It is hard to imagine that you have not heard of its close relative, the sea star. Sea stars are one of the exceptions; these particular invertebrates have not escaped public notice. Even small children can recognize the sea star (sometimes called "starfish"). You can spot them everywhere from children's books to jewelry, and a visit to just about any public aquarium will reveal a touch tank with one or more of these animals. Of course, many species of sea stars can be found in the intertidal zone along our shores, and that proximity also raises their profile.
Ophiosamus, perhaps a couple hundred million years old!
Brittle stars, on the other hand, are not seen clinging to the surfaces of wave-swept rocks. The brittle star is a close genealogical relative of the sea star, yet it is an animal that is rarely seen at all by the average person.
Brittle stars are members of the phylum Echinodermata ("spiny-skinned") which includes the sea star, sea urchin, sand dollar, sea cucumbers, crinoids and their kin. More than 2,000 species of brittle stars have been described to date, though specialists who study this group almost certainly would tell you than many species remain undiscovered. That's not surprising, given the ecology of these animals. The group is represented in the fossil record dating back into the Early Ordovician, about 480 million years ago.
External anatomy of the brittlestar (by Mathias Oddo)
The mouth of a brittle star.
Brittle stars are exclusively marine, benthic organisms. That is to say, they are found only in the saltwater environments, and, at least as adults, they are associated with the seafloor. Of course, as the oceans cover 71% of our planet, that's still quite a bit of territory. Brittle stars can be found from the high intertidal zone to depths exceeding 8000 meters! In some areas, they occur in incredible abundance, reaching hundreds to thousands of individuals per square meter (see Hughes, 1998).
The general body plan of a brittle star is not terribly different from that of sea stars. Most (but not all) species in the group possess a well defined central disk from which five arms radiate. The animal possesses an endoskeleton that consists of bony elements called ossicles. In most species, the arms break relatively easily, perhaps as a defensive mechanism. Like other echinoderms, the brittle stars possess a network of canals, collectively called the water vascular system, that extend from the central disk through the arms. The system is used for locomotion as well as the transport of nutrients and wastes. While the water vascular system does permit them to shuffle water into and out of their tube feet, these animals are able to use their arms to move rather quickly along the substrate, whether toward a food item or to shelter (see video). The digestive system includes a central mouth that is surrounded by an interesting feeding structure composed of five jaws. The gut of the brittle star is incomplete, meaning they lack an anus and therefore the mouth is a two-way street. (Be thankful that our guts are complete.) Brittle stars appear to have an incredibly ability to respond to stimuli such as light and chemicals in their environment. Not everything is understood about their sensory systems, yet some interesting finds have been made in recent years. For instance, close examination of the endoskeletal features in the species Ophiocoma wendtii show that its upper (aboral) surface consists of many tiny, calcitic lenses that might collectively function like a compound eye (Aizenberg et al., 2001). In other words, the whole surface of the animal could be functioning as a single eye.
Brittle stars feed in a variety of ways. Some species are predators, but most species are either scavengers or suspension feeders, capturing organic matter that they encounter on the seafloor (e.g., see Loo et al., 1996 and the video here).
Larva of a brittle star.
Species of brittle stars produce both asexually and sexually. When it comes to sexual reproduction, these animals spawn, releasing their gametes into the water column where fertilization can then occur. Their life cycle includes a pelagic larval stage, the general term for which is "pluteus larva." Naturally, variations exist among species in everything from larval forms to the finer details of the life cycles. The common thread is that a larva persists in the water column for some time before settling to the substrate and continuing its development into an adult. For an example of a study of their reproductive strategies, see McGovern (2002).
Brittle stars on the seafloor at depth.
If you have ever walked in a rocky intertidal zone or you have gone diving on a reef, you undoubtedly have been within reach of the brittle star, perhaps without ever knowing it. Of course, some species are more conspicuous, particularly those that feed by extending their thin arms into the water column in order to capture their meals.
Dense aggregation of brittle stars.
With half of the planet's surface covered by expansive deep-sea plains, it is probably safe to assume that brittle stars are among the more common macroscopic organisms on Earth. They're just largely out of reach, out of sight, and therefore out of mind.
That's too bad, because they are fascinating organisms that hold physiological and behavioral secrets we have yet to uncover. The brief notes here barely scratch the surface, so I encourage you to explore the group further!
Several brittle stars on a sea star in California waters.
Diver above brittle-star-covered rocks in California (from CA Diver Magazine).
Among the invertebrates, the phylum Cnidaria is one of the best known to the layperson. This group includes everything from sea anemones to corals and jellies. The cnidarians are named for their cnidae; these are the stinging cells unique to their tissues. Every cnidarian possesses cnidae, but the species vary considerably when it comes to the structure and number of these cells. You can wander along the beaches of California and touch a giant green anemone or moon jelly without detecting a sting at all, yet one chance collision with a box jelly in Australian waters can send you to the hospital writhing in pain.
Originally, I thought I might shed some light on the true jellies (also called "jellyfish," though they are not fishes). While the jellies do share a fairly basic body plan, they exhibit a truly extraordinary variety of size, shape and color. However, the objective is to highlight groups that fly (or float) under our radar, ones that get little attention in mainstream media and even in our educational institutions.
The morphology of the by-the-wind sailor.
Physalia physalia washed ashore. Watch your step!
One group of cnidarians, the siphonophores, deserves more attention. The siphonophores often are mistaken for jellies. It might not apply in every case, but jellies typically can be identified by their conspicuously bell-shaped medusa. Siphonophores, on the other hand, do not exhibit this familiar shape. In fact, siphonophores are not individual organisms but colonial cnidarians. The "body" is created by a large number of individuals, referred to as zooids, that usually have highly specialized functions. Different zooids of a given colony function in feeding and reproduction. Some zooids, packed with cnidae, serve in defense.
Group of Velella velella, the by-the-wind-sailor.
Often, the colony is associated with a gas-filled float called a pneumatophore and specialized "swimming bells" called nectophores. The floats help the colony to maintain its position at the surface, or elsewhere in the water column, and the nectophores help to propel the colony. Many siphonophores lack structures for swimming and simply "go with the flow.
The blue button, Porpita porpita.
"Physalia physalis, the Portuguese Man-o-War, is probably the most famous of the siphonophores because it is relatively common and issues a painful sting and the marks to go with it. Most people refer to the Portuguese Man-o-War as a jelly, but it is actually a siphonophore. It has a large, highly distinctive float that permits it to remain at the surface and trail the colony through the water. By doing so, the tentacles and zooids can capture small prey items such as small fishes and crustaceans. It is an impressive sight, with an impressive sting that earns the colony respect on first contact.
Velella that have drifted ashore in masses.
Other small, surface-dwelling siphonophores include the blue button (Porpitaporpita) and the by-the-wind sailor (Velellavelella). These drifters are at the mercy of the currents and winds. On occasion, the by-the-wind sailor washes ashore in great numbers, where the individual colonies die. The first time I happened upon dried masses of these on the beach, I felt more like I was walking through large drifts of dead leaves (see picture of a smaller aggregation).
Rhizophysa, 3 views of one colony.
Many of the species discovered to date are fragile deep-sea colonies. Discoveries of new species of siphonophores are not uncommon. Some species can be observed in relatively shallow waters, where divers might have a chance encounter. These include species such as Rhizophysa, which can extend its feeding tentacles to increase its size from four or so inches to a couple of feet. When it comes to size, we cannot escape mention of the species Prayadubia, the giant siphonophore. Colonies of this midwater species can achieve lengths of 40 meters. In other words, if this was a single organism, it would technically be the longest animal on the planet (longer than every dinosaur, except perhaps Diplodocus).
A bioluminescent deep-sea species, found beyond 700 meters.
The steady work of expeditions using manned or submersibles and remotely operated vehicles (ROVs) undoubtedly will reveal additional diversity in this group. Studies of these strange creatures at the lab bench should provide some insights in the fields of developmental genetics and perhaps in related areas such as cell communication.
Now, siphonophores do not at all hold up well in captivity. One will not likely see future aquarium displays of by-the-wind sailors and frail deep-sea species. Still, with the increasing pool of high-quality images, I can imagine an awe-inspiring gallery with an accurate 130-foot silicon model Praya hanging above.
Praya sp.
Apolemia lanosa, taken by ROV Tiburon at 1150 meters.
When it comes to species diversity, the insects reign over all other animals. Roughly 900,000 species of insects are known, and the actual diversity is thought to be many times this number. Another way to get the point across might be to say that insects account for more than 50% of all species described on Earth.
Yes, it's real. A metallic weevil.
Scanning electron microscope image of grain weevil.
Now, this includes all of those insects we see on a regular basis, from ants and bees to the flies. The variation is so great that choosing a place to start is difficult. A famous quote comes to mind. When the famous geneticist J. B. S. Haldane was asked by some theologians what he could infer about God from his studies, he replied "God must have had an inordinate fondness for beetles. He made so many of them."
A male giraffe weevil.
The elephant weevil.
The beetles do show amazing variety. You are familiar with the ladybird beetle (a.k.a. ladybug), and you can likely picture others such as the scarabs. Do weevils come to mind? The weevil is a beetle that occurs worldwide and is represented by more than 60,000 species. They tend to be small, often less than half of a centimeter in length, though some can grow to several centimeters.
Weevils are generally fairly easy to recognize; they possess a unique form. Like the other insects, the weevils have compound eyes. It is what emerges from between these eyes that enables them to be more quickly recognized. Weevils have an elongate snout, probably more properly referred to as a rostrum. A pair of antennae also arise from the rostrum, but it is at the end of the rostrum that you find the mandibles and other mouthparts that perform the business of eating plant material.
Usually, this impacts us not at all, but various species of weevils do affect our crops. There is a wheat weevil. There is a flour weevil. There is a maize weevil. There is a cotton weevil. Getting the picture? I thought something like alfalfa might be weevil-free, but a quick search indeed revealed the alfalfa weevil.
What about fruits? Aha! As it turns out, various fruits are attacked by weevils as well, from the strawberry root weevil to the mango seed weevil.
A chestnut weevil.
What about dry fruits like nuts? They were no exception. Weevils affect everything from peanuts to chestnuts to hazelnuts. As long as that last one does not interrupt my supply of Nutella, all will be okay. Interestingly, these weevils drill into the protective shell of the nuts and lay eggs inside. When the larvae hatch, they feast upon the nut inside until they are well developed enough to emerge.
One in particular, the boll weevil, wreaked havoc on cotton harvests in the United States. If you are interested in that history, perhaps you should start with the Boll Weevil Monument in Alabama.
The red palm weevil.
Of course, there is much more to the weevil than its negative impact upon our crops. They play an important role in terrestrial food webs, both in their action as herbivores and as food items themselves for species of other insects, birds and small mammals.
Speaking of worms (see previous post), your body might be serving as housing for some right now. The likelihood of this, of course, is related to factors such as the region in which you live, your diet, travels, hygiene and health care. Still, there is a group of invertebrates that includes many parasitic species for which humans serve as hosts. They are called roundworms, or nematodes.
Agreement on the number of species of nematodes is lacking. A quick search on-line will reveal estimates from 12,000 to more than 20,000 species. It also reveals an understanding among invertebrate taxonomists that the actual species diversity of the group is substantially higher. Meldal et al. (2007) cite estimates of 0.1 to 1 million species; well, that helps. If we remain incredibly conservative, we can still say that the species-level diversity of these animals is more than double that of the mammals. Most species of nematodes are free-living and inhabit sediments in terrestrial, freshwater and marine environments. In these sediments, nematodes can be extraordinarily abundant. One study of forest soils in different areas in China found these worms to be present in densities ranging from several hundred to several thousand individuals per 100 grams of soil (Zhang et al., 2012). Very similar estimates exist for sediments in other reaches of the world.
In a TED Talk in 2007, the famous naturalist and ant biologist E. O. Wilson said: "Consider the nematode roundworm, the most abundant of all animals. Four out of five animals on Earth are nematode worms -- if all solid materials except nematode worms were to be eliminated, you could still see the ghostly outline of most of it [Earth] in nematode worms. About 16,000 species of nematode worms have been discovered and diagnosed by scientists; there could be hundreds of thousands of them, even millions, still unknown." It seems that he was drawing from the a statement made by biologist Nathan Cobb in 1914: In short, if all the matter in the universe except nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites." Yowsa. Can we just agree on something simple: Wherever we might be, we're surrounded by nematodes worms. What are they doing? As you might guess, their life cycles vary considerably. Some nematodes feed upon plant tissues. Some specialize on fungal cells. Other nematodes consume bacteria. Their activity in sediments is enormously important on a global scale when it comes to the transfer of energy and the cycling of elements through these environments. In fact, the degree to which they modify the chemistry of sediments makes the Nematoda some of the "most valuable players" when it comes to our planet's animal fauna.
A specimen of Trichinella spiralis.
The role of nematodes as parasites probably does not hold the same weight, but they do impact a wide array of other species. Those species of nematodes that infect plants include some groups that affect our crops. So, I'm sure you are wondering which species pay visits to the human body.
Trichinella spiralis encysted in skeletal muscle.
Let's start with Trichinellaspiralis. There's a reason you want to cook meat such as pork well. The larvae of this roundworm bore through the intestinal wall of a host, such as a pig, and are then carries via the circulatory and lymphatic systems to striated muscle. These larvae then encyst in the muscle. Should a human consume muscle with viable cysts, the larvae are released in the stomach and then mature in the small intestine in a period of 1 to 2 weeks. The larvae then enter the person's lymph or blood vessels and encyst in muscle cells. Of course, that's not the end of the story; the infection comes with less-than-desirable symptoms and health consequences.
The "filarial" nematode parasites might disturb you a bit more. Multiple species of these "thread-like" nematodes in the superfamily Filaroidea affect humans. Consider the species Wuchereria bancrofti. This species, and a couple of others, cause a disease known as lymphatic filariasis. Humans become infected when an infected mosquito takes a blood meal and introduces larvae into the bloodstream, where they migrate to lymph nodes, most commonly in the legs and genital regions. The larvae continue to develop over the course of a year and then, as adults, reproduce in the lymph vessels. The female can produce thousands of the early-stage larvae, called microfilariae, which are released into the bloodstream. Their presence in the bloodstream ensures that they can be ingested by any mosquito that should feed on the infected human, beginning the cycle anew. Having an abundance of parasitic worms in your lymphatic vessels is not something that the body takes to kindly.
Two Haitian women with elephantiasis.
The presence of these nematodes can lead to marked inflammation of the lymph nodes and vessels and obstruction of these pathways. The consequences range for severe fevers and chills to "elephantiasis." The image provided here is one of the more kind. In men, the filariasis often affects the legs and scrotum.
A nematode infection beneath the cornea of the human eye.
If that is not enough, imagine having a subcutaneous nematode infection in you eye. That's exactly where nematodes such as Loa loa and Onchocerca volvulus like to migrate. These nematodes can cause blindness. Just avoid being bitten by the African flies that are their vectors.
A lovely pile of Ascarislumbricoides.
Better known nematodes are the intestinal parasites of the genus Ascaris. The species A. lumbricoides causes a disease called ascariasis. We can get these worms by eating food or drink that has been contaminated by fecal material from an infected individual. The eggs ingested via the contaminated food then hatch in the small intestine, with larval forms burrowing through the intestinal wall and entering the bloodstream where they migrate to the lungs. The larvae manage to work their way into the alveoli (air sacs) and into the trachea ("wind pipe"), where they are coughed up and then swallowed for a second time. This brings some back to the gut, where they develop into adult worms. Those people infected might not show any symptoms, depending on the number of worms, but they can cause malnutrition, respiratory illness and other complications (among them, death).
Whipworms, each about 4 cm in length.
Given improvements in technology and practice in the areas of food safety and sanitation, you are not likely to experience the infections mentioned above. The human pinworm (Enterobiusvermicularis), however, is one for which you might be a candidate. Humans, most often children, ingest this nematode's eggs via contaminated foods or hands. The larvae hatch in the small intestine and migrate to the colon. Adults then lay eggs around the anus and are passed with feces.
Whipworm (Trichuris trichiura) has a very similar life cycle. The spread of this parasite is facilitated when people defecate outside and/or when human feces is used as fertilizer. Eggs reside in the soil and can contaminate water or foods. Whipworm might infect up to 800 million people worldwide, most commonly in tropical countries; that's more than twice the population of the United States.
I am going to leave hookworm, yet another human-loving nematode, for you to investigate separately. If you have a pet dog, you might opt to avoid letting him or her "kiss" your face. Now that I have mentioned dogs, you will also want to be sure to give your pet canine a regular dose of heartworm medication. It is not some scam on the part of veterinarians; the dog heartworm, Dirofilaria immitis, is nasty business. As a biologist, I have seen my fair share of preserved specimens. One of the most impressive collections was the heart of a German shepherd that was literally bursting with adult heartworms.
Of course, nematodes do infect other invertebrates as well. I came across the interesting story of the Myrmeconema neotropicum, a roundworm that so far is only known to infect a particular species of South American ant. When infected, the posterior-most section of the ant's abdomen enlarges and develops an appearance that resembles the red berries commonly found in the tropical forest it inhabits. The infected ants, with their now bright red abdomens, crawl to an elevated position. The thought is that fruit-eating birds confuse the infected ants for berries and consume them. Researchers are examining the possibility that the birds then spread the nematode parasite via their egg-laden droppings.
From the free-living masses that writhe through sediments to the tiny ant-infesting nematodes to the 9-meter-long Placentonema gigantissima that resides in the placenta of the sperm whale, nematodes have radiated into every niche imaginable. You probably will not see one at all on your next visit to the museum, but you might wonder if someone standing next to you is carrying a few.
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