Today we’re going to take a little break from our normal routine of talking about delicious seafood. Instead, we’ll
give you a quick lesson on some of the creatures from the deep that won’t be finding their way to your plate any time soon. Actually, they won’t be finding their way anywhere, as they’re all but immobile. Of course, we’re talking about Sea Sponges and Sea Anemones. We’ll start out by introducing you to the sponge.
Contrary to what a certain cartoon which shall remain nameless may have led you to believe, sponges do not work at undersea fast food restaurants, nor do they live in pineapples. That’s pretty obvious. What may not be as obvious is the fact that a sponge is an animal and not a plant. They are an animal with a very unique anatomy and live very sedentary lives. In regard to their bodies, a sponge has a very porous body, at the center of which is a mesophyl, which is made up of a very gelatinous substance1. It lies in between two cell walls that are very thin and possess channels which exist to help the sponge constantly circulate water throughout its body.
The sponge is unique among most animals in that it does not have a nervous system, circulatory system, or even a digestive system. This is why the water circulation that we mentioned in the previous paragraph is so important. By moving water throughout its system, the omnivorous sponge is able to suck in both small sea animals and plants in order to feed. This circulation also allows the sponge to excrete when it forces water out. Most importantly, the influx and expulsion of water provide and help to release oxygen from the sponge. Without this function, they would be unable to survive.
Even though a sponge is largely immobile, immature sponges in the larval stage are able to swim and move about
freely. These larval sponges are called parenchymula2 and are the main means by which the population of sponges is able to be spread out throughout the world. Some adult sponges also have the ability to move, but they rarely use it. Even when they do, they don’t exactly take off at blinding speeds and don’t cover great distances. The average mobile sponge moves at a pace of approximately 0.16 inches per day. To put it in perspective, it would take a sponge about 139 days to travel from the baseline of a basketball court to the three point line. Don’t look for them in the NBA any time soon.
Sponges are creatures of great longevity, with many of them living beyond the age of 200. Because of this and their many unique traits, sponges are very popular among tropical aquarium keepers looking for impressive and diverse specimen for their tanks. Aside from marine collectors, sponges have been of use to humans in other ways throughout the years. While the kitchen sponge you use to clean your dishes is highly unlikely to be made of a real sponge from the ocean, the opposite was true many years ago. However, there has been a resurgence in the use of sponges when it comes to bath sponges.
Humans aren’t the only animals who have been known to exploit sponges. Dolphins who live off of Australia’s western coast have frequently been known to uproot sponges from their positions and attach them to the lower portion of their bodies. They do this in order to cushion and protect their lower body areas when they are trawling the craggy ocean floors in search of food.
The other marine creature of little movement who we’d like to introduce to you today is the Sea Anemone. Like the sponge, they are a creature that likes to stay right where they are. (Not that they really have much choice.) Unlike the sponge, anemones are not motile creatures at any point in their lives, even during their immature stages of development.
Anemones are among some of the handful of sessile sea species known as polyps. What this essentially means is that they are mostly cylindrical in form with a disc-like shape at the top of their bodies. This is where an anemone’s head is located, and in the center of this disc structure likes the mouth. Overall, their bodies give them a vase-like appearance, which is offset only by the vast array of tentacles they have protruding from their heads.
Most anemones are bright in color and look almost like flowers. Don’t let their exotically beautiful and delicate
appearance fool you though. These guys are predators and are more than capable of holding their own in battle. They are largely carnivorous in terms of diet, and prefer to feast on small marine animals. They are opportunistic hunters who wait for their pray to swim by and then suddenly strike out at them with their tentacles. When they do this, they release their venom which stuns their prey. Once this is done, they use their tentacles to pull their now helpless prey into their mouths3 and chow down. Though a typical anemone will only be venemous enough to kill small sea creatures, some have a reserve of poison that is strong and powerful enough to kill a human, given that it is able to sting one a few times.
Even though the anemone is a largely immobile creature, it can at times make very small movements, either by contorting its body or by using its single foot, which is commonly referred to as a basal disc4. This is located at the bottom of the cylindrical portion of an anemone’s body and is mainly used in the process of attaching to long-held locations on rocks or just beneath the ocean floor. Once they’ve picked their spot, they almost never leave it unless they are pulled away by a predator or by somebody working for a tropical aquarium business.
Anemones are different from sponges in that they do not lack a nervous system. Even though they have nervous systems, they possess very primitive ones that lack a centralized brain. Most of the activity of their nervous systems is devoted to holding their place in the water and capturing prey. Since the anemone does not have any specifically designated sensory organs, it is not fully understood how it is that they become aware of the presence of their prey in order to capture it4.
Many anemones can live to over 50 years of age. They are a victim of their own longevity in a way, because it is one of the key reasons why they are so popular as fixtures in tropical aquariums. Though they don’t rely on humans for survival in any way, they do share a co-dependency with various forms of sea algae. The algae leak into the body of the anemone and once they’re inside, they’re safe from potential predators and also are guaranteed a certain amount of sunlight. They need this in order for the process of photosynthesis to take place. When they undergo this process, they help the anemones by way of supplying food and much-needed oxygen.
As we briefly mentioned in passing before, anemones are considered to be sessile sea creatures. The same can be said for the sponge. In order to be classified as sessile, an animal must meet a few different criteria. Both creatures qualify for this title by virtue of the fact that they spend a majority of their lives rooted to a substrate and usually are unable to move about at will5. While this may make for a boring life, it does have its benefits. They do not have to expend much energy and can survive on a relatively small amount of food. They may have to diet, but they get to skip the whole exercise part.
We hope you’ve enjoyed our quick lecture on sponges and anemones and that you’ll now have a greater understanding of these creatures when you see them in tanks, bath tubs, and if you’re lucky enough, in the ocean.
1. Author Unavailable
2. Author Unavailable
3. Gomick, Alan J
The Sea Anemone
4. Author Unavailable
5. Merck, J
Adaptations to Sessility