Who are the polychaetes? You are probably quite familiar with the earthworm. It is a member of the phylum Annelida; collectively, the species in this group are called the "segmented worms." The segmented worms occur worldwide, from polar regions to the Tropics. In size, they range from less than a millimeter to greater than a meter in length. The group includes many close relatives to our garden variety earthworm, and even some more notorious worms -- the leeches. Many species of segmented worms are found in terrestrial and freshwater ecosystems. Mesenchytraeus provides an example from a more extreme environment; they live in glacial ice and get into trouble if exposed to temperatures much higher than 0° C. However, the vast majority of the diversity in the phylum Annelida, about 8,000 species, are ocean-dwelling (marine) forms called polychaetes.
The king rag worm, Alitta virens.
Polychaete worms occur throughout the world ocean. Some species can be found in the water column, but most of the worms are benthic, meaning they are associated with the sea floor. Their life history often includes a larval form that exists in the water column for a time and then settles to the bottom.
I selected the polychaetes for mention here because the average person probably associates the word "worm" with a bland, slimy creature that creeps its elongate body quietly through the Earth beneath our feet. Therefore, they are out of sight and out of mind, and that's all the better for us.
The reality is that the "worm" is a catchall term that applies to a number of wildly different and highly successful groups of animals. Additionally, many of the species exhibit beautiful forms (in the eye of the beholder) and unique features.
Closeup of the king rag worm.
They vary considerably in their feeding ecology. Most polychaete worms are free living, but some are parasitic, even invading other worms. You will be happy to know that none of them infect humans; you need not worry about being colonized the next time you hit the surf.
Scanning electron microscope image of a scale worm's mouth.
Many polychaetes, like the earthworm, are deposit feeders. As they move through sediments, the sediments move through them, and the worms digest any organic material among the grains. The king rag worm is one example, working its way through mudflats and achieving lengths greater than a meter.
Whale-bone worms, burrowed into the bone of a dead whale.
Many other species are predatory and bear jaws. In some species the jaws are eversible, and snap at everything in their path. Check out this bobbit worm clip, linked here and inserted at the bottom of this post. The BBS series The Blue Planet also includes footage of a polychaete crawling through a sponge in search of a meal, snapping its jaws at the small crustaceans within. To the naked eye, their feeding structures do not seem terribly daunting, but examination of these worms with a light microscope or scanning electron microscope reveal a more horrific, alien world.
If you want an even stranger example, some polychaete worms specialize in dining on the bones of dead marine mammals. The worms burrow into the bones of whales and other vertebrates that have fallen to the sea floor, where they feast and play an important role in the process of decomposition.
A sabellid worm growing among a coral colony.
Suspension feeding is a strategy for some species. This is practiced by the sabellid fan worms, for instance, which live in a tube of their own construction. (This in itself is accomplished in an interesting way.) Then, the worms extend their elaborate feeding structure into the water column to catch the unlucky passersby.
A Christmas tree worm, the body of which is within a burrow in the coral.
Some polychaetes make there homes in the structures produced by other organisms. Christmas tree worms, for example, live in coral colonies and add small touches of color to the reefs. These worms, as well as the fan worms mentioned above, are capable of detecting differences in light and can quickly retract their feeding structures. They're clearly the inspiration behind some of the "plant" life in James Cameron's movie Avatar.
Of course, there is more to an animal than the manner by which it obtains food resources. The path to reproduction in polychaetes can be a strange one too.
Large aggregation of Christmas tree worms, differing in color.
Unlike flatworms, most polychaete worms have separate sexes, and, in most cases, fertilization occurs externally. Some species are asexual and reproduce by budding. Myrianida, for example, is a worm that at times appears to have a tail, only the tail actually consists of new worms that are produced asexually by the parent. There is a bit more to this story, of course, so I encourage you to look into it! The Palolo worm is another interesting example, where the rear end of the animal consists of segments packed with sperm and egg (called epitokes) that rise to the sea surface where the gametes are released. These nutrient-rich segments are actually collected for food in some regions, yum!
Myrianida pachycera and its tail of worms.
Tomopteris, a deep-sea swimming, bioluminescent species.
As is the case with other sexually reproducing species, following fertilization of the egg by a sperm cell, the egg soon divides to produce new individuals of many hundreds of cells. Many marine invertebrates exhibit a life cycle that includes a free-living, swimming larval form. Many species of polychaetes produce larvae called trochophores, some of which are non-feeding larvae that continue to develop from the nutrients remaining from the egg's yolk. I could carry on with images and descriptions of reproductive modes among the polychaetes and the diversity of bizarre larval forms.
Plenty of other species would have been great candidates for this page as well (e.g., the parchment worm Chaetopterus). While you can investigate this on your own, taking a peek at what is known about the thousands of species have been described, keep in mind that many species await discovery. On a regular basis, we make new finds everywhere from the muddy floor of a local coastal bay to dark, deep-sea waters that remain largely unexplored. We might see a doubling of the known species diversity in this group in the coming decades. This is not too surprising.
Our technology is better, giving us a better reach into new environments. Still, more goes into the discovery of new species than simply being in the right place. You must have someone with knowledge of the group and who can recognize the form as unique. On top of that, the person must have some level of excitement about the find and the willingness to spend time doing the work of formally describing it to the scientific community. But how in the world does someone develop an interest in worms in the first place, especially when they are just slimy garden creatures that hide in the soil?
I have a friend who will argue that sea slugs are the world's most beautiful animals. I am not so sure that the word "slug" is one that you mind immediately links with "beauty," but we are not talking about your everyday garden slime. The sea slugs, or nudibranchs (for "naked gill"), consist of several thousand species, the vast majority of which inhabit the tropical marine habitats. These animals are close relatives of the snails. They are hermaphrodites, as each individual possesses fully functioning male and female reproductive systems. Sea slugs exhibit a wide array of color patterns and behaviors, and some of their characteristics are unique among Kingdom Animalia.
Glaucus atlanticus
One species, Glaucus atlanticus, lives in the water column. It floats upside down at the sea surface and feeds upon jellies and their close relatives. They are able to ingest and incorporate the stinging cells of the jellies without trigging the cells and then use them for their own defense.
Flabellina iodinea
The Spanish shawl (Flabellina iodinea) can actually be found in local waters off California. These sea slugs consume tissue from animals such as hydroids and anemones. In doing so, they ingest stinging cells without causing them to discharge. The Spanish shawl can then later use the stinging cells of its prey for its own defense.
Placida sp.
In Biology, one learns that plants, algae and some bacteria are able to synthesis their own organic compounds utilizing energy from sunlight. That is, such organisms are photosynthetic. Plants and algae have special structures inside of their cells called chloroplasts where the process of photosynthesis occurs. In the same breath, teachers will explain that animals do not have this ability and must consume organic molecules from other organisms. While all of this is true, a few of the sea slugs do present an unusual case. Species from the genus Placida feed on algae. They are able to ingest the chloroplasts from the algal cells, avoid destroying them, and then store them. The chloroplasts then continue to function in the body of the sea slug, making these sea slugs the only animals on the plant to be "solar powered."
Some sea slugs are predators that have no trouble ingesting other members of their group. Navanax is a large predatory slug that occurs off southern California. It is no uncommon to catch it in the act of ingesting another nudibranch. The video here shows yet another predatory species in action.
The variety in shape and color pattern is immense, so it actually is a bit difficult to ignore the thousands of beautiful species that we have failed to highlight here. The mission is more to reveal their existence. If these have caught your eye, you can explore the group further by reading through the short articles at Sea Slug Forum. That's right; there is a forum dedicated to these creatures.
I must start with the amphipods. For a moment, I tried to avoid it so that I would not feel so predictable, but I could not let them go. It was a member from this group, an animal no greater than two centimeters in length, that dragged me into the fields of taxonomy and marine science.
What are these? Amphipods are crustaceans, close relatives to the ones you might find on a dinner plate such as crabs, shrimp, and lobsters. The are about 6,000 species in this group. That means there are more described species of these mysterious things called amphipods than there are mammals. Amphipods are ubiquitous in the marine realm. I'll show you some that you can find within 60 seconds of stepping onto the beach and some you will likely never see, unless you have plans to visit the ocean depths.
The beach hopper is one of thousands of different species of amphipods.
Remember walking along the beach, perhaps a little too close to that rotting pile of kelp, when you saw some tiny organisms jump about in front of you? Beach hoppers are amphipods. I confess that I am no big fan of these particular "pods," but they are among the more conspicuous members of the group. Of course, they also play a role in making those piles of algae disappear so that you can better enjoy your experience at the beach. And can you really look at the picture and tell me it's not cute?
Alicellagigantea from deep waters.
If we look at a couple of close relatives from the deep sea, you'll see that not all amphipods are created equal. Alicellagigantea is a touch bigger, reaching lengths of more than a foot. Aren't you glad that the beach hoppers are the small ones? Despite their impressive size (for an amphipod), these look pretty bland to me.
Epimeria rubrieques, a poorly known deep-sea species.
There are some other forms that have more eye-catching ornamentation. Check out the spines on Epimeria, for instance. I don't know why it needs such ornamentation or what it is doing way down there in the ocean depths, but it has one cool suit of armor (one that presumably remains unseen by its neighbors, as there is no light where it dwells).
If we do head upward, toward the sea surface, we can take a look at another kind of amphipod. Some species are associated with other floating organisms, like jellies and salps. "Hyperiid amphipods" actually live inside these larger, floating animals.
Phronima emerging from a salp
Members of the genus Phronima are great examples. You will have to trust me that it looks a touch scarier when you peer at it through a microscope. Evidently, this animal was the inspiration for the queen in the movie Alien.
Whale lice aggregating on fluke of dead gray whale.
Ventral surface of a whale louse from humpback whale.
I could share many more examples, from amphipods that burrow into kelp to terrestrial forms you might find in soil with potted plants. However, I will finish this post with mention of some epibiotic forms that drew my attention as a college student and that continue to interest me today. These are the skeleton shrimp and the whale-lice. Yes, that's right: whales have lice. Of course, they're crustaceans, not the insects that normally come to mind when you hear the word lice. They earned that name because of their unusual life history. Whale lice are found exclusively on the surface of whales, dolphins or porpoises. They do not swim at any stage of their life cycle; they spend their entire life clinging to the skin of their blubbery mammalian hosts. In this lineage, natural selection then has channeled the mutations occurring over millennia into some pretty interesting anatomy. Whale lice are dorso-ventrally flattened and possess recurved claws on their limbs, for it pays to be able to stay on the whale. The sometimes occur in large numbers. Interestingly, a given species of whale louse is specific to a given host species. For instance, the gray whales that migrate along the West Coast of North America every year are associated with only three species of whale lice, and these three species are not found anywhere else.
A close relative of the whale lice is the skeleton shrimp, so called because of their narrow, elongate bodies. They also have been referred to as the "praying mantis of the sea" because of their large forelimbs (relative to body size) and characteristic posture.
Rendering of new skeleton shrimp from California.
There are about 350 species of skeleton shrimp known to science and many more waiting to be discovered. I have found them living on everything from seaweed to sea grasses to sea stars to stone fish. They cling to other organisms with their rear limbs and then scavenge for food with their front two pairs of limbs and rasping mouthparts. To the naked eye, they could pass for pocket lint. Under the microscope, some look more like Godzilla. The skeleton shrimp often occur in abundance. The groom the organisms to which they are attached but serve as food for small fishes. The exhibit a variety of behaviors that make them interesting to watch, from their quick, inchworm-like movements to male-to-male combat.
Caprella californica.
I just shared seven of the 6000+ species in this group. We'll call it quits at that. These animals play enormously important roles in marine ecosystems, admittedly most often by serving as links in marine food webs. Still, with the diversity of their form and the wide array of habitats in which they are found, they make my short list of invertebrate animals that deserve mention.
One step into the Natural History Museum of Los Angeles County yields a view of the "dueling dinosaurs," the skeletons of a Triceratops and a Tyrannosaurus posed in battle. Should you wander into the main lobby of the Natural History Museum at the Smithsonian, you will see a large African elephant. What might you see when you visit the Field Museum in Chicago? Why, a Tyrannosaurus and an elephant, of course.
Dueling Dinosaurs, Natural History Museum of LA County
African elephant on "center stage" at the Smithsonian.
It is not terribly surprising. When the ceiling of the entrance lobby is 40 feet from the floor, a sea star on a pedestal is not quite a feast for the eyes. An 18-foot by 38-foot lizard certainly does the trick.
The purpose of these institutions is to inspire wonder about the natural world and to educate the public about natural history. Like any business, they also want to get people in the door, and dinosaurs do just that. Recent expansions of the Smithsonian and the Natural History Museum of LA County could have been designed with an emphasis on just about any aspect of our natural world and its change through time. Still, each of these institutions put dollars and focus exactly where one might expect: dinosaur halls.
What do I have against dinosaurs (and elephants)? Nothing. They are fascinating animals. They simply do not, in my opinion, deserve such a disproportionate amount of our attention. Is our interest in different forms of life on Earth directly related to size?
Kids know only what we show them.
We recognize about 350 species of dinosaurs today, though many more likely roamed the Earth over time. We cannot argue that they were not important; after all, their demise brought opportunity to our early mammalian ancestors, the ability to radiate and diversify. The thought is that we (humans) might not be here otherwise. Nevertheless, 350 species is taxonomic peanuts. Scientists estimate that there are roughly 80,000 species of snails on the planet. Let's compare. 350. 80,000. 350. 80,000. There are approximately 60,000 species of spiders. We have recorded another 60,000 species of crustaceans. The list continues.
Of course, we show bias toward other organisms as well. What of Flipper? Dolphins exhibit incredible behaviors and learning ability. It helps that they are not only represented by fossil relics and that we can witness their behaviors firsthand, even swim with them for a penny or two. I will admit there are some marvelous forms, such as river dolphins, but the diversity of living species in this group is not impressive. All told, whales, dolphins and porpoises represent about 80 species.
The cookiecutter shark. Read how they got their name!
Aside from pets, another group comes to mind: sharks. Even within this group, we are biased, most of the attention going to the "man-eaters." How many sharks can you name? Ten? There are nearly 400 valid species of sharks, and I will admit that many of them are quite interesting. Favorites include the cookiecutter shark (Isistiusbrasiliensis) and the wobbegong sharks (family Orectolobidae). For most people, however, it is probably the picture of a single species that the word "shark" conjures, the great white.
A wobbegong shark, cryptic against coral.
It makes you wonder: What else is out there? Well, of more than 30 phyla (major groups) of animals on Earth, all of the vertebrate organisms, those with a backbone, fit neatly into one subphylum (Vertebrata). All of the fishes. All of the frogs, toads, newts and salamanders. All of the snakes, lizards, turtles and tortoises. All of the birds. All of the mammals, from shrews to bats to primates to lions, tigers and bears (oh my!).
Collectively, the vertebrate animals represent about 5% of the animal species described to date.
Most people rarely consider the other 95% of animal life on the planet. These other species often occur in great abundance, serving as critical links in food webs. Some of them build ornate structures and exhibit unusual behaviors. Many of them occur in most unusual habitats. The majority of invertebrate species are harmless, but some cause disease. A few of them are just as deadly as the toothy vertebrates described above.
We can examine some of these life forms here, not so much as an attempt to convince you that they belong in a museum lobby as to give you a slightly broader window on biology and to suggest that some lesser known forms are worthy of notice.
In each upcoming post, I will steer you away from the large, vertebrate animals in order to highlight some of the spineless wonders that might otherwise remain unknown to you.
Plenty of other species would have been great candidates for this page as well (e.g., the parchment worm Chaetopterus). While you can investigate this on your own, taking a peek at what is known about the thousands of species have been described, keep in mind that many species await discovery. On a regular basis, we make new finds everywhere from the muddy floor of a local coastal bay to dark, deep-sea waters that remain largely unexplored. We might see a doubling of the known species diversity in this group in the coming decades. This is not too surprising.
