DIVE DRY WITH DR. BILL #790: THEY ARE "STARS" TOO
I'm reminiscing... thinking about the way much of biology was taught back when I was in high school (just after the Middle Ages). The organizational scheme was largely based on taxonomy... the different classifications to which we humans have assigned the other species coexisting with us on Planet Ocean. Fortunately some of my biology classes at Harvard were organized based on ecological relationships. Ecology was becoming a mainstream branch of science back then, although even at my alma mater it had to do an often losing battle with the molecular biologists for office space.
Discussing species based on their taxonomy can often be quite informative. This is especially true today when species identification is based not just on a few physical or behavioral characteristics, but on molecular genetics. Yep, today biologists have a greater understanding that the diverse fields formerly recognized and taught needed a better synthesis for us to understand the complexity of the natural world. The molecular biologists who study DNA now apply that knowledge in determining the evolutionary origins of various species and how they are related, and thereby classified.
Today I'll focus on a group of marine critters in the phylum Echinodermata. Those of you who still remember your Biology 1 class probably recognize this group as the "spiny skinned" critters. Perhaps the best known members of this group are the sea stars (formerly known as starfish until the PC crowd intervened). Lesser knowns include sea urchins and sea cucumbers. The group I'm looking at today is the ophiuroids, commonly known as brittle stars.
Brittle stars can be extremely common even in our waters. However, unlike most of their sea star relatives, they generally hide under rocks and in crevices where they are less likely to be noticed. It's not that they are afraid of us divers. They hide from potential predators like sheephead and rock wrasse which often snatch them up if a rock is overturned.
Now the ophiuroids, like the sea stars and sea urchins, have armor plating on them for protection. Okay, so it really isn't much in the way of armor since their exoskeletons are made of calcium carbonate and related minerals. The wrasses don't seem to mind this, but I'm sure most humans would find brittle stars to be far too crunchy for the little meat inside!
Like most echinoderms, ophiuroids have five part radial symmetry. They have a central disk with usually five arms extending off of it, similar to most sea stars. These arms aree very flexible and allow the brittle star to crawl along the bottom, often fairly quickly if they are exposed when a diver overturns their rock. Their movement has earned them another common name, snake stars. The arms are often covered by spine-like extensions which may not only give them traction over muddy bottoms, but also serve as a weak defense.
The mouth sits on the underside of the central disk. It actually serves as both a mouth and an anus. I've met a few people who share that trait and I call them potty mouths. Brittle stars scavenge and feed on detritus although some will capture small prey like crustaceans and worms. We have a species in our waters, Ophiopsilla californica, that burrows into the sand and extends its arms above the substrate to capture plankton.
On several of my night dives in the past, I've observed spawning in a species of brittle star known as Ophioderma panamense. They are commonly called serpent stars due to their long, thin arms. At night, while hungry sheephead and other wrasses are sleeping, this brittle star comes out from under its rock and climbs up to the top of the reef. They will even move up into the "canopy" of the lower growing seaweeds to release their eggs and sperm.
On these dives I've observed dozens and dozens of these brittle stars out in the open. They synchronize their behavior so lots of the girls and boys come out of hiding at about the same time. This is important because they are broadcast spawners. The ovaries and testes in these brittle stars release their gametes through tiny genital slits on the underside of their central disk. The more engaging in this, the better as it helps ensure there will be fertilization and a new generation of these fascinating brittle stars.
© 2018 Dr. Bill Bushing. For the entire archived set of nearly 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Brittle star on diver's glove and oral opening on the underside of the disk also showing genital slits (white arrows); "spines" branching out from the arms and an Ophioderma releasing eggs on top of the reef.
I'm reminiscing... thinking about the way much of biology was taught back when I was in high school (just after the Middle Ages). The organizational scheme was largely based on taxonomy... the different classifications to which we humans have assigned the other species coexisting with us on Planet Ocean. Fortunately some of my biology classes at Harvard were organized based on ecological relationships. Ecology was becoming a mainstream branch of science back then, although even at my alma mater it had to do an often losing battle with the molecular biologists for office space.
Discussing species based on their taxonomy can often be quite informative. This is especially true today when species identification is based not just on a few physical or behavioral characteristics, but on molecular genetics. Yep, today biologists have a greater understanding that the diverse fields formerly recognized and taught needed a better synthesis for us to understand the complexity of the natural world. The molecular biologists who study DNA now apply that knowledge in determining the evolutionary origins of various species and how they are related, and thereby classified.
Today I'll focus on a group of marine critters in the phylum Echinodermata. Those of you who still remember your Biology 1 class probably recognize this group as the "spiny skinned" critters. Perhaps the best known members of this group are the sea stars (formerly known as starfish until the PC crowd intervened). Lesser knowns include sea urchins and sea cucumbers. The group I'm looking at today is the ophiuroids, commonly known as brittle stars.
Brittle stars can be extremely common even in our waters. However, unlike most of their sea star relatives, they generally hide under rocks and in crevices where they are less likely to be noticed. It's not that they are afraid of us divers. They hide from potential predators like sheephead and rock wrasse which often snatch them up if a rock is overturned.
Now the ophiuroids, like the sea stars and sea urchins, have armor plating on them for protection. Okay, so it really isn't much in the way of armor since their exoskeletons are made of calcium carbonate and related minerals. The wrasses don't seem to mind this, but I'm sure most humans would find brittle stars to be far too crunchy for the little meat inside!
Like most echinoderms, ophiuroids have five part radial symmetry. They have a central disk with usually five arms extending off of it, similar to most sea stars. These arms aree very flexible and allow the brittle star to crawl along the bottom, often fairly quickly if they are exposed when a diver overturns their rock. Their movement has earned them another common name, snake stars. The arms are often covered by spine-like extensions which may not only give them traction over muddy bottoms, but also serve as a weak defense.
The mouth sits on the underside of the central disk. It actually serves as both a mouth and an anus. I've met a few people who share that trait and I call them potty mouths. Brittle stars scavenge and feed on detritus although some will capture small prey like crustaceans and worms. We have a species in our waters, Ophiopsilla californica, that burrows into the sand and extends its arms above the substrate to capture plankton.
On several of my night dives in the past, I've observed spawning in a species of brittle star known as Ophioderma panamense. They are commonly called serpent stars due to their long, thin arms. At night, while hungry sheephead and other wrasses are sleeping, this brittle star comes out from under its rock and climbs up to the top of the reef. They will even move up into the "canopy" of the lower growing seaweeds to release their eggs and sperm.
On these dives I've observed dozens and dozens of these brittle stars out in the open. They synchronize their behavior so lots of the girls and boys come out of hiding at about the same time. This is important because they are broadcast spawners. The ovaries and testes in these brittle stars release their gametes through tiny genital slits on the underside of their central disk. The more engaging in this, the better as it helps ensure there will be fertilization and a new generation of these fascinating brittle stars.
© 2018 Dr. Bill Bushing. For the entire archived set of nearly 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Brittle star on diver's glove and oral opening on the underside of the disk also showing genital slits (white arrows); "spines" branching out from the arms and an Ophioderma releasing eggs on top of the reef.
