Tube worms are sessile animals
that use branched tentacles to
trap passing zooplankton.
Welcome to the third and final installment of this series on marine filter feeders. If you need to get caught up, part I is here and part II is there.
That first diary is a discussion about plankton and the second diary focuses on animals that use siphons to filter this plankton from the water. Now we’ll take a look at a few other strategies that slow or non-moving animals use to filter feed. And I failed to note earlier that many texts and websites will use the term "suspension feeders" for animals that filter plankton from the water. The two terms are interchangeable.
Mucous Nets
As we saw last week, mucous on the gills are responsible for actually ensnaring the plankton that is drawn in from a bivalve or tunicate’s siphons. Earlier this year I posted an essay on the reproductive behavior of the slipper limpet, and in that diary briefly mentioned that it traps its food using a mucous net. Here’s how it works.
Adult slipper limpets are sedentary and usually grow in stacks of up to a dozen individuals. When threatened they will clamp the large foot down against the shell or rock they are attached to and seal the edges of the shell against the substrate. However, when they need to eat they will loosen their grip and allow the front portion of the shell to rise up a bit, allowing a flow of water to enter beneath the left side of the shell, the current being driven by cilia within the animal’s body.
To increase the surface area available to trap the plankton from the incoming water, the gill surface is stretched out into a long, thin organ that runs nearly the length of the body. Mucous nets surrounding the gills trap incoming plankton as the water passes back out of the right side of the body. Once filled with food the mucous net is rolled into a long thread and enters a food groove which leads to the mouth. The entire plankton-mucous structure is then swallowed whole and the mucous is recycled back to the gills.
Go to this site and scroll down to the bottom to watch a video of a slipper limpet slurping up its plankton-filled mucous net.
Nematocysts
Nematocysts are the stinging cells found in all Cnidarians (jellyfish, corals and sea anemones). These cells work like little harpoons and are triggered when something touches them. The tentacles of all cnidarians have thousands of these cells covering their surface, and all contain venom. The potency of the venom depends on the type of prey each individual species feeds on. Those that feed on fish have extremely strong stinging cells which are needed to quickly stun and kill larger animals. Structurally, cnidarians are pretty delicate, so a thrashing fish is not good for the body. It needs a quick kill. These are the species that give jellyfish a bad rap when swimmers are stung by them. (Note that the stinging ability is mainly used for food gathering and not for defense. Protection is simply a useful side effect.)
As you can see from the diagram below, the cell is filled with a coiled wire-like structure. This is tipped with a barbed spear at the end and has a venom producing gland at its base. When the cell is touched by anything organic (plankton, your finger) it triggers this "harpoon" to shoot out of the top of the cell and embed itself into the flesh. One cell won’t do much damage, but thousands working together will instantly paralyze its prey and immobilize it.
Most cnidarians have a very mild stinging ability because the amount of venom needed to kill a plankter is quite small. Sea anemones are attached to the bottom by a basal disk that sticks tightly to rocks or shells. The top, or oral, surface contains a centralized mouth surrounded by hundreds or thousands of finger-like tentacles. When the tentacles are extended they will ensnare any type of plankton that happens to come into contact with them. Once the tentacles are filled with food the anemone will pull them into its body and transfer the food into the mouth.
Sea slugs (which are mollusks, related to snails) are not only immune to the stings of the anemone, but actually contain chemicals that mask its biological makeup in order to fool these cells into not exploding. These slugs can actually feed on an entire anemone without triggering the nematocysts. More incredibly, while most of the anemone is transferred to the stomach to be digested, the sea slug is able to divert the undischarged stinging cells to its gills (the feathery objects sticking out of the animal’s back, as you can see below). The anemone’s stinging cells are then incorporated into the gill structure of this seemingly defenseless, shell-less mollusk. And guess what happens when a predator tries to eat the sea slug?
Non-stinging Tentacles
Other animals are able to filter plankton from the water using tentacles without any kind of stinging cells. Two groups I’d like to mention are the comb jellies and the sea cucumbers. Comb jellies are often confused with jellyfish but are not even remotely related to them. They are members of a completely different phylum of animals known as Ctenophores.
Ctenophores have trailing tentacles (usually only two) that ensnare plankton using specialized "sticky" cells known as colloblasts. Rather than bringing the food into the mouth as jellyfish do, comb jellies will actually rotate their bodies towards the extended tentacles, transferring the food into mouth. Note that, like jellyfish, comb jellies are plankers themselves. These are filter feeding plankton that feed on other plankton.
Sea cucumbers have elaborate, branching tentacles (they are Echinoderms, so these tentacles are based on multiples of five) that are extended into the water column. These tentacles are covered with a thin layer of sticky mucous, and again it’s the slime layer that actually entraps the plankton. When an individual tentacle branch is filled with food it is inserted into the animal’s mouth and the food is wiped clean. As the tentacle is pulled from the pharynx a new layer of sticky mucous coats the structure. This tentacle is then extended back out again and the process is repeated with each of the other five extremities. I’ve been trying to get my son to film this process using some of my local species, with no luck so far, but I did find something that shows this feeding process extremely well here.
Not all sea cucumbers feed this way. Some species are detritus feeders, meaning they basically eat dirt. That will be another diary.
Other diaries in this series can be found here.