Q: Just how “simple” are sponges? What does it mean that they’re “simple” organisms?
A: Sponges are simple in that they don’t have organs, much less defined body parts or brains. But they do have specified cell types that serve different functions, including choanocytes (which beat their flagellae to move water and nutrients through the sponge), sclerocytes (which make the hard spicules that comprise the sponge’s skeleton), and archeocytes, which can become other kinds of cells.
Q: I’ve heard that sponges are valuable to the biomedical industry; why is that?
A: Sponges are able to recognize “self” cells and reject “nonself” cells at least as accurately as the human immune system can. Scientists are studying the means through which they do this in order to understand how immunity evolved and also to see if they can apply this new understanding to human tissue grafts.
A: Good question. Sponges actually form their own “reefs,” complex habitat in and around which a number of other species live. They’re also a food source for a number of organisms, including sea slugs. And some coral-reef sponges are the home for species of eusocial snapping shrimp—that is, shrimp that have colonies and social/sexual structures like those of ants or honeybees. Shrimp live within these sponges almost like bees in a hive.
A: Polychaetes exist in an enormous variety of habitats, from intertidal areas down to deep-sea hydrothermal vents. You could find errant polychaetes on a beach or mudflat, usually under rocks; you could find sedentary, tube-building polychaetes on a number of surfaces, from intertidal rocks to the pilings of piers to coral reefs to the very deep sea.
A: That’s how we think of jellyfish—and that’s how they’re usually visible—but for part of their lives, scyphozoan cnidarians (the majority of what we think of as “jellyfish”) go through a stage of development where they look like sea anemones instead or like coral polyps. When medusae—the bell-shaped form of jellyfish—reproduce sexually, they create a bunch of larvae. The larvae then become polyps, which look like anemones. These polyps then divide into a bunch of genetically identical medusa. So jellyfish get the benefits of sexual reproduction and genetic difference, but then they also get more copies of themselves out there through their asexual phase. Hydrozoans, another class in the same Cnidarian phylum, have the same alternate life cycles, but because they are mainly present in their polyp form, that’s how we think of them. True sea anemones and corals (all anthozoans) never go through a medusa phase.
A: It really depends on the kind of jellyfish and the kind of prey. For us humans, most jellyfish stings will only hurt for a little while; however, some species, especially certain kinds of box jellyfish (cubozoans), can be fatal to humans.
A: When horseshoe crabs crawl up the beaches to spawn, there is usually a lot of competition among the males for a female mate. Some males actively shove at each other in order to be the one who’s directly “clasping” the female, but other males hover around near the female without actively engaging in fighting and without “clasping” anybody. And yet these males also fertilize a fair number of females’ eggs. They exhibit an alternate mating strategy that works well in the crowded spawning environment.
A: It really depends on the animal, though there’s speculation that some can live for a very long time (over 50 years). It’s certainly worth noting, though, that any lobsters you’re actually seeing on a plate or in an aquarium are typically more than 5 to 8 years old (since this is how long it takes for juveniles to reach sexual maturity).
A: Sexual dimorphism (literally, two shapes) is when individuals of one sex have different physical characteristics than those of the other sex. Some very typical kinds of dimorphism are size or coloration differences. In a number of marine species that produce eggs, females tend to be larger than males because larger females can produce many eggs. In some species where courtship behaviors occur, members of one sex or the other will have special traits that let them fight or show off—as in the case of fiddler crabs, where males have a hugely outsized claw that they use to show off for the females.
A: Good question. Although barnacles look like mollusks—especially when you just see their shells on rocks—they’re actually crustaceans. When they’re larvae, they look a lot like the larvae of crabs and other crustaceans, and even after they metamorphose their feeding appendages are jointed in the way of arthropods.
Q: What kind of importance do chitons have ecologically?
A: Most chitons are intertidal, and their grazing and predation influence the composition of microalgae communities as well as the composition of species that rely on those communities. Their movement—which often follows “trails” through tidepools—can also affect the locations of relatively sessile species like sea anemones.
A: Some snails eat plants, but many species are carnivorous or scavengers. Many snails drill holes into the shells of other organisms (often mussels, clams, or barnacles, although sometimes other snails) and then liquefy and suck out their prey.
A: A radula is a trait that most snails possess; it has often been called a “toothed tongue,” and although that helps give you a visual on it, it’s not really what it is. It is, however, a structure in snails’ mouths that is covered with rough, hard material made of chitin; the snail extends this structure and uses it to rasp algal film off of rocks, or rasp at seaweed, or drill through the shell of another organism (with the help of a little release of acid to make the drilling go faster). So it functions as a snail’s teeth, in a way, but the structure itself is very different.
A: This answer varies hugely both among classes and within them. By and large, cephalopods live a very short time—rarely more than 4 years, and usually only 1—while some clams are known to have lived over 100 years. Snails may live anywhere between 3 and 20 years, but very little is known about the precise age of various species.
A: It depends on your definition of smart. Insofar as their ability to solve problems we give them in the lab, yes, they seem very capable: Octopi can learn how to unscrew glass jars in order to get to food inside them and can learn by watching one another; cuttlefish can make their way through mazes, learn to recognize different symbols, and match their patterns of camouflage to human-made patterns (like chessboards).
A: “Dioecious” and “gonochoristic” both mean that a species is composed of individuals of separate sexes—like us. “Monoecious” or “hermaphroditic” means that a species is composed of individuals that are either both sexes at once or that change sex.
Q: Are Cuvieran tubules of sea cucumber part of their organs?
A: No, these are special sticky tubules that only some sea cucumbers have. When sea cucumbers feel threatened, they can eviscerate—basically throw up most of their organs (and then grow them back later). Cuvieran tubules are extra predator defenses that cause a predator to become sticky and tangled up.
A: Starfish can and do move their arms, but they—and sea urchins and sea cucumbers—also move by using their many tube feet. Sea urchins can also use their spines (which they can move) in order to move. Brittle stars tend to move by using their long arms, which they can crawl with or use to snake across the ocean floor.
A: Jawless fish still have rings of teeth, and they use a lot of suction in order to clamp onto prey and eat their flesh. Hagfish are often scavengers, and they will clamp onto dead things and, in order to get the right amount of leverage to pull a chunk of flesh free, will tie themselves into knots (which also cleans themselves of their slime). Isn’t nature wonderful?
A: Scientists still don’t have all the answers on singing behavior in whales, but at least one of the reasons is in order to attract mates. Male whales sing during breeding season in order to get females interested in them; they may also compete, either for mates or more generally, via singing.
A: It’s hard to remember, because we don’t really think of baleen whales as hunters, but both baleen and toothed whales eat other animals; in the case of toothed whales, they eat fish, seals, penguins, giant squid, and even other whales; in the case of baleen whales, they eat sand eels, krill, and other small animals that they strain out of the plankton in vast quantities.
A: Good question. Otters can play a major role in kelp-forest ecosystems in particular. Sea otters eat sea urchins which in turn in eat kelp. If there aren’t enough sea otters around to keep sea-urchin populations in check, the kelp forests can be decimated by enthusiastic sea-urchin grazing.
Q: Why do fish often have bioluminescence patches or structures?
A: Bioluminescence serves a number of different functions for fish, from helping create distractive patterns that camouflage them or confuse predators to lighting their way to luring in prey to (potentially) serving in courtship displays.
A: Whale oil was highly prized for use in lamps and as a lubricant; whale bones were used in scrimshaw; whales’ baleen plates were used in women’s corsets; the oil from sperm whales’ spermaceti organ was also very popular as a lubricant, and sperm whales were hunted frequently for this. (Whales also, like many mammals, have a bone in their penis, and this bone, called the dork, was often claimed by sailors on whaling vessels as a prize.)
A: It’s true that adult ascidians can look a lot like sponges—but in fact they are very different organisms that evolved much later than sponges and are in the same phylum (Chordata) as vertebrates like us. The best time to see this relationship between ascidians or sea squirts (which admittedly look like tiny sacs of water) and vertebrates is during the larval stage of their life cycle, when they have a notochord much like that of vertebrates. It’s worth keeping in mind, with these species, that the whole idea of “higher” or “lower” organisms is a mistaken concept—all of the forms that survive today exist because they are well-suited to their environments, and more recently evolved species don’t necessarily look like us!
A: In a number of species that engage in competition around courtship, some males fight or otherwise compete with one another for the privilege of mating with females. But some males pretend to be females so that they can “sneak” by the competing males and mate with females while the other males are busy showing off. (Cuttlefish are one of the kinds of animals that have been found to do this.)
A: These are two kinds of (mainly marine) arthropods that are in some ways the reverse of one another in terms of their body plans: Isopods look as if they’ve been flattened from above (as if someone stepped on them), while amphipods look flattened from the sides (as if somebody squeezed them on either side). Both are present throughout various ocean habitats, from intertidal areas to the deep sea. In fact, an alarmingly huge type of amphipod has recently been found in the deep sea.
Q: Have horseshoe crabs really been around for millions of years?
A: Sort of. Horseshoe crabs are called “living fossils,” but it’s important to remember that one, the animals living today have not lived for millions of years (they usually live between 16 and 19 years), and two, species that were slightly different but looked very similar existed millions of years ago. So while it’s true that the general horseshoe-crab body plan was obviously an effective one, since it hasn’t altered much in all this time, it’s not accurate to say that these horseshoe crabs are exactly the same as the kinds that wandered the earth long before mammals evolved.
Q: So are shrimp and lobsters basically giant sea insects?
A: Well, they’re all in the same phylum, and current phylogenetic data suggest a fairly close relationship, but crustaceans aren’t insects—they’re just related to them. If that makes you less interested in eating lobster, well—I’ll still eat yours.
Q: Why are barnacles always packed so close together on rocks?
A: Barnacles’ optimum habitat is usually a fairly small space—a certain area of the rocks in the intertidal zone. (Other zones might not work for a lot of reasons, like too much or too little wave action, species that can outcompete them, or too many predators.) So tons of barnacle larvae all settle in the same area, and as they grow the space gets pretty crowded. In fact, many of the juveniles that settle don’t reach adulthood, because there’s just not enough room for them on the rocks.
A: I’ll use the example of a dolphin—experts at echolocation—to answer. Dolphins project sound—clicks—forward, and their “melon” (the round ball part of their head) and their nasal sacs help amplify and focus the sound. The clicks shoot out and, thanks to the excellent conductivity of water, can travel quite far; when they hit an object, they bounce back to the dolphin, and the reflected sound travels from an area in its jaw into its middle ear, so that it “hears” the shape of the area around it.