in size from 0.01 mm to the gargantuan size of 32 mm in length. Ostracods are found in marine, freshwater and moist terrestrial environments. Ostracods first appeared in the Ordovician Period and approximately 20,000 fossil and 10,000-15000 living species have been described so far.
The ostracods I’m most interested in are entocytherid ostracods which live as ectosymbionts of crayfish and other crustaceans. These tiny creatures just fascinate me.
All entocytherid ostracods have Edward Scissorhands-like claws on their walking legs which help them adhere to their hosts,
but various genera have modifications to their shells, and other appendages that make little sense to me (see https://naturalsciencesresearch.wordpress.com/2017/02/24/a-few-words-about-wo/).
I could go on an on about various fun facts related to “seed shrimp” including: some ostracods are bioluminescent, other species reproduce by cloning, and a 425 million-year-old fossil ostracod penis was found from a rock in England. Many more fun facts about these non-shrimps can be found here.
Hopefully, after reading this blog you’ll have learned that a shrimp, by any name is not necessarily a shrimp.
In writing this blog I visited all sorts of online sites and references including:
- Bauer, R.T. (2004). Remarkable Shrimps: Adaptations and Natural History of the Carideans. University of Oklahoma Press, Norman: xvii+282 pp.
- Biology of Shrimps via Wayback Machine and Discovery Centre at Museum Victoria
- Caridean Shrimp Anatomy by Charles & Linda Raabe
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