Cones

Part of the attraction of the cone shell probably has been an aura of danger. The animals bearing the cone-shaped shell belong to a taxonomic group (superfamily) that once had the label "Toxoglossa," which translates roughly to "venom tongue;" and in many species of cones the apparatus that led to this name is sufficiently large to pose a threat to a collector or observer handling the animal. Among the photographs here are two showing cones that are relatively common in the Hawaiian Islands, each with its venom-tongue, or proboscis, extended. The proboscis is a hollow extension of the gut, and just inside its tip is a tooth, also hollow, that functions much like a hypodermic needle. The tooth is rigid, being made of the same type of material (chiton) that forms the shells (exoskeletons, including jaws and stingers) of insects. When the tip of the proboscis contacts a prey animal, the tooth is thrust into the prey and venom is squeezed through it ( link to pdf version of a recent paper on cone toxins).

Some cones catch fish

In the striated cone the venom apparatus is especially large, and the venom is especially toxic to vertebrates (presumably including humans). In fact, this species was used as the principal example during World War II to alert United States military personnel serving in the tropical Pacific to the danger of cone stings, which from some species have occasionally been fatal to humans. However, it was ten years after the end of World War II when a graduate student (Alan J. Kohn) from Yale demonstrated for the first time the use to which the striated cone actually puts its venom apparatus. It harpoons fish! The striated cone is relatively common and has been known to biologists for several centuries. It was among the first mollusk species to receive a scientific name, having been labelled Conus striatus by Karl von Linne in 1758. The fact that the remarkable fishing skills of this animal could remain hidden for so long is testament to the seductive power of the shell to distract from the biology of the animal within. It took the curiosity of Dr. Kohn (now an emeritus member of the Zoology faculty at the University of Washington) to overcome that power. His observations on the striated cone were merely a small part of his studies of the distribution and feeding habits of Hawaiian cones. In his doctoral dissertation, a classic work in animal ecology, he showed us how the habitat and food resources are shared systematically by the many cone species living in a given area. Most of the cone species that he studied catch and eat marine worms, with each species tending to select kinds of worms not eaten by the other species. However, at least three of the Hawaiian species catch and eat marine mollusks (including other cones), and at least three catch and eat fish. The venom apparatus plays a key role in catching and subduing the prey of a cone, whether it be worm, mollusk, or fish.

Sniffing for prey

A key role in finding the prey to begin with is played by the cone's siphon, which is very much analogous to the human nose (perhaps even more analogous to the nose of an elephant, but without the prehensile ability). Just as air is taken in through our nose and, on its way to the respiratory organs (our lungs), it passes over the chemical-sensitive tissue (olfactory mucosa) that endows us with a sense of smell, water is taken in through the cone's siphon and, on its way to the respiratory organs (the cones ctenidia, or molluskan gills), it passes over chemical-sensitive tissue (osphradium). The chemical sensitivity given to the mollusk by its osphradium allows it to detect objects at a distance by virtue of the chemicals they emit. Thus, although one might argue that the mollusk is "tasting" the water, the sense it derives from the osphradium is very much analogous to our sense of smell. Furthermore, perhaps because it can swing its siphon about, much like the elephant swings its trunk, the cone seems much better able than we to detect the direction of the source of an "odor." Each cone also possesses a pair of primitive eyes. At the very least, these provide sensitivity to changes in light intensity and to motion; but their importance in complicated tasks such as locating prey is not known.

Fish-eating cones in captivity

The Hawaiian fish-eating cones are easy to keep and in fact will live together peacefully in a single aquarium tank. Given a choice of hard surfaces, small (dead) coral heads, and sand in a tank, the cat cone and the striated cone usually choose the sand, resting beneath it with only the tips of their siphons visible. In the case of the cat cone, this is interesting because the animal commonly is found on hard substrates in the wild. The dusky cone usually will choose the hard substrate (including the aquarium filter), but occasionally will take up a position beneath the sand. On the near-shore reefs around Kihei, it commonly is found in dead coral heads, especially in rose coral (Pocillopora meandrina). If they are hungry, all three species will go on fishing expeditions whenever fish are placed in the tank. In fact they will set out on fishing expeditions if one simply pours in a little water from a container with fish in it, but leaving the fish behind.

The dusky cone is a bold hunter

As soon as fish or fish scent is introduced into the tank, the cone extends its siphon and begins to move it back and forth slowly. Then it usually extends its proboscis slightly. In the cases of cat cones and striated cones, only the siphon and proboscis usually are visible above the surface of the sand at this time. The proboscis often is attractive to fish, seemingly looking like a small worm or some other potential food item. If a fish is sufficiently tempted to nip the proboscis of the striated cone or the cat cone, the animal withdraws its siphon and proboscis to the safety of the sand, the fish escapes unscathed, and the fishing expedition is postponed for fifteen or twenty minutes. If the fish nips the proboscis of the little dusky Cone, however, the animal extends its proboscis even farther, seemingly tempting the fish to strike again. Eventually, the fish usually attacks the tip of the proboscis, where the barbed tooth is hidden. Immediately the tooth is thrust into the fish, venom is injected, and an intense struggle begins.

The dusky cone is very small, so the struggling fish often will lift it right off of its foot and swing it about wildly in the water. In the meantime, the cone has begun to pull in its proboscis, just as a human fisherman would reel in his line. In this case, however, the reeling-in process often carries the fisherman toward the fish rather than vice versa. Reaching the fish, the cone attempts to swallow it even as it continues to struggle; but the action of the venom is nearly complete by now, and the struggle soon ceases. Like the striated cone and the cat cone, the dusky cone is able to swallow a fish even slightly larger than its own shell. When it does so, it obviously cannot withdraw its stomach into the safety of the shell until the fish is digested, so it is vulnerable. At this point, the dusky cone retreats into the nearest available hiding place, beneath the sand or deep into a crevice.

The larger cat and striated cones are timid

While the dusky cone will use its proboscis as bait, the larger cat cone and the very large striated cone do not. Instead, the latter two depend upon stealth to ambush or stalk their prey and often will withdraw in response to the slightest disturbance. For example, I once observed a striated cone about to harpoon a fish when the fish's gill moved, brushung the cone's proboscis. The cone left the fish intact and retired from the field for nearly twenty minutes. In searching for fish, the cat cone or the striated cone often leaves its sandy shelter and moves very slowly over the substrate, probing with its proboscis and sweeping slowly from side to side and front to back with its siphon. Once the cone is close to a fish, only the proboscis continues to move, and it does so very slowly. The tip of the proboscis seems to be endowed with tactile and chemical sensitivity that allows the cone to distinguish its prey from everything else. While the cone is probing for a fish, the tip of the proboscis may touch sand, dead coral, rock, even the flesh of another mollusk, and simply passes on as the search continues. When the tip of the proboscis touches a fish, however, the harpoon is thrust and the struggle begins.

Unlike the dusky cone, the striated cone or the cat cone will wait for the venom to take effect before it begins to pull in its proboscis and retrieve the fish. In the meantime, the cone may be yanked right off of the sand and, by the end of the struggle, may find itself upside down or resting on its rear (large) end. Throughout the struggle, the proboscis seems to function like a fishing line or, more precisely, the line connected to the detachable head of a fishing spear, usually denying the fish sufficient leverage to break free from the harpoon and thus keeping it tethered for ultimate capture. The teeth of fish-eating cone species not only are barbed at the pointed end but have a bulb-like enlargement at the other end, giving the animal something to grip with the muscles at the tip of the proboscis. In spite of this adaptation, if the fish is sufficiently strong (or large), it will pull the harpoon-like tooth out of the grip of the cone's proboscis and escape temporarily, until the venom overcomes it. Then the cone often will probe with its siphon, apparently sniffing out the whereabouts of the lost prey. When it finds the fish, the cone will attempt to swallow it just as if it were still pinned to the end of the proboscis.

Swallowing a fish

As a consequence of its fishing method, the dusky cone often harpoons the fish in a way that allows the animal to swallow it headfirst, which clearly is the easiest way to manage all those spiny fins. The cat cone or the striated cone often harpoons the fish in the flank and then attempts to swallow it sideways, a difficult task that sometimes seems to involve more of a struggle than catching the fish to begin with. In carrying out the task, the animal clearly uses strong muscles ringing its lips (buccal lips) to fold the fish and allow it to be swallowed. The striated cone can modify its fishing pattern to the extent that it will seek out and swallow a fish that it has not killed itself. In doing so, it probes with its siphon but does not use its proboscis at all (i.e., it does not attempt to harpoon the fish before swallowing it).

Danger to people

The dusky cone and the cat cone are among the cone species reported to have used their harpoons to sting people. All cones should be handled with great care, always from the rear (large, or "crown") end. Human fatalities have been reported from the stings of two species, the geography cone (Conus geographus), which has not been found living in Hawaiian waters, and the cloth-of-gold cone, which is relatively common in Hawaii. The stings of several other species have produced symptoms ranging from very mild to nearly fatal. The nearly fatal case involved the penniform cone which is common in Hawaii and sometimes is mistaken for the cloth-of-gold cone.

Cloth-of-gold in captivity

With its graceful shape and intricate and colorful pattern, the shell of the cloth-of-gold cone is especially popular among collectors. The animal is easy to keep in an aquarium tank. I kept the one shown in this collection of photographs for over twelve years. Here are some anectdotal observations on that animal. It would devour almost any other sea snail that was placed in the tank with it. I qualify this generalization only because the animal I kept alway was hesitant to eat herbivorous marine snails, but not the least bit hesitant to eat carnivorous ones. Even when it was hungry, it often would ignore small herbivores for several days or weeks, yet immediately attack a canivore placed in its tank. When it was hunting, it used its siphon extensively, appearing to sniff all about, both in the open water and along the bottom in its search for the scent of the prey. Moving about very rapidly, it easily overtook other gastropod mollusks. Having done so, it extended its proboscis and used it in close coordination with its siphon to probe the captured animal. As soon as the tip of the proboscis touched the fleshy part of the prey, the cloth-of-gold injected its venom, which appeared to act immediately. Then, with the very front of its foot, it deftly grasped and rotated its prey into an aperture-up position for eating. The animal swallowed its prey whole, using the strong muscles ringing its buccal lips to pry the prey animal from its shell, leaving the shell itself perfectly clean inside. Shell collectors who have difficulty cleaning their newly acquired shells would do well to enlist the services of the cloth-of-gold. With a little patience, they even will be able to retrieve the horny operculum of the prey, which the cloth-of-gold regurgitates (serious collectors always save the operculum along with the shell).

An eating routine

Before beginning its meal, this cloth-of-gold routinely went through a short but intriguing action sequence. First it slowly raised the front of its own shell, away from its foot. Then it snapped the shell abruptly back to its normal position, forcing a strong stream of water to flow over the aperture of the prey. This process was repeated two or three times before the meal was started. Observing a behavioral pattern such as this, the modern biologists often will ponder its "adaptive value," the advantage it provided that led to its selection during the evolution of the animal. One possibility in this case is getting the sand out of the food. The prey often is captured on a sandy bottom, in which case its foot usually is covered with sand. The shell-snapping routine blows most of that sand away. In spite of an obvious possibility such as this, however, experience has taught the prudent biologist to remain open-minded about the adaptive value of any structure or behavioral action.

Mucus trails

Any gardener who has gone out early in the morning must have observed the trails of slugs and snails that were abroad during the night. A sea snail or slug leaves the same sort of trail (a very thin layer of mucus) on the bottom of the sea as it wanders about. Some predatory mollusks simply search out the mucus trails of their potential prey, then follow them. Occasionally a wandering sea snail would pass close to the resting cloth-of-gold. When that happened, the cone often would use the tip of its proboscis to follow along the mucus trail of the wanderer. There is little doubt that the tip of the proboscis is endowed with sensitivity, probably both tactile and chemical. The chemical sensitivity in this case is more akin to our sense of taste than to our sense of smell, requiring contact between the "tongue" of the animal and the material being sensed. Thus, the cloth-of-gold appeared to be "tasting" and/or "touching" its way along the mucus trail.

Moving

Although there apparently are many variations on it, the basic theme in the movements of sea snails along the bottom seems to be grasp and slide. For example, the entire front of the foot might grasp the bottom and the entire rear of the foot then could slide forward over mucus-coated bottom as the muscles of the foot contract. Next, the rear of the foot could grasp the bottom and the front of the foot could slide foward as the muscles of the foot expand (relax). In some sea snails, this sort of action is carried out alternately by the right and left halves of the foot. In others, successions of waves of grasping and sliding move over the foot, so that at any one moment several parts of the foot are grasping and other parts are sliding. The mucus layer seems to be a key feature in all of the various models of travel. Cones deposit a particularly rigid and persistent mucus trail, so rigid that it ocasionally can be seen leaving bridges over small gullies along the path the animal has taken. On two occasions I have observed a cone descend in midwater supported only by its mucus trail, just as a spider would descend in midair supported only by a strand of silk. Once it was a small penniform cone, descending in an aquarium tank from the glass lid to the floor, and the other time it was a small cat cone taking the same route. Both species can be found in habitats where such a maneuver might provide an effective shortcut. On the other hand, if the cone either could not remember or could not sense remotely the location of the bottom, the maneuver could lead to a very long descent, carrying the animal far from its normal haunts. Both descents that I observed were head-first, and in each case the cone was continually probing with the front of its foot, as if attempting to locate the bottom

Eating and being eaten

It is true in most ecological systems that the hunter generally also is hunted, and this process continues until one reaches the top carnivore, an animal so big or so tough that, as an adult, it is safe from all predators (but, of course, not from parasites). The cone is not a top carnivore. Snorkeling along the seaward edge of an Hawaiian fringing reef or around the edges of a patch reef, one occasionally finds the freshly crushed shell of the striated cone or the cloth-of-gold, sparkling on the surface of the sand. Having broken apart the shells of large cones to examine the animal within, I can tell you that it is indeed a difficult task. The animal that can crush a large cone shell is very strong. Fortunately for us beachcombers, some predators apparently can devour cones without damaging their shells. Snorkeling near Makena (Maui), we found the unbroken shell of a large Striated Cone. The condition of the shell indicated that the animal had been eaten very recently. Resting a few inches away was a very large Triton's trumpet (Charonia tritonis), that we surmised was the diner, although the Triton's trumpet is reputed to be a devourer of starfish and sea urchins rather than mollusks. At the time, we were interested more in observing and photographing the animal than in analyzing its stomach contents, so we settled for one striated cone shell in excellent condition (but without the operculum), one live Triton's trumpet, and at least one unresolved question. Another unresolved question is how predators can attack and eat cones without being fatally stung. It is well known that many species of cone use their harpoons in defense as well as in hunting. Is the venom of a given cone species so specific that it affects only certain kinds of animals, leaving other kinds free to prey upon the cone? Whatever the answer may be, cones are eaten; and their shells roll in with the waves in fairly large numbers.

Cone shells on the beach

By the time it is beached, a cone shell almost always has lost both its thin outer skin (periostracum) and the luster protected by that skin. However, the bold, often beautifully-colored patterns on the cone shell sometimes are preserved; so beachcombing for cone shells can be rewarding. If a beachcomber is fortunate enough to find a cone shell still covered with its dull, brownish periostracum, then he simply can soak the shell in pure household bleach to remove the skin and expose the still-lustrous surface beneath. Cone shells (especially those whose original owners inhabited reefs or solution benches) also may be encrusted with calcareous algae and various sedentary animals. Such a crust may be difficult to remove. If it lies over intact periostracum, then a long soak in household bleach may do the job. Otherwise, a lot of patient scraping will be required; and in the end the collector often will find that the encrustation has marred the surface of the shell. Small, beached cone shells very often are broken into tiny pieces, but one part of the shell-- the large end (or crown) seems to be especially resistant. Therefore, one often finds large numbers of eroded cone-shell crowns, looking very much like small buttons, littering the beach. Sometimes erosion leaves holes in the centers of the crowns, in which case the cone-shell crowns become puka shells, suitable for stringing into a necklace.