DIVE DRY WITH DR. BILL #763: MR. LIMPET
Divers are ecstatic when they encounter a form of shell-less snail known as a nudibranch. Even landlubbers "lub" snails. They chow down on escargot and abalone. However, other than as an object of beauty or an epicurian delight, snails often get no respect. Of course I focus not on individual groups of marine critters but on entire ecosystems, so every species has interest to me. Yes, even snails.
In today's column I want to focus on a group of snails often encountered in the intertidal by even those who fear venturing into the deeper depths. I'm talking about limpets, especially the species known as the kelp or seaweed limpet, Discurria insessa. Back in the old days when I first arrived on Catalina Island, this species was known as Notoacmea insessa but then the molecular biologists decided to reclassify it. I can hardly keep up with their changes!
These snails have a shell not unlike an abalone. It is wide open on the bottom with no protective trap door or operculum. They rely on their muscular "foot" to hold tight to the substrate to prevent predation and dessication. In the case of the kelp limpet, the substrate isn't an intertidal rock but the stipe of a feather boa kelp, Egregia menziesi. Attaching to kelp may sound like a slippery slope, but these limpets burrow into the feather boa for added protection.
This species has a fairly wide geographical range from southern Alaska to Baja California. Not coincidentally, this is also the range of the feather boa kelp. Although this snail was often thought to live only on feather boa kelp, I have observed it on Pterygophora californica off San Miguel in the northern Channel Islands.
The shell of the kelp limpet is less than one inch long. It is fairly smooth with fine radial lines, thus reducing any resistance to turbulent water motion that is experienced in the immediate subtidal where feather boa lives. It is said to have a fairly high peaked shell, but I've seen ones that are fairly low in shape.
The deep depressions within which these limpets shelter are created when the limpet chows down on the feather boa. It will also munch on other small algae which may attach to the kelp. In turn it may be eaten by sea stars (aka starfish) such as the ochre star, Pisaster ochraceus.
Although these snails may spawn any time of year (my preference), such activity apparently peaks in spring and summer. You know, when a young limpet's fancy turns toward... you know what! Eggs and sperm are cast out into the surrounding ocean and fertilization occurs externally. Within as little as a few days, the resulting larvae are ready to settle down on an unsuspecting feather boa. They generally choose older feather boa "plants" that have already reproduced.
The host kelp grows most rapidly during summer. However during fall and winter, growth declines and storms often decimate the feather boa. The depressions created by the kelp limpets further weaken the alga. This is not a particularly wise strategy on the part of the limpet since it accelerates the demise of its host. The high winter mortality greatly reduces both the limpet and kelp populations, but in spring both usually recover.
Of course this particular species of limpet has chosen an unusual "habitat." Many other species of limpets live on rocky shores, feeding on various algae that grow there. Most are fairly small although the related owl limpet, Lottia gigantea, may grow to 3 1/2 inches.
Back in the early 1970s when I was teaching at the Catalina Island School (Toyon Bay), I took my students on a "survival" hike to Little Harbor. They were supposed to eat off the land and sea for two days. For my dinner, I prepared a stew of owl limpets and cactus (nopales). I didn't see many of my students preparing similar dishes. It turned out most of them had stopped at the Airport-in-the-Sky on the way for buffalo burgers!
© 2018 Dr. Bill Bushing. For the entire archived set of over 750 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: The kelp limpet on feather boa (courtesy of Stef Flax) and a close-up of the limpet (courtesy of Jerry Kirkhart); kelp limpet on Pterygophora kelp and owl limpet on rocky substrate in the intertidal.
Divers are ecstatic when they encounter a form of shell-less snail known as a nudibranch. Even landlubbers "lub" snails. They chow down on escargot and abalone. However, other than as an object of beauty or an epicurian delight, snails often get no respect. Of course I focus not on individual groups of marine critters but on entire ecosystems, so every species has interest to me. Yes, even snails.
In today's column I want to focus on a group of snails often encountered in the intertidal by even those who fear venturing into the deeper depths. I'm talking about limpets, especially the species known as the kelp or seaweed limpet, Discurria insessa. Back in the old days when I first arrived on Catalina Island, this species was known as Notoacmea insessa but then the molecular biologists decided to reclassify it. I can hardly keep up with their changes!
These snails have a shell not unlike an abalone. It is wide open on the bottom with no protective trap door or operculum. They rely on their muscular "foot" to hold tight to the substrate to prevent predation and dessication. In the case of the kelp limpet, the substrate isn't an intertidal rock but the stipe of a feather boa kelp, Egregia menziesi. Attaching to kelp may sound like a slippery slope, but these limpets burrow into the feather boa for added protection.
This species has a fairly wide geographical range from southern Alaska to Baja California. Not coincidentally, this is also the range of the feather boa kelp. Although this snail was often thought to live only on feather boa kelp, I have observed it on Pterygophora californica off San Miguel in the northern Channel Islands.
The shell of the kelp limpet is less than one inch long. It is fairly smooth with fine radial lines, thus reducing any resistance to turbulent water motion that is experienced in the immediate subtidal where feather boa lives. It is said to have a fairly high peaked shell, but I've seen ones that are fairly low in shape.
The deep depressions within which these limpets shelter are created when the limpet chows down on the feather boa. It will also munch on other small algae which may attach to the kelp. In turn it may be eaten by sea stars (aka starfish) such as the ochre star, Pisaster ochraceus.
Although these snails may spawn any time of year (my preference), such activity apparently peaks in spring and summer. You know, when a young limpet's fancy turns toward... you know what! Eggs and sperm are cast out into the surrounding ocean and fertilization occurs externally. Within as little as a few days, the resulting larvae are ready to settle down on an unsuspecting feather boa. They generally choose older feather boa "plants" that have already reproduced.
The host kelp grows most rapidly during summer. However during fall and winter, growth declines and storms often decimate the feather boa. The depressions created by the kelp limpets further weaken the alga. This is not a particularly wise strategy on the part of the limpet since it accelerates the demise of its host. The high winter mortality greatly reduces both the limpet and kelp populations, but in spring both usually recover.
Of course this particular species of limpet has chosen an unusual "habitat." Many other species of limpets live on rocky shores, feeding on various algae that grow there. Most are fairly small although the related owl limpet, Lottia gigantea, may grow to 3 1/2 inches.
Back in the early 1970s when I was teaching at the Catalina Island School (Toyon Bay), I took my students on a "survival" hike to Little Harbor. They were supposed to eat off the land and sea for two days. For my dinner, I prepared a stew of owl limpets and cactus (nopales). I didn't see many of my students preparing similar dishes. It turned out most of them had stopped at the Airport-in-the-Sky on the way for buffalo burgers!
© 2018 Dr. Bill Bushing. For the entire archived set of over 750 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: The kelp limpet on feather boa (courtesy of Stef Flax) and a close-up of the limpet (courtesy of Jerry Kirkhart); kelp limpet on Pterygophora kelp and owl limpet on rocky substrate in the intertidal.
