Skeleton Shrimp

InvertebrateCosmopolitan

Sep 29

Written By Alexander Julius Jensen

Caprellidae spp.

Skeleton shrimp are skeletally thin and often have ghostly transparent bodies. They grasp algae with their spindly rear legs, while their front legs form large "claws" used for grooming, defence, and capturing prey. Luckily, they only grow to a couple of centimetres (~1 inch) long.

The Skeleton, the Ghost, and the Marine Mantis

The skeleton shrimp is a gaunt and ghostly creature of the sea. It goes by a few ghastly names — all of them fitting, if not truly accurate.

The body of a skeleton shrimp is, unsurprisingly, skeletal; comprised of seven segments, appearing like an exposed spinal cord with spindly limbs, claws, and antennae protruding from the vertebra. At its rear, the shrimp has several legs that curve and terminate in tipped hooks. Like one monstrous hand, it uses these legs to grasp and anchor itself onto varying surfaces, from algae and bay grass to other animals like sponges and hydroids.¹ At its other end, its head is crowded with sets of projecting antennae that sway in the current, used by the shrimp for swimming and filter feeding. A pair of front limbs form bulky, powerful "claws", which are held tucked against the body, akin to the posture of a praying mantis — giving this shrimp the nickname of "praying mantis of the sea” (not to be confused with mantis shrimp).

Its alternative name is the "ghost shrimp". The exoskeletons of most species are pale white or transparent, and some reinforce this ghoulish appearance with eyes coloured blood red. But there are also species with bodies tinted tan or brown, or with a reddish hue, while others still have the power of invisibility; able to change their colour to blend in with their background.

Haunted Seas

These ghost shrimp haunt every ocean worldwide. They look like they'd belong to the dark world of the deep abyss — with all the other malformed monstrosities at the ocean's bottom — and while some skeleton shrimp do live there, most species are actually found at shallow depths, clinging to other organisms, which themselves cling to rocks, piers, and pilings.

The shrimp crawl across their hosts like armoured inchworms; grasping on with their front legs, moving up their hind body, then grasping with their hind legs, and advancing with their forebody. While they can swim — either by using their antennae or rapidly bending and straightening their bodies — most of their travelling is done passively, stuck to materials that get swept away in the currents.

Skeleton shrimp are a vital part of their ecosystems, as they eat detritus and other food particles floating through the water, each acting like a tiny water filter. But they also hunt; not through active pursuit — a lanky critter fervently flexing its way through the water column doesn't make for the most effective predator — but through ambush. A skeleton shrimp sits deathly still on its perch, with its front claws poised much like a praying mantis. Its transparent, camouflaged, stick-skinny body is nearly unnoticeable. Any unfortunate copepods ² or aquatic worms that venture too close meet their end in the hooked grasp of the skeleton shrimp. Fortunately, these gaunt ghouls grow no larger than 2 or 3 centimetres (~1 inch) long and pose no threat to people.

Mid-molt Mating

Like most exoskeletoned invertebrates, the skeleton shrimp needs to moult its hard outer armour in order to grow. Oddly, the intermediate, vulnerable stage between a moult and the growth of a new exoskeleton is the only time a female skeleton shrimp is able to breed. To determine the state of a female's exoskeleton, a male will inspect her by "tickling" her body with his copious antennae. If two males simultaneously find a newly moulted and receptive female, things can get ugly. From their usual placid selves, the males turn vicious, like reanimated corpses from a horror story. They fight with their powerful front claw-limbs and the victorious combatant wins a mating with the female. And, in some species, he also gets to be the female's post-mating meal — a prize worth fighting for.

After enjoying her snack, the female goes about transferring her eggs, against common advice, into one basket — that being her brood pouch; one of the leaf-like projections around her midsection. The eggs hatch only four days later into fully developed, but miniature adult forms. For about 12 hours they stay in their mother's pouch — you can think of the pouch as an enclosed tutorial level in a video game, where the newborns learn the basics, like how to coordinate their lanky bodies — before they're released into the wider world. Still, the young refuse to leave their mother. They cling to the outside of her body, scraping and eating microscopic foodstuffs from the surface. She puts up with this for about a week, before actively removing her children from her person and flinging them off to make their own way in the vast ocean. An ocean full of dangers, like predatory anemones, fish, and actual shrimp.

Shrimp? Mantis? Amphipod!?

All of the names used to describe this critter are very evocative, and, on the surface, very fitting. But the skeleton shrimp is more enigmatic than it appears and all of its spooky titles, instead of helping, only serve to mislead from the truth.

I've been calling this animals a skeleton "shrimp", and it does look vaguely like a shrimp, if not a very emaciated one. But the skeleton shrimp is not a true shrimp — nor does it have a true (internal) skeleton. And although it shares the hunting habits — and hungry, post-mating habits — of praying mantises, there's no close relation there either. But, admittedly, the skeleton shrimp is closer to true shrimp than mantises.

All of these animals belong to a group known as arthropods (in the phylum Arthropoda) ³, the most diverse phylum of all animals. Within this phylum, the mantises belong to a subphylum known as Hexapoda, and, nested within that, a class known as the insects (Insecta). While skeletons shrimp — along with true shrimp, crabs, and lobsters — belong to a subphylum of crustaceans (Crustacea), and, on a lower taxonomic level, the class Malacostraca.

A phylogeny of the Malacostraca — a class of crustaceans. Skeleton shrimp belong to a group known as amphipods (Amphipoda), rather than true shrimp (Caridea).

But here, skeleton shrimp diverge from true shrimp. True shrimp belong to the order Decapoda, with crabs, lobsters, crayfish, and prawns. Meanwhile, skeleton shrimp belong to, and are, amphipods (order Amphipoda). There are over 10,500 species of amphipods. Most of them are tiny aquatic detritivores or scavengers, with bodies like compressed shrimp; kind of bean-shaped. The gangly skeleton shrimp, making up the family Caprellidae, are the odd group out.

If your eyes glazed over at the web of taxonomic names and hierarchies, I don't blame you. The gist is that skeleton shrimp and true shrimp are both crustaceans, but, within that group, they belong to different orders. The true shrimp are closely related to crabs, lobsters, and the like, while skeleton shrimp are actually amphipods — making up a unique family of gaunt, long-bodied critters among a larger group of small, squat-bodied critters.

As to their other name, it's needless to say that the "ghost shrimp" are not actual spectres of the sea — just pale little amphipods with exceptionally lanky proportions.

¹ Both sponges and hydroids belong to a category I term "dubious animals" — indeed, many people don't even know that they are animals, rather than plants or members of some other grouping. But animals they are.

From top to bottom; sponges, hydroids, and copepods.

Sponges make up the phylum of Porifera, which consists of anywhere from 5,000 to 15,000 known living species of sponge. They are generally colourful, immobile and, amorphous creatures, and can often be found as parts of coral reefs where they filter water, collect bacteria, and process carbon, nitrogen, and phosphorus. Their "poop" is also eaten by other organisms of the reef. As mentioned before, they are animals (in the kingdom Animalia), and it's possible that the sponges were the first group to branch off on the tree of life from the last common ancestor to all animals — in other words, sponges are the "sister group" to all other animals.

Hydroids look like feathery plants but perform very animal-like behaviours, such as carnivory — a hydroid uses tiny stinging cells in its tentacles to capture small plankton, which it carries to its mouth area and ingests whole. There are over 3,700 known species and while some are solitary, others form colonies that can resemble one larger organism — as large as 50 centimetres to 1 metre (1.5 to 3.3 ft) in height. They belong to the phylum Cnidaria, best known for jellyfish. Indeed, during the last of their three life stages, hydroids look like little floating jellies. Prior to that, a hydroid's initial life stage is a minuscule free-swimming larva, then a sessile (attached to a surface) and usually colonial polyp — this polyp then releases the gamete-producing male or female jellies (or 'medusa'). Some species don't undergo this last medusa stage.

² Copepods are another group of odd and tiny aquatic crustaceans. They live pretty much everywhere with water, salty or fresh, including oceans and seas, rivers and lakes, swamps and bogs, ponds and puddles, or just anywhere that's somewhat damp, like atop moss or beneath leaf litter — even subterranean caves, polar-ice waters, and hydrothermal vents aren't free of copepods. You likely have copepods in your yard or local park. The reason for such omnipresence is copepod diversity; they form a class (Copepoda) with over 21,000 described species.

Most copepods are between 1 and 2 mm long, shaped like teardrops, with transparent exoskeletons, two pairs of conspicuous antennae, and, depending on habitat, anywhere from two eyes, to one eye, or, in some cave-dwelling species, no eyes at all. They can be free-living — part of the plankton floating through the water column — benthic — living below the soil or sand — and/or parasitic — mooching off of other organisms.

³ The arthropods are characterised by exoskeletons, segmented bodies, bilateral symmetry (with right and left sides being symetrical), and jointed appendages. A few familiar arthropods include crustaceans (crabs, shrimp, isopods, and barnacles), arachnids (spiders and scorpions), myriapods (millipedes and centipedes), and insects (ants, flies, fleas, beetles, butterflies, and bees).

These critters represent around 75% of all animals on Earth, and play an irreplaceable role in basically every ecosystem; as scavengers, nutrient recyclers, pollinators, and a food source for other animals.

Where Does It Live?

⛰️ Usually in shallow waters, clinging to algae, grasses or other organisms on rocks, piers, and pilings. Some species are also found in deep oceans.

📍 The world over.

Size // Tiny
Length // Between 1 - 4 cm (0.5 - 1.5 in)
Weight // N/A
Activity: Mostly nocturnal 🌙
Lifestyle: N/A
Lifespan: 3 - 4 months (females up to 1 year)
Diet: Omnivore
Favorite Food: Detritus 𓇼
Phylum: Arthropoda
Class: Malacostraca
Order: Amphipoda
Family: Caprellidae

There are hundreds of described species of skeleton shrimp (in the family Caprellidae) — found the world over, mostly living at shallow depths.

Many species have transparent bodies, others vary from tan to brown to reddish, and a few have the ability to change their colour to blend into their surroundings.

Although skeleton shrimp are usually gripping onto something — algae, sea grass, or other animals like sponges — they can swim; either by using their antennae or by rapidly flexing and straightening their bodies.

These critters are also known as the "praying mantises of the sea". When hunting, they sit motionless and wait for an unfortunate worm or copepod to get close, then use their mantis-like foreclaws to attack the prey.

Also like praying mantises, a female skeleton shrimp may eat the male as a post-mating meal.

Female skeleton shrimp can only mate while growing a new exoskeleton after a moult — before it regrows too hard. Males will fight aggressively for a chance to mate with a female.

The female keeps eggs in her brood pouch — the leaf-like projections on the middle segments of her body.

The eggs hatch into fully developed, but miniature, skeleton shrimp (shrimplets?).

They stay in the pouch for about 12 hours, learning to coordinate their lanky bodies, and when they emerge, they cling to the surface of their mother for about a week, scraping microscopic organisms off of her body for food. Eventually, she forces them off and flings them into independent life.

The skeleton "shrimp" aren't true shrimp, but belong to another group of crustaceans known as amphipods (order Amphipoda). Most of the over 10,500 amphipod species are squat and sort of bean-shaped. The skeleton shrimp are the long and lanky, odd ones out.