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Dive Dry with Dr. Bill

#531: Sea Elephants and Butterflies

Or, if you prefer to be scientific... heteropods and pteropods. No, this week's column has nothing to do with the issue of gay marriage and DOMA currently being reviewed by the Supreme Court. However, I will go on record as saying that I would like my gay and lesbian friends to have equal rights under the law. After all, why shouldn't they be as miserable as my straight friends who have married?

Last week I wrote about the jelly-like creatures we've been seeing in our waters lately, so I thought I'd elaborate further on a few that most people never experience. Heteropods and pteropods are names for different groups of snails that swim out in the open water with the plankton. The "pod" in their name refers to their feet, although no podiatrist worth their salt would confuse a snail's foot with a human one (except maybe mine).

Snails are members of the group referred to by us scientific nerds as gastropods. That name translates from Greek into "stomach foot" because the foot that snails use to crawl on land or in the ocean is located just below the digestive tract. My stomach is located above my feet too, although it is starting to droop down towards them.

These are groups I'm not terribly familiar with so I spent a good deal of time researching them on the Internet. After doing so, I think I am even more confused than I was before I started! As one who received his formal training in marine biology many decades ago, I've used the words pteropod to describe planktonic snails like the sea butterflies and heteropod to describe those like the sea elephants.

The scientific name pteropod was first created in 1804 by Georges Cuvier for a group of free swimming snails that dwelt in the open ocean. It means "wing foot." In more recent times scientists replaced that name with two different ones, the Thecosomata and Gymnosomata. Don't worry, you won't be quizzed on those! And the heteropods are now known as the Pterotracheoidea. Now even as a marine biologist, I prefer the simpler names... and simpler times.

So let's look at one of the thecosomate "pteropods," the sea butterfly Corolla. Thanks to friend, diver and underwater photographer extraordinaire Kevin Lee I am able to include images he has taken of one species. This group produces large feeding webs of sticky mucus top capture both plant and small animal plankton to munch on. These webs may be as large as six feet in diameter!

Their relatives, the gymnosome "pteropods," swim actively in the plankton and prey on their cousins the thecosomes. Their mouths are equipped with the typical feeding apparatus of snails, the radula, with its many teeth-like scraping plates. Their mouth parts may also include hooks, adhesive tentacles and a jaw to grasp and swallow prey.

As for the heteropods (or pterotracheiods if you prefer), I was able to film one from Cabrillo Mole as I waited for a boat. Diver Anastasia Laity allowed me to use one of her images so you could see what they look like in the water. The species is named Carinaria cristata and is normally found further north off central California but wanders into our waters at times.

Carinaria is a "vicious" predator that feeds selectively on salps, the open water tunicates I talked about last week. They will also munch on other plankton such as copepods, but they do not prefer them. I guess copepods must be like brussel sprouts to them. They will even cannibalize their own species, frequently taking smaller individuals as a tasty snack. Since they are predators, and feed higher on the food chain than the sea butterflies, heteropods are present in lower numbers than the more common pteropods.

The "heteropods" are primarily subtropical, but also enter our waters. Their snail feet are flattened laterally and flap about serving like fins to propel the predator. Although the foot is located on their ventral side, they swim upside-down. Fairly well-developed eyes allow them to image their prey. They posses a feeding radula and a trunk-like proboscis that gives them their common name of "sea elephants."

No report on the critters of the briny deep would be complete without a little talk about s## (er, I mean reproduction). These jelly-like open ocean molluscs believe in internal fertilization rather than just playing the lottery by casting sperm and eggs out into the cruel, cold ocean. Males often have very pronounced... ah, er, um... you know whats.

Thecosome pteropods like the sea butterflies begin life as males and then turn into females. If you remember Dr. Bill's Sex Education 101 class, that's referred to as protandrous ("first male") hermaphroditism. On the other hand, the gymnosome pteropods have both sex organs at the same time and are known as simultaneous hermaphrodites. The pterotracheiods or heteropods are sexually dioecious, having separate genders. Studies of the species Carinaria cristata in the California Current indicate that there are nearly equal numbers of males and females during mating season around May.

Of course these planktonic snails have had to undergo a few evolutionary modifications to survive drifting in the water column. Wearing a heavy shell for defense like most marine snails would sink them in a second so their shells, if present, are tiny. They are also thin and less calcified, sometimes even composed of a cartilage-like substance to reduce weight. The gelatinous, almost transparent bodies make it more difficult for predators to see them in the open ocean. Crawling on a normal snail's foot would be useless, so theirs are adapted to provide propulsion in the water column. So now you know all I know about these strange snails!

© 2013 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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A heteropod or sea elephant I filmed from Cabrillo Mole and Anastasia Laity's shot of one in the water;
Kevin Lee's images of the pteropod Corolla spectabilis (sea butterfly)

This document maintained by Dr. Bill Bushing.
Material and images © 2013 Star Thrower Educational Multimedia