And we are back, looking at how animals move around in the ocean. As a follow-up to Locomotion in the Ocean (part one), we continue to look at how marine animals move around. As can be expected, ocean animals are not limited to fins, flukes and flippers. Some animals also use methods like jet propulsion or rely on a multitude of feet and/or legs to move them along. In the marine environment, where an animal is more often than not the prey AND the predator, it is very important to be able to move quickly and efficiently.

Jet propulsion

According to Wikipedia, jet propulsion is a method of aquatic locomotion where animals fill a muscular cavity and squirt out water to propel them in the opposite direction of the squirting water. Animals that have opted for this method include: octopuses, squids, salps and jellyfish.

Some have opted for the rear-rear method, where water is drawn in from the rear of the animal, and also expelled from the rear. Octopuses are known for crawling along the seafloor, or up rocky and coral reefs, but when needed, they can use jet propulsion to quickly move away in bursts of speed. Jellies have also opted for this method of locomotion.

Then there is the front-rear method. Animals, like barrel-shaped salps, make use of this method. Looking at a salp, one would think that the gelatinous mass is just that – a string of jelly. Well, it turns out that for some part of its life, a salp swims alone, but at some point, salps join up and form long strands, or wheel-shaped organisms. When relaxed and not threatened, these salps all swim at their own pace, but when they (as a group) are threatened, they synchronise their strokes to evade predators and other dangers. Clever salps indeed!

A colony of salp in the Red Sea. Credit: Lars Plougmann [CC BY-SA 2.0]

Tube feet

You might be surprised when you learn which animals have opted for tube feet. There is the usual suspect – sea stars, but then there are also sea urchins, sea cucumbers and sand dollars. These animals are all echinoderms and tube feet form part of their complex water vascular system. Each foot is a hollow tube with a powerful sucker at the end.

It is also good to note here that echinoderms have a water vascular system shaped a bit like a mandala – one line connected to the next, connected to the next, connected to the next, and all coming together to make a pretty picture. The water vascular system uses hydraulics to move water into the tube feet, and by doing that, move the feet and get them to stick. Water is drawn into the animal through the sieve plate or madreporite.  From there, it flows down a drain (the stone canal) into the ring canal. The ring canal basically runs all along the circumference of the central disk of the animal. From there, the water is equally distributed into the lateral canals.

The many tube feet of a local spiny sea star.

Lastly, the water is carried to the radial canals which each end in a tube foot. At the base of the foot there is a small sac called the ampulla, which holds the water. When the ampulla contracts, water fills the foot, and causes the foot to extend. The suction cup at the tip of the foot forms a seal around the substrate. The formation of the seal is the indication to the ampulla that it can relax, and the water is drawn back into the sack. This creates vacuum pressure which causes the suction cup to stick and the tube foot to shorten, pulling the animal forward just a bit. For a sea star, or any other tube-footed animal to move, it has to go through this process continuously.

A sea urchin may look like it's only covered in spines, but it actually has hundreds of flexible tube feet too. Credit: Kyle Burns/Two Oceans Aquarium

Legs

Fantastic fins, tubular feet and finally, we get to the leggy legs. The obvious contenders in the category are crustaceans. You know - crabs and crayfish. Then there are also horseshoe crabs, that have legs, but aren’t crabs. We even have a mammal, the sea otter, that has legs and the occasional fish (more on that later), and also very small animals with legs like sea lice. But let’s talk about crabs and their legs.

We have all seen crabs scuttling sideways across the beach. A crab’s wide, flat body design forces it to have this peculiar way of walking. Their multiple legs are located on either side of their bodies and the leg joints bend outwards. Therefore, with the combination of the location of their legs and the way these legs bend, crabs are forced to move in a sideways manner. There is of course always the exception to the rule. Some crabs, like the blue swimming crab has modified hind legs. These legs are paddle-like and the crab uses these to swim.

We also have to talk a bit about fish that seem to have legs. The red-lipped batfish is a wonderful example of a fish where some of its fins have developed into walking appendages. These fish are actually not great swimmers and tend to use their modified pectoral, pelvic and anal fins to “walk” across the ocean floor. Just like the batfish, frogfish also use their fins more like feet. Frogfish generally don’t like moving around that much, but when they do, they use their modified pelvic fins to move across the ocean floor.

In the end, how a marine animal chooses to move is greatly dependent on where in the ocean it spends most of its time, what kind of prey it consumes, and what kind of predators it needs to contend with. Some animals, like migrating whales, or Yoshi who is taking us on a worldwide adventure, move over great distances. Others like sea sponges, stay put because it is the right thing (for that animal) to do.  While we don’t always completely understand why animals move the way they do, we can simply marvel at the wonders of the ocean and see how such diverse species manage to make things work for themselves, and the ecosystem.

Tube sponges off the Malaysian coast, prefer to stay in one place and let their food come to them. Credit: Bernard Dupont [CC BY-SA 2.0]
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