Gastrotrichs from St. Lucia Beach, South Africa. You definitely want to click to embiggen. Creative Commons M. Antonio Todaro, Matteo Dal Zotto, Sarah J. Bownes, Renzo Perissinotto. Click image for source and license.

This is the fourth post in the Wonderful Things series.

As we saw last time, the thin strip of sand found on beaches is home to many organisms that can dwell no where else. But the strip swept by waves -- the intertidal -- may be the richest part of all. Living between the wet sand grains is a whole universe of microscopic life: the meiofauna.

The sand meiofauna are a collection of tiny organisms bigger than about 30 to 50 micrometers (John Q. Bacterium is about 1-2 micrometers long) but smaller than about 1 mm long. Almost every major group of marine invertebrates has a representative species living here, and they are beautiful, diverse, and surprisingly complex. They must withstand a harsh environment, where the waves toss about the sand like boulders, and they are exposed alternately to drying air and salt water.

They have adapted in similar ways to the narrow, shifting, dark environment. They're often miniaturized, elongated, and flexible; they adhere to their substrate, or have special ways of moving or reinforcing their bodies; and they lack pigments or eyes. They are also hugely obscure. Though many scientists have studied them, word does not seem to have reached the outside world. So for today, I will share a little about just three:

  • Mud dragons, the Kinorynchs. Their name means "moving snout", and I think you can see why:

    Kinorynchs use the pressure of fluid in their bodies (they have what's called a hydrostatic skeleton) to force their head up and out of its retracted postion in order to allow its spines to grab sand or sediment. As their head retracts, their body is pulled forward. When its head is fully retracted, neck plates cover it. When it's fully extended, nine sharp teeth or "oral styles" for grasping prey extend. You can see them if you look at the tip of the head when it's fully extended. They eat bacteria, algae, silica-shelled microbes called diatoms, and detritus.

  • Gastrotrichs (literally, "stomach hairs", for the cilia that line their bellies). These animals occur almost no where else but wet beach sand.

    Though they look quite alien to our eyes, these little guys have a lot in common with you and me. They have nerves and large brains wrapped around their pharynx, or throat. They also have muscles along their bodies that help swim and steer. Muscles move their bristles and adhesive tubes. The cilia on their bellies direct food like bacteria, algae, foraminiferans, diatoms, and the ever-popular generic organic schmutz to their mouths. The pharynx pumps the food into a proper gut, which vents its products via an anus.

    Unlike you and me, their bodies have adhesive tubes projecting at intervals along their sides and, especially, on their bum. They use these tubes to glue themselves to sand grains, which may be why the gastrotrich in this video had that embarrassing bit of detritus stuck to it like toilet paper. Like dragons, their bellies are soft and vulnerable, so they press them protectively to sand, plants, or sediment. Their exposed sides and backs bear defensive bristles, scales, or even multi-pronged spines. You could see the spines on the gastrotrich in the video quite clearly.

    Four-pronged spines on the body of the gastrotrich Thaumastoderma ramuliferum. Scale bar: 20 micrometers. Creative Commons M. Antonio Todaro, Tobias Kånneby, Matteo Dal Zotto, Ulf Jondelius. Click image for license and source.

    Charmingly, some gastrotrichs move like inchworms. They possess paired glands on their adhesive tubes: glue glands sit next to releaser glands filled with solvent. To move, they glue their front end to a surface, hitch up their backsides, pull forward, glue down, then secrete solvents to release the front.

  • Water bears, the Tardigrades. This one seems to be backing away from a pushy Paramecium. Notice its eight little clawed feet at the end of their stumpy, non-jointed legs. They can also sport pegs or sucking disks.

    These creatures are often found on land living in the thin film of water on water-absorbing surfaces like mosses, liverworts, lichens, algae, forest litter, and, oddly enough, roof shingles. For that reason, they're also called "moss piglets". These guys happen to be some of the most bad-ass survivalists on the planet, probably as a by-product of the ability they share with mosses to survive being dried out for a really long time. They can survive the vacuum of space. AND they're incredibly cute.

    In water, they live everywhere from the abyss to mountain hot springs. The marine water bears -- including those that live in beach sand -- may not have the same survivalist abilities as their land-based kin because they don't have to fight dessication.

    Tardigrades' dark secret is that they're often vampires. These tardigrades feed on plants and small animals like rotifers, nematodes or even other tardigrades by piercing them with a needle-like stylet and and drinking the contents. If you review the film, you can see the long, sharp stylet in the tardigrade's head. Like a toddler covered in chocolate or cookie crumbs, you can also see the telltale remains of this tardigrade's most recent meal: a plant or photosynthetic microbe. Its gut is positively green with chloroplasts.

Further Reading

Meiofauna of Sandy Beaches

Meiobenthology: The Microscopic Fauna in Aquatic Sediments, By Olav Giere