DIVE DRY WITH DR. BILL #839: BRITTLESTARS
Anyone who has taken a high school biology class (especially if it was one of the ones I taught back in the 60s and 70s) should know about the invertebrate phylum Echinodermata. If not, I've got a nice tall dunce cap for you to wear when you sit in the corner. Of course you know about starfish. Hopefully you also know about sea urchins. For the B students you may remember sea cucumbers. Those with top grades may even remember brittle stars, basket stars and crinoids.
And for the truly exceptional, you may even remember extinct forms such as the helicoplacoids. Back in my Harvard days I took invertebrate paleontology with professors Bernard Kummel and Stephen Jay Gould (who went on to wider fame as an author). During the lab in that class, I discovered a new species of helicoplacoid in our fossil collection. Of course as a lowly undergraduate, the task of scientific naming went to a far more advanced professor at UC Berkeley, J. Wyatt Durham.
My subject for today is the ophiuroids, more commonly known as the brittle stars. Here off Catalina we see them far less frequently than starfish, sea urchins and sea cucumbers. That is because they are very "smart" (to be anthropocentric). They know to keep hidden under the rocks during the day or a hungry sheephead might chow down on them. Personally, I prefer to get my calcium from milk. By the way, basket stars are also in this class but they are a very rare sight in our waters unless you dive deep. And if you do (in a submersible), you will probably see that brittle stars are also very common on the bottom.
Ophiuroids usually have five arms, like the five part radial symmetry of most echinoderms. Unlike their sea star relatives, they have long slender arms and a central disk that is distinct from the arms. Rather than using tube feet to move about like sea stars, they use their long, thin arms (often covered with spines). The arms also lend these critters the name serpent stars (although I know a snake when I see one). Calcium carbonate (calcite) plates protect both the central disk and arms.
Brittle stars do possess tube feet and a water vascular system like starfish. However, they do differ a bit from those of sea stars. Their primary function is to transfer tasty tidbit to the oral opening. Their mouth is surrounded by five "jaws" that are often "toothed." It leads into the esophagus and stomach. The mouth also serves as an anus to eject wastes. I know some humans whose mouth performs the same dual function. Their nervous system consists of a main nerve ring in the central disk with radial branches extending into each arm.
So what tasty morsels do brittle stars choose when they are starved? They generally scavenge or chow down on detritus. I'm sure glad I'm not a brittle star! Some, like the soft sediment dwelling Ophiopsila californica burrow into the sand with their five arms extended to trap small plankton and organic matter. They are fairly common in our waters. I was surprised to learn that this species, as well as several other ophiuroids, can bioluminesce. This may be to ward off potential predators (although they have no eyes to see one coming).
IMHO one of the most beautiful of ophiuroids is the spiny brittlestar, Ophiothrix spiculata. It comes in a fairly wide variety of colors. I rarely see it on my dives off Catalina but it is very common on Anacapa, one of the northern Channel Islands, often covering the bottom. Sheephead predators are less common there. It is also found covering many of the offshore oil rigs in SoCal.
So you also want to know more about the sex life of these interesting critters? I thought so. In most species the sexes are separate but some are hermaphroditic (having both sex organs) or even protandric (beginning life as males and turning into females). They are broadcast spawners, releasing their sperm and eggs into the surrounding waters and hoping they win the lottery.
On some of my night dives in the past I was pleased to find one of our larger local species, the banded serpent star Ophioderma panamensis, spawning. At night they crawl out from under the rocks, amble up to the highest point on the reef that they can and rise up on their long arms. They then release their gametes into the water. To me the amazing thing is that this spawning was well synchronized as I found a large number of them doing it simultaneously... and it only lasted about 20-30 minutes before they crawled back down!
© 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Brittle star and underside of arms showing lateral spines; beautiful Ophiothrix spiculata on hand and mass of them on oil rig platform; burrowing brittlestar Ophiopsila californica in open and cluster in sand; Ophioderma panamense on rock and spawning on reef at night.
Anyone who has taken a high school biology class (especially if it was one of the ones I taught back in the 60s and 70s) should know about the invertebrate phylum Echinodermata. If not, I've got a nice tall dunce cap for you to wear when you sit in the corner. Of course you know about starfish. Hopefully you also know about sea urchins. For the B students you may remember sea cucumbers. Those with top grades may even remember brittle stars, basket stars and crinoids.
And for the truly exceptional, you may even remember extinct forms such as the helicoplacoids. Back in my Harvard days I took invertebrate paleontology with professors Bernard Kummel and Stephen Jay Gould (who went on to wider fame as an author). During the lab in that class, I discovered a new species of helicoplacoid in our fossil collection. Of course as a lowly undergraduate, the task of scientific naming went to a far more advanced professor at UC Berkeley, J. Wyatt Durham.
My subject for today is the ophiuroids, more commonly known as the brittle stars. Here off Catalina we see them far less frequently than starfish, sea urchins and sea cucumbers. That is because they are very "smart" (to be anthropocentric). They know to keep hidden under the rocks during the day or a hungry sheephead might chow down on them. Personally, I prefer to get my calcium from milk. By the way, basket stars are also in this class but they are a very rare sight in our waters unless you dive deep. And if you do (in a submersible), you will probably see that brittle stars are also very common on the bottom.
Ophiuroids usually have five arms, like the five part radial symmetry of most echinoderms. Unlike their sea star relatives, they have long slender arms and a central disk that is distinct from the arms. Rather than using tube feet to move about like sea stars, they use their long, thin arms (often covered with spines). The arms also lend these critters the name serpent stars (although I know a snake when I see one). Calcium carbonate (calcite) plates protect both the central disk and arms.
Brittle stars do possess tube feet and a water vascular system like starfish. However, they do differ a bit from those of sea stars. Their primary function is to transfer tasty tidbit to the oral opening. Their mouth is surrounded by five "jaws" that are often "toothed." It leads into the esophagus and stomach. The mouth also serves as an anus to eject wastes. I know some humans whose mouth performs the same dual function. Their nervous system consists of a main nerve ring in the central disk with radial branches extending into each arm.
So what tasty morsels do brittle stars choose when they are starved? They generally scavenge or chow down on detritus. I'm sure glad I'm not a brittle star! Some, like the soft sediment dwelling Ophiopsila californica burrow into the sand with their five arms extended to trap small plankton and organic matter. They are fairly common in our waters. I was surprised to learn that this species, as well as several other ophiuroids, can bioluminesce. This may be to ward off potential predators (although they have no eyes to see one coming).
IMHO one of the most beautiful of ophiuroids is the spiny brittlestar, Ophiothrix spiculata. It comes in a fairly wide variety of colors. I rarely see it on my dives off Catalina but it is very common on Anacapa, one of the northern Channel Islands, often covering the bottom. Sheephead predators are less common there. It is also found covering many of the offshore oil rigs in SoCal.
So you also want to know more about the sex life of these interesting critters? I thought so. In most species the sexes are separate but some are hermaphroditic (having both sex organs) or even protandric (beginning life as males and turning into females). They are broadcast spawners, releasing their sperm and eggs into the surrounding waters and hoping they win the lottery.
On some of my night dives in the past I was pleased to find one of our larger local species, the banded serpent star Ophioderma panamensis, spawning. At night they crawl out from under the rocks, amble up to the highest point on the reef that they can and rise up on their long arms. They then release their gametes into the water. To me the amazing thing is that this spawning was well synchronized as I found a large number of them doing it simultaneously... and it only lasted about 20-30 minutes before they crawled back down!
© 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Brittle star and underside of arms showing lateral spines; beautiful Ophiothrix spiculata on hand and mass of them on oil rig platform; burrowing brittlestar Ophiopsila californica in open and cluster in sand; Ophioderma panamense on rock and spawning on reef at night.
