Dive Dry with Dr. Bill

#165: The PEN is Mightier than the SWORD

It is time once again to take pen to paper... or more accurately finger to keyboard. Although writer's cramp is far less likely using the computer, this time a pen would be more appropriate. "Why, Dr. Bill?" you ask. Because the subject of this week's column is none other than the elusive sea PEN... far mightier than the SWORDfish (although much less tasty I assume)! Speaking of which, I hope some of my readers were able to enjoy the sessions of the 4th International Billfish Symposium held in Avalon recently. Guy Harvey's underwater footage of billfish attacking baitballs was fantastic. I'm developing an unexpected interest in these fish after attending some of that scientific conference... but strictly as a scientist, not an angler.

Thanks to Scuba Luv, I've been able to dive sites near the Isthmus like Bird Rock, Ship Rock and Eagle Reef with greater frequency. I find myself spending more time at the deeper depths (100+ feet) over the sandy bottoms at the base of these rock formations. Recently I wrote about the brittle stars buried in the sand at Bird Rock. This column focuses on a field of sea pens I observed off Eagle Reef. The day was overcast so not much light penetrated to the sandy depths, and the contrast was poor. I was scanning the area for unusual critters like those brittle stars when I just made out the upright feather-like shape of a white sea pen. Then I started seeing more and more of them... a biological bonanza!

Sea pens are members of the phylum Cnidaria which includes the corals, sea anemones and jellyfish (oops, we're supposed to call them sea jellies now in this biologically PC world). Unlike the permanently attached corals or the drifting planktonic sea jellies (note I didn't say jellyfish), they have a quite different structure due to their habitat and lifestyle. The organism is actually a colony of polyps, like a gorgonian or reef-building hard corals, rather than a distinct individual. Much like a marine condominium. The white sea pen looks like the old quill pens that were used in drafting our own Declaration of Independence, Constitution and Bill of Rights that seem to be fading away under our current national administration. However, I won't go into politics at this point since the election is over. Note: my views do not necessarily reflect those of the paper's publisher!

This sea pen is whitish to gray in color with numerous branches extending off to either side of the central axis. The branches have tiny feeding polyps called autozoids with eight feather-like tentacles that capture plankton and organic matter drifting by. There are also specialized polyps called siphonozoids that circulate water past the feeding polyps. Of course water currents along the bottom also accomplish this. The central axis, which is actually a modified polyp, may reach two feet in length.

This sea pen is known from British Columbia to Isla Cedros off Baja California. It is found on soft bottoms (sand or mud) with the central axis buried in the substrate. Sea pens may be abundant in partly sheltered shallow bays and mudflats. Although common in those shallow habitats, they may be found to depths of 200 feet or more (well beyond my current dive limits!). The buried end of the sea pen has a bulb-like peduncle. Using peristaltic movement, similar to the way we move food through our intestines or a caterpillar crawls on a leaf, the peduncle is drilled into and secured under the sand or mud. The upper part of the central axis which is above the bottom is known as the rachis. Don't worry, you won't be tested on these technical terms.

Reproduction is accomplished by external fertilization. Sperm and eggs are released into the surrounding waters where, hopefully, they will meet one another some enchanted evening. This form of mating doesn't hold much interest for me... in fact, my days of successful reproduction hopefully ended nearly 22 years ago with my son Kevin! The eggs hatch into larvae which develop tentacles and settle to the bottom within a few days. This short duration in the plankton suggests that sea pens are not capable of long-distance dispersal during the larval stage.

Since I was filming at depth, and possibly slightly narced due to the increased nitrogen in my body, I forgot to test one very interesting phenomenon associated with these critters. If they are disturbed, for example by touching them, they bioluminesce producing cold greenish light just like a firefly. Other relatives like the sea pansy, subject of a much earlier column, will do the same thing. Some scientists feel this sudden emission of light may startle a predator and cause it to flee. One common predator, the sea slug Armina, does not seem to be fazed by this and may take large bites of the polyps before the entire sea pen retracts below the substrate, another of its defense strategies. Other sea slugs or nudibranchs, and even a unique brittle star called the basket star, are also known to feed on sea pens. I just wanted to get my filming done before my bottom time ran out and I ended up doing a deco (decompression) dive, so I didn't touch them at all.

Speaking of disturbance, I'm off in Florida for my father's memorial service after this column is sent to the paper. I sure hope that we will be spared weather-wise. As you all know, the Gulf region has experienced a bumper crop of hurricanes. I hope Hurricane Gamma, Delta and Epsilon don't decide to interrupt our celebration of Dad's life. I'll be glad to get back to the land of earthquakes after spending time with my mother and sisters.

© 2005 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Buy my "Munching and Mating in the Macrocystis" DVD so you can take Dr. Bill home with you... we'll both be glad you did!

White sea pens in the sandy bottom at the base of Eagle Reef.

This document maintained by Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia