DIVE DRY WITH DR. BILL #782: THE SLENDER, WHITE OR ELONGATE SEA PEN
Another marine species I have not written about in many years is the slender, elongate or white sea pen, Stylatula elongata. I don't see these often in the dive park but have found individuals and clusters of them at other sites around the island. They can be found on sandy, muddy or silty soft bottoms in areas with current. Members of the genus are found along the margins of the eastern and western Pacific Ocean and the western Atlantic coast.
This species is known from Alaska and British Columbia to San Diego. Another species S. gracilis, is found further south from southern Baja California to Panama and may actually be the same species. A second species that can be confused with the white sea pen is Virgularia ssp. and I'd need a microscope to detect the differences.
The sea pen is a cnidarian, related to jellyfish (aka sea jellies), anemones, corals and the like. Their closest relatives in the group are the gorgonians or sea fans. As such it bears polyps with eight tentacles surrounding the oral opening. These can be seen clearly in the enlarged image taken by my friend underwater photographer Kevin Lee. The tentacles are used to snag planktonic critters out of the surrounding waters to munch on. in turn these sea pens are chowed down on by nudibranchs such as Tritonia diomedea and Armina californica and certain species of sea stars.
Sea pens are actually colonies of individual polyps. The colony may reach 16" in length. Some polyps develop into specialized structures. These include the rigid stalk or rachis and its enlarged bulbous base or peduncle. This serves to anchor the sea pen into soft sediment. Other polyps populate the branches off the central stalk. They may be specialized to generate water flow (siphonozooids), to feed (autozooids which possess stinging nematocysts) and reproductive polyps.
Some species of sea pens are gonochoric, that is having only one gender per individual. Others are hermaphroditic, having both sex organs in the same individual. I try not to pose the question to the marine critters I dive with... "don't ask, don't tell." The gametes are spawned through the oral opening and upon fertilization become drifting larvae. Some do not spend much time in the planktonic stage and therefore are not good at dispersing over long distances. If the growth rings observed in cross sections of the stalk or rachis actually represent annual growth, some sea pens may live to 100 years.
Sea pens generally prefer somewhat quiet waters. However, if they are dislodged by bottom currents, they are capable of relocating and burying their peduncles back into the substrate. Although I've never observed this personally, some species are able to generate a greenish light through bioluminescence. Most of my encounters with them are during daylight so I may not be able to perceive this light. I'll have to try on some future night dive.
Now today's youth with their Bic and Uni-Ball pens may wonder why we apply the common name "sea pen" to these cnidarians. Well, if you took US history in school, you may be aware that in our early history feathers were often modified to serve as writing instruments to create documents such as the Declaration of Independence and the Constitution. Heck, in my youth I even used a feather pen and ink bottle, but never penned any document so crucial!
© 2018 Dr. Bill Bushing. For the entire archived set of over 775 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Examples of the slender sea pen from Catalina waters; image of sea pen polyps by Kevin Lee and the rachis at the base which embeds into the soft bottom thru the peduncle.
Another marine species I have not written about in many years is the slender, elongate or white sea pen, Stylatula elongata. I don't see these often in the dive park but have found individuals and clusters of them at other sites around the island. They can be found on sandy, muddy or silty soft bottoms in areas with current. Members of the genus are found along the margins of the eastern and western Pacific Ocean and the western Atlantic coast.
This species is known from Alaska and British Columbia to San Diego. Another species S. gracilis, is found further south from southern Baja California to Panama and may actually be the same species. A second species that can be confused with the white sea pen is Virgularia ssp. and I'd need a microscope to detect the differences.
The sea pen is a cnidarian, related to jellyfish (aka sea jellies), anemones, corals and the like. Their closest relatives in the group are the gorgonians or sea fans. As such it bears polyps with eight tentacles surrounding the oral opening. These can be seen clearly in the enlarged image taken by my friend underwater photographer Kevin Lee. The tentacles are used to snag planktonic critters out of the surrounding waters to munch on. in turn these sea pens are chowed down on by nudibranchs such as Tritonia diomedea and Armina californica and certain species of sea stars.
Sea pens are actually colonies of individual polyps. The colony may reach 16" in length. Some polyps develop into specialized structures. These include the rigid stalk or rachis and its enlarged bulbous base or peduncle. This serves to anchor the sea pen into soft sediment. Other polyps populate the branches off the central stalk. They may be specialized to generate water flow (siphonozooids), to feed (autozooids which possess stinging nematocysts) and reproductive polyps.
Some species of sea pens are gonochoric, that is having only one gender per individual. Others are hermaphroditic, having both sex organs in the same individual. I try not to pose the question to the marine critters I dive with... "don't ask, don't tell." The gametes are spawned through the oral opening and upon fertilization become drifting larvae. Some do not spend much time in the planktonic stage and therefore are not good at dispersing over long distances. If the growth rings observed in cross sections of the stalk or rachis actually represent annual growth, some sea pens may live to 100 years.
Sea pens generally prefer somewhat quiet waters. However, if they are dislodged by bottom currents, they are capable of relocating and burying their peduncles back into the substrate. Although I've never observed this personally, some species are able to generate a greenish light through bioluminescence. Most of my encounters with them are during daylight so I may not be able to perceive this light. I'll have to try on some future night dive.
Now today's youth with their Bic and Uni-Ball pens may wonder why we apply the common name "sea pen" to these cnidarians. Well, if you took US history in school, you may be aware that in our early history feathers were often modified to serve as writing instruments to create documents such as the Declaration of Independence and the Constitution. Heck, in my youth I even used a feather pen and ink bottle, but never penned any document so crucial!
© 2018 Dr. Bill Bushing. For the entire archived set of over 775 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Examples of the slender sea pen from Catalina waters; image of sea pen polyps by Kevin Lee and the rachis at the base which embeds into the soft bottom thru the peduncle.
