Starfish or sea stars are probably the most celebrated of all tidepool life. It does not matter what shore, dock, rocky ledge or beach you find yourself upon, you can usually find sea stars. They are easy to find, easy to keep and no matter how many you find you always have to pick up another to give it a good close look. They are fascinating!
The Northern Sea Star is the name of our most common star here in Maine. They come in all sizes and colors. A good challenge for classes at the beach is to see who can find the smallest star of all. It is surprising - once you look in the right places very small stars can be found. The smallest ones we have located are usually in the pouches of another species - the brood star. This particular species actually spawns and 'broods' its young. There are usually dozens clasped in this oral area beneath the star. When found it appears they are feeding, but rather they are just protecting their young. They could be mistaken for a food source or part of its stomach, but if you you closely enough you will see little orange stars clustered within. Now those are really small stars. Small 1/2" to 1'4" stars can be found under rocks, in crevices, or attached to the holdfasts of seaweeds. Another nice thing about small stars is that they can be dried out for classroom use without any chemicals or stinky smell and they keep well for years.
I am not sure what causes the Northern Sea stars to vary so much in color, but I have an idea that it may depend on their diets. We have colors like tan, brown, red, pink, purple, and orange. The largest ones are often a purplish color with orange highlights. In the early summer the stars fatten up - their gonads (sex organs) ripen and they are ready to spawn. It is at this time that my kids and I have great fun wading at low tide to find these beautiful pinkish-purplish monster stars. The largest one we have found here in Cobscook Bay was 28" from tip to tip.
Once you find a good place for stars you can always count on returning to find them again and again. Stars love structures such as pilings, rocky ledges, overhanging crags, docks, and cobbly sea bottoms. Besides grouping up for spawning purposes in early summer the best place to find them is on good feeding grounds. For sea stars this means a place with lots of blue mussels or perhaps oyster beds. Once you find a mussel bed you can find stars actively feeding. The way to see if they are feeding is to look for stars that look all 'balled-up'. The center of the star will be very visible but some of the arms will not. They will be folded around the prey, tube feet in action doing their best to open the shell of the hapless bivalve. If you pick the star up and flip it over you will often see the large watery stomach structure hanging out all over the place. As the water drips from the animal and it's prey the stomach will often be drawn back into the cavity from which it was extended. It is great fun to see. Students will find some in various stages of feeding. Many times the star has already finished it's mollusc milkshake and only the empty shells remain between its arms. Burp!
If you look closely at a live sea star it is easy to quickly find the points of interest. Starfish have the radial symmetry plan with their 5 arms stretching outwards. Tucked in at the base two arms is a tan or orange madreporite or sieve plate. It looks a bit like an eye without pupil in it. This is how water enters the star's hydraulic system. It helps her/him pump up tube feet so that it can move. Another feature to observe is the very spiny hard skin that stars have. It is like a very rough sandpaper. If the star is live - lay it on a rock and watch the very slow movements of the animal on this surface. Stars do not have eyes but rely on touch. You will see the very tips of the stars arms curve upwards to 'reach' for another rock, or piece of seaweed upon which it could move. It is feeling its way around to see where it is. Kind of like a person in their dark bedroom feeling for the light switch. If you watch stars underwater you will see that at the tip of each arm are 6 - 10 small elongated tube feet that are constantly 'feeling' around for new structures upon which to crawl. These tube feet are very very thin and hairlike and are like a snail's or lobsters antenna.
Flip the star over to see the grooves in the bottom of each arm. Deep inside the groove are hundreds of larger tube feet that are used to move. The tip of each has a bit of a suction cup - these all work together to give the star traction on the seafloor moving it along. Each side of the groove have little spines that actually move - these are called - you guessed it "movable spines". They are finer in size than the immovable spines that cover the topside of the animal.
At the very center of the animal is the mouth or oral cavity. When feeding, the stomach will protrude and/or retract through this hole. If observant it is sometimes possible to look closely at the mouth and see a bit of the stomach up inside the hole - especially if the animal has been feeding recently. Like any coldwater invertebrates - sea stars do not have to feed very frequently and probably do not do much feeding in the winter when temperatures are low. Once spring comes they must gorge themselves to provide nourishment for their reproductive organs. Upon dissecting an large sea star in May or June - you will find a HUGE digestive gland and BULGING gonads that are ripe with sperm and eggs. When the temperature is right males and females release their sex cells into the water simultaneously and larval stars are born! Voila - the miracle of life. I have witnessed this only a couple of times in tidepools in the summer. It looks like tiny confetti or milk oozing from the topside of the animal. Seems to come right out through their skin.
Sea stars are a lot of fun! Kids love them and I guess I am just a kid. I have found and held literally thousands of different stars over the years. Each one is worth looking at and they all seem to be unique in some ways. Occasionally we will find a star with 6 or 7 arms. I have seen arms that split half way down resulting in a y shape or what appears to be a side growth arm - a mutation of sorts. Thats all now for Northern Sea stars. Another time we can talk or highlight the other 7 species of stars that are found in the cold Atlantic waters. - Tidepool Tim
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