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RON SHIMEK |
| This small chiton, Mopalia porifera, clearly shows the eight shells typical of the group. Note the ornamentation of the girdle and shells. |
Now that you’ve had your dose of culture and history for the day, back to living chitons.
When looked at from the bottom, there is a large central foot used for creeping and adherence to the substrate. The head is found in front of the foot, but really only consists of a short squat protuberance with the mouth at the tip. Outside of the head and foot is the underside of the girdle, which is often fairly wide and can be placed tightly against the substrate.
Figure 1
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RON SHIMEK |
| This diagram shows the bottom surface of a chiton as if it was crawling up the side of an aquarium. All major organs or structures are labelled. Water enters at the incurrent fold in the girdle and passes out the excurrent fold. |
Still looking at the animal from below, a groove or space can be seen surrounding the head and foot, separating them from the girdle. As this groove lies under dorsal mantle it is called “the mantle cavity.” Numerous small leaf-shaped gills are located on either side of the animal in the mantle cavity. They originate from the side of the body above the foot. The anus is on a papilla and opens into the mantle cavity at the midline of the body at the rear of the animal, and two small apertures, one for the kidney and one for the gonad, are also found on each side of the animal opening from the body into the mantle cavity. These uro-genital apertures are also found near the rear of the animal.
Water is pumped through the mantle cavity by the action of millions of small, microscopic hair-like “cilia” found covering the gills. The basic water flow is from the front to the back, which prevents fouling of the gills by forcing the waste products, gametes and feces out the rear of the mantle cavity with the water flow.
The internal morphology of chitons is relatively simple. They have a long tubular gut that is looped and coiled on itself in the body cavity. The gut has a lot of glands that dump digestive enzymes, mostly carbohydrases, into it. Such long guts and accessory organs are typical of herbivores no matter where they are found. Plant material is not particularly digestible and long guts and lots of chemicals are necessary to digest it completely.
The business end of the chiton contains a radula designed to rasp up hard food. (See last month’s column for a description of the basic radula.) The basic radula is present in the chitons, as well as the grazing snails, and is indeed very characteristic of the mollusks, in general. Parts of chiton radulae are often highly mineralized with iron salts, such as magnetite and hematite. This makes the chiton radular teeth much tougher than most biogenically produced minerals, and most chitons use them to graze on calcareous algae.
The algae are rasped from the substrate by the radula and passed into the mouth cavity where they are bathed in a mucus solution. This results in a slurry of mucus and very fine particles consisting of algal fragments, including calcareous tissue, as well as pieces of the substrate. This slurry is passed through the gut where the tissue remnants are digested away. The remaining material passes out the anus as feces, often appearing as a pure white pellet of compacted calcareous sand and mucus. In effect, chitons turn coralline and other algae into chiton tissue and sand.
As befitting their basic “go slow,” more-or-less sedentary, existence, at first glance, chitons appear to lack many of the sensory attributes, such as eyes and sensory tentacles, characteristic of other mollusks, particularly the gastropods. However, they are not as lacking in stimulation as they may seem.
If you examine the back of a chiton with a hand lens or powerful magnifier, you will see that the shells are covered with rows of small dots. These dots are referred to as “aesthetes,” and they are peculiar extensions of the underlying dorsal mantle up through the shell. This makes chiton shells fundamentally different from the shells of all other mollusks, which lack such perforations. Furthermore, it has been demonstrated that the aesthetes that extend up through the shell pores are sensitive to light. One might take note that chitons truly lead an aesthetic existence.
Additionally, the dorsal surface of the girdle is often covered in scales, hairs or granules. All of these structures have been shown to contain nerves and to be fundamentally responsive to stimulation. However, with all of the sensory input available to them, with few exceptions it is hard to demonstrate overt responses in the chitons to most environmental variables. They generally show no overt responses to any indirect stimuli, such as changes in light intensity or temperature. They may respond to touch, desiccation and other direct stimuli, such as variations in water movement. Generally, as befitting a truly “mellow” animal, such responses are slow and deliberate.
The chiton nervous system is well developed, but significantly unspecialized when compared to some other mollusks, such as snails or cephalopods. There is a nerve ring surrounding the mouth region and longitudinal nerves that run posteriorly through the animal. The large aggregations of ganglia that are the brains of many other mollusks are lacking in these animals, and their behavior appears correspondingly simple.
Reproduction in chitons follows a basic pattern that is similar to other “primitive” mollusks. The sexes are generally separate, and hermaphroditism is rarely reported in this group. Temperate chitons spawn once a year — reproductive data concerning the tropical chitons are sparse. Both genders spawn into the water, and, consequently, there is no sexual dimorphism.
The eggs are large and yolky. After fertilization the embryos develop quickly into a rapidly swimming non-feeding larva. This larva develops seven grooves across its back where the first seven shells will form, and settles out of the plankton, generally on some rock made tasty by the addition of a coralline algal coating.
After settling, there is no drastic metamorphosis. The animal gradually changes into a miniature chiton. Seven shells develop initially and the eighth follows within a short period. Juvenile chitons on the rocks grow slowly and it may take quite a while to reach a few millimeters in length.
Growth has not been measured in tropical chitons, but in one common eastern Pacific species, Katharina tunicata, the growth rate appears to be about 1 millimeter (1/25th of an inch) per year. As Katharina may exceed 4 centimeters (1 1/2 inches) in length, it is reasonable to assume that the animals may live at least 40 years. This species likely becomes sexually mature at about 15 to 20 millimeters (6 to almost 8 inches) in length, so at about the age of 15 to 20 years.
The largest chiton, Cryptochiton stelleri, may reach lengths exceeding 30 centimeters (12 inches). No one knows how old they get, but many researchers think they may be quite long lived; perhaps living a century or more. Cryptochiton stelleri are unique for another reason. Unlike all other chitons their shells are totally embedded in and out of sight in the girdle.
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RON SHIMEK |
| Two chitons are visible in this picture taken in the waters of northern Puget Sound, Washington. The brown chiton is Tonicella insignis and the partially obscured animal to its left is Tonicella lineata. |
Chitons are herbivores specializing on coralline algae, although they also eat diatoms, filamentous algae, and, occasionally, are grazing carnivores. Coralline forms are found in both the red and green algae (Rhodophyta and Chlorophyta, respectively) and very common components of coral reefs. In fact, the term “reef-forming” coral is really a misnomer, as reefs are not formed by corals but rather by coral and other fragments cemented together by coralline algae, primarily green coralline algae.
Green coralline algae are very common in hobbyist systems as well. Most of the time, we recognize these when they appear on the sides of the aquarium, but much of the gray-green coloration of live rock is due to these algae. Of course, red coralline algae are also found in hobbyist systems and provide a pleasing, albeit largely abnormal, color to mini-coral reef aquaria. In natural reef systems, red coralline algae seldom are found in well-illuminated areas. Their red color comes from accessory photopigments that allow them to dominate low-light environments, such as deep water or cave interiors. In brightly lighted areas, other organisms can generally grow faster than the red coralline algae and overgrow or smother them.
In all of these tropical habitats, as well as in temperate rocky shore habitats, chitons are a common part of the fauna. In temperate areas, grazing by chitons is a major and well-documented biological process. In fact, the rasping of chitons may well be a measurable erosive force on some beaches, as the chitons rasp up measurable amounts of the rocks along with their algae.
Tropical chiton natural history and ecology are poorly known, but it seems likely they have an important effect there as well. The rasping ability of chitons is such that they are generally able to cut into virtually all of the algae in areas where they are found. In an aquarium, the rasping ability of some of the larger chitons may be very impressive.
I once kept a few individuals of the large Pacfic Northwest chiton, Katharina tunicata, in one of my acrylic research tanks as a way to keep down excess algal growth. In a short time, I was able to detect grooves rasped into the acrylic walls of the tank. Visions of tank collapse danced like sugar plums in my dreams, and the chitons were liberated to the beach...
In the best studied habitats where chitons are common, the rocky shores of the Pacific coast of North America, intertidal chitons help maintain the biological diversity of the habitat. Their grazing opens up habitat patches for the recruitment and settlement of many organisms, but also particularly their prey algae. As with many herbivores and predators, chitons tend to eat in one area for a while and then move on to a new area to start to eat, even if there is plenty of food found at the first site. This results in a series of “eaten” patches separated by regions where the algae are intact. This “eat a bit and move” behavior keeps the habitat from becoming dominated by one or another alga. Instead, by their grazing chitons help maintain a diverse ecosystem composed of a mosaic of mini-patches of algae.
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RON SHIMEK |
| The white object in the center of this image is the underside of the girdle of an individual of a species of Placiphorella, the carnivorous chitons. Small tentacles are visible on the edge of the girdle and if touched, they will cause the animal to slap the girdle down capturing prey. The animal is oriented vertically in a rock crevice. |
Chitons probably have the same effect whereever they are common. This could mean that chitons are very important indeed to reef aquarists and to ecologists. Many corals and other organisms, such as snails, are known to settle from the plankton onto coralline algae. By increasing the variety of coralline algae found in a reef area, the chitons could be directly influencing the variety of coral species found there by providing a series of different settling clues for different coral species.
In most reef aquaria coralline algae grow rather well. Indeed, it is difficult to keep them from growing, for when we create the conditions we perceive as necessary for coral growth, we are really creating the conditions for coralline algal growth, as corals are only a small component of the natural algal (= coral) reefs that we are trying to imitate. In hobbyist systems, coralline algal grazing by chitons may indeed result in some algae getting eaten. There are generally a relatively wide variety of such algae present. However, and as they typically reproduce well, these opened patches get filled in rapidly and often by a different species than was there initially. This, in turn, results in a more natural appearing mosaic of algal types.
In closing, I should mention perhaps the oddest behavior in all of chitondom, that of active (sort of) predation. Chitons of the genus Placiphorella have a large expanded girdle on their front ends. The chitons attach themselves to rocks and lift the front girdle up into the water. Numerous small tentacles dangle from the girdle and whenever these are touched by an amphipod or other small crustacean, the girdle is slammed flat to the rock smashing the bug, which is then devoured. These chitons are totally carnivorous and have forsaken the herbivorous diet for one juicy meat.
For pictures of the carnivorous chiton, Placiphorella, follow this link.
For pictures of several chitons follow these links:
www.clever.net/kerry/creature/lchiton.htm and
www.geocities.com/Yosemite/Trails/9029/pc4.htm
Follow this link to a photo of the chiton without visible shells.
For photos of African — mostly Atlantic —chitons, follow this link.
Follow this link to photos of Australian chitons.
For photos of tropical chitons follow these links:
troyb.com/photo/gallery/003-29-chiton.htm and
www.1am.mus.ca.us/~dgeiger/polyplacophora.html.
Finally, follow this link to a photo of a woman wearing a chiton.
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