Small animal neighbors on our docks

Sponges and tunicates at Schoonmaker Marina |  photos and post by Michael Konrad, South 40 Pier

Living on the water means having animal neighbors that land residents infrequently see. The terns, gulls, egrets, herons, pelicans, and cormorants are familiar to us all, and if you live near deeper water, you may see seals. Schools of small fish swim in both shallow and deep water around our floating homes, causing the birds and seals to go crazy when they find them. We usually don’t see these fish because most are just an inch or two long, and their colors match algae in the water. Fish that don’t have good camouflage don’t live long.

However, there is a much larger group of animals attached to our docks, but not noticed because they are small and don’t seem to move. Mussels are filter feeders, as are most of these animals, with hidden internal pumps constantly drawing water in through slightly open shells and across sticky surfaces to trap very small prey. Tunicates are another group of animals that pump water through their bodies to trap food in a filter. They are often thought to be sponges or algae because some are bright orange or yellow. Barnacles constantly extend and contract small hairy legs from their shells to catch even smaller animals, but if you see a barnacle and bend down to look more closely it will see your shadow and quickly pull its legs in.

Most of these dock animals have very different juvenile and adult shapes and behaviors, analogous to a juvenile caterpillar and adult butterfly. Marine juveniles swim freely in the water for a few days, but you are unlikely to notice them because they are less than a mm (1/16 of an inch) in size. But the little fish see and eat them; they are their main food. The big fish we eat were once little fish, and these little fish depend on these small dock animals. We are on the top of a food pyramid. As you go up this pyramid the total mass (weight) in each layer of small animals has to be larger than the total mass of each layer of larger animals above, because the efficiency of an animal converting prey to body growth is inherently low.

The most enjoyable way to appreciate how many different small animals live on our docks is just to look at a few. The photo above was obtained using a GoPro camera held by hand a foot below the water at a finger dock at Schoonmaker’s Marina. The surface of the concrete float has animals living on top of animals, with algae (marine plants) mixed in. The yellow stringy things are sponges but most of the rest are species of tunicates. Tunicates, a.k.a. sea squirts, are basically muscular tubes filled with water, and when you poke them, or when they swallow an irritating object, they squirt this water out. There are many smaller species mixed in this dock crowd, but they are tiny, covered by neighbors, and cannot be recognized at this magnification. All the tunicates are pumping several times their body volume of sea water every minute to filter out sufficient prey. They have a rich sex life that would drive Gov. DeSantis crazy, since each is both male and female and fertilize their own eggs as well as those of their neighbors.

Encrusted mussel

This next photo is a single mussel encrusted with many smaller animal species. The mussel was pried off the dock, transferred to the lab and placed in a large Pyrex dish with sea water and allowed to recover from the trauma. Gradually the animals extended arms and heads and could be photographed using a macro lens. All the surrounding life was carefully painted over as black using Photoshop™.

The mussel is the large black oval which starts at the center and extends down and to the left. We can’t see here that its shell is cracked open a few mm to allow it to pump water in and out.

The large white tunicate above “1” is a Ciona, one of the most common animals seen in the first underwater photo.

The cluster of orange tubes to the left of “2” is a colony of small tunicates that were produced asexually from a single founder animal, and thus are all clones with DNA containing the same genetic information.

The white arms of an anemone can be seen to the left of “3”, its body is hidden by the dark red shell of a colony of Bryozoa. The Bryozoan shell contains a rectangular array of tiny tubes, each containing a hydra, a miniature version of an anemone. At higher magnification we would see the red arms of the hydra waving in the water to catch even smaller prey.

Below “4” are the feathery orange arms of a tube worm spread like a fan around the opening of the tube shell that curves around and under the muscle shell and contains the body of the worm. A small white arc marks the edge of the tube. The orange fan serves as a gill, and traps small prey which are swept into its intestine by cilia. If I am careless and vibrate the tray or cast a shadow the worm will retract this fan into its tube and seal the opening with a tough operculum in less than a second.

Skeleton shrimp

You can see many light yellow hair-like things extending from the edge of the mussel under “5”. These are Caprellidae, a.k.a. skeleton shrimp. An enlarged Photoshopped image of one is seen in this last photo. They are Arthropods (marine insects), and have a hard outer shell and jointed legs, just like shrimp. Usually, they will be seen clinging to the dock with the six small legs at the end of their body and swaying back and forth, using their claws to grab objects that drift by. If it feels like food, they pass it to smaller arms and finally push it into their mouth. The small lobes just above the six lower legs are gills. Females have much larger lobes that form a basket and holds and protects fertilized eggs until they hatch into tiny Caprellidae and are released to fend for themselves. Caprellidae can be up to 20 mm long, but most are around 2 mm.

If you can increase your magnification, you can see more and more animal species, because most animals are small. As you might guess, I could spend hours describing more small animals on this mussel. Is there a lower limit to size? At about 0.1 mm, animals start looking like a collection of lumps since all life is made of lump-like cells, which are about 0.01-0.02 mm (10-20 microns) in diameter. If you could see objects 1/100th the size of these cells you would begin to discern some of the huge molecular machines that convert food into energy and use that energy to move. We now have specialized microscopes that can do just that!

You can go to your local bookstore (in Sausalito: Books by the Bay at 100 Bay Street, or Waterfront Wonders at 314 Caledonia Street) or to find books that describe animals like these. While there you might take a look at the fascinating book Life on the Dock, which amazingly has pictures similar to the ones in this article. The astronomer and well-known TV personality Neil deGrasse Tyson often says at the end of his shows “…keep looking up”. The less known biochemist and marine biologist Michael Konrad says at the end of this article, “…on the dock, keep looking down”.