Last week, the Echinoblog Mail Dept. received a great letter (a REAL paper letter!) from an aspiring young person with questions about STARFISH!
Her name is Brianne (name changed to protect privacy) and she's an intrepid 6th grader from an elementary school somewhere in Colorado!
Her teacher, JH is applauded for encouraging her to write me the letter and for fostering Brianne's interest in science!
Her letter read as follows (corrected for spelling..)
Our teachers asked us students to write to an expert on a topic, that we chose, and I chose to write mine on starfish. I chose starfish cause starfish are just a mystery; I want to learn more about starfish.
I'm looking forward for your response on starfish, on the questions. Thank you I knew you were the right one to write to.
Thank you Brianne!
First-let me make sure that you understand that when I say "starfish" this is a different word that describes the same animal as "sea star" (Class Asteroidea in the Phylum Echinodermata) and just so that we are clear, a starfish/sea star is NOT a fish in the same way that say, a goldfish is a fish.
Starfish belong to a group of weird spiny-skinned animals called ECHINODERMS. Which includes starfish, sea urchins and all of their relatives!
Brianne's questions follow.... (some questions were related and these have been grouped together)
1. How do starfish eat?
There are many ways that starfish eat. But most of them feed by extending their stomach out THROUGH their mouth and onto whatever they happen to be feeding on.
Food might be anything from organic slime to algae to animals that either don't move very much(such as mussels or clams) or may never move (such as sponges). Some, as we'll see below are fierce predators.
This video shows a small bat star under high speed so that you can see it moving!
Do you notice that kind of brownish colored jelly that seems to grow and than shrink? That is the stomach extending through the mouth and onto the surface of the clear glass aquarium!
But that is not the only way that starfish eat!
2. Do starfish bite?/Do Starfish have teeth? No and Yes.
This will seems strange at first... starfish do not bite! But SOME starfish DO have teeth!
Since most starfish feed using their stomachs, there's no real need for them to bite and chew the same way that say, a dog might.
BUT there are some starfish that have a lot of spines located around their mouth that might be important to helping them eat. See those red circles below? Those are indicating spines present on a few special species that like to eat a particular kind of sea sponge. They aren't for chewing but they might be important for eating in ways that don't know much about yet.
For the sake of being clear, let me say that starfish do NOT BITE. You don't ever have to worry about a starfish ever hurting you in that way. They may have teeth and a stomach-but they don't really have jaws....
3. Can starfish live on shore? Yes (kind of..)
Most starfish species live at depths where they are always underwater, but there are many species that live right at the edge of the ocean but most do not live exactly on the sandy beaches (although some do live near sandy beaches! and become easier to see when the tide goes out)
In this video from British Columbia, you can see many examples of the colorful Ochre Star (species = Pisaster ochraceus) living where the waves crash onto the rocky reefs.
On the other hand, there are other species which live only on muddy or sandy bottoms but in very shallow places near the coast. In the United States, you are likely to find these kinds of species on the tropical Atlantic coasts of Florida and Texas.
But here for example is a video of the intertidal area in Singapore! If you move the video to the time at 3:02 there are sea stars/starfish that live in very shallow mud and sand.
4. Do starfish move? YES. absolutely they do!
However to a human being's eyes-they move VERY slowly, if at all. However, if you put a camera on them and SPEED UP the recorded motion from several hours or minutes you can see all kinds of movement and behavior!
here is an example of such movement as several bat stars move over the seafloor
Some species, such as the Sunflower sea star on the Pacific coast of the US moves so quickly that you can see them without a camera..
5. Can they swim?
ADULT starfish don't swim and live their lives, crawling around entirely on the bottom of the sea floor.
But juvenile starfish (called larvae) have a very different body shape than adults! Starfish undergo a change before they become the large adult-bottom living form in a similar way that slow-moving caterpillars transform into flying butterflies.
When starfish/sea stars are "babies" (=larvae) they look like this and are covered with many small hairs that constantly move and help the larvae to swim and move. So that one day, the larvae will settle on the bottom to become an "adult" starfish.
Larvae are VERY tiny. This one is about the size of the tip of your pencil!
In this case, the tiny larvae above, settles down and grows up into the big adult over the course of several years (almost 2 feet across!).
6. Do starfish have eyes? Yes..kind of..
Although starfish/sea stars DO have something similar to eyes, they are not quite the same. There are organs on each of the armtips that permit starfish to sense light and perhaps heat.
Many special sensors lie on the top of their body that allows them to sense light. Many species are sensitive to light and will move off if it is too hot.
Note the GREEN ARROWS below! Those are where the "eyes" (technically called eye spots) are located.
However, "seeing" is not as important to a starfish as it is to a human being. Starfish bodies are open to the sea water around them. They don't have blood like we do, they just let the ocean fill every part of their body!
Because the ocean is always "inside" them, they can "smell" or detect food immediately. They can detect anything good or bad that might be around them. This includes "smelling" bad things like poisons and uncomfortable things about the water (such as if it is freshwater -starfish only live in sea water). But they can also "smell" predators and prey immediately!
Many predators that feed on starfish can immediately be sensed by a starfish when their "smell" gets into the water. If a starfish doesn't like the smell, it will move away from it very rapidly..
7. Do starfish have gills? Yes!
Starfish breathe by absorbing oxygen from special finger-like structures that are found on their body surface.
If we enlarge the area in the blue circle to the box below...
We see all of those fuzzy patches between all of the white granules/spines. Those fuzzy patches are each a single finger shaped balloon like structure called a papula (plural is papulae). Those are the gills.
Each one of those patches can contain dozens of papulae, which are extended when the animal is comfortable and feels secure. Each of these permits the animal to collect oxygen for the starfish to breathe. They are generally found only on the top and side surfaces, generally not on the underside.
8. Are starfish mammals or do they lay eggs?
Starfish are members of the phylum Echinodermata. A phylum represents the largest natural grouping of different 'types' of animals. For example, phylum Mollusca includes squids, clams and snails and phylum Arthropoda includes insects, crabs, spiders and centipedes. Humans and mammals belong to the phylum Chordata-also called the chordates.
Echinoderms are the "spiny skinned" animals-and include starfish, their close relatives the brittle stars, sea urchins, sea cucumbers, and feather stars. Echinoderms are on the same "side" of the family tree of the animals but are quite distant from chordates (which includes the mammals).
Echinoderms have evolved in a VERY different way and in a much different direction than Chordates (mammals). And so, their biology is also much different.
Their reproductive/egg-laying biology is one example of how different they are..
Here is a video of a starfish from the tropical Pacific (in southern Japan). Not all starfish assume this unusual "standing on arm tips" pose..See those white threads being released by the animal??
This species is releasing reproductive cells-called gametes into the water.
Male and female gametes join together in the water and grow into a series of different small larval stages that eventually settle out and grow into an adult.
Here is video that shows the many different stages as they grow and develop...ultimately becoming an adult.
MOST times, eggs and baby starfish form in the ocean. BUT in some UNCOMMON cases there are species of starfish that will actually KEEP their eggs or juveniles until they grow to an adult size...Again..this is NOT typical.. but it does happen. These eggs and juveniles are usually kept around the mouth...9. Do starfish have brains? No, but...
So, starfish have what is called radial symmetry (specifically pentaradial or five part symmetry). That is, if you were to cut the animal in half it does not have an exact and opposite side.
The arms all project out from the middle the way that spokes on a bicycle wheel project from the middle.
As a result, there is no head or true central location where the nerves meet.
In its place, is what's called a radial nervous system. The nerves form a ring around the mouth in the disk and radiate out along the arms.
Although the nerves seem simple, this style of nervous system has likely been around in starfish for a very long time.
Although its not as apparently developed as the brain or nervous system in a human (or some other mammal) the behavior in sea stars is surprisingly complicated. Under time lapse cameras, biologists have observed that sea stars are capable of a great deal of behavior that one would not expect from such a seemingly simple nervous system!