While hard to find in the winter, hermit crabs are suddenly back in large numbers. I saw the tiniest hermit crabs occupying the smallest snail shells that I have ever spotted. It almost required a magnifying glass to watch these minis. Their house suppliers, the periwinkles and top snails, were also back in huge numbers.Under any given rock there were a multitude of intertidal organisms including fish (eel-like looking gunnels and small tide pool sculpins), sea cucumbers, sea stars, hermit crabs, kelp isopods, sand fleas, worms and really cute tiny chitons.
Chitons are a member of the mollusc phylum and share some characteristics with snails and limpets. They have a muscular foot on which they scoot along and which helps them attach firmly to their rocky habitat. They also have a rasping tongue called a radula, which is only found among molluscs. The radula is a fascinating invention of nature looking much like a miniature chain saw, but in the case of the chitons it is made with a mineral called magnetite, apparently the only known use of that mineral in animals.
On the chiton’s top side are eight interlocking plates or valves. These are held together and surrounded by a fringe called a girdle. The plates give the animal protection from impacts coming from above while still allowing flexible movement. Chitons can roll into a little ball like a pillbug when pried off the rocks. If the animal dies or gets eaten, the connecting tissue no longer holds the valves together and the individual plates can often be found washed up on the beach or carried onto land by birds and otters. These plates are often called “butterfly shells” because of their shape.
Most common in our tide pools is the Black Chiton Katharina tunicata, also called Black Katy. It gets to about three inches in size and is a traditional food of Alaskan Native people of this area. The butterfly shells have been found in mittens dating back 3,000 years and more. While I would advise against eating any filter feeding molluscs like mussels and clams because of the danger of paralytic shellfish poisoning, chitons, limpets and snails are considered safe to eat because they graze algae off the rocks rather than filtering the water for food. Of course, one should always keep in mind that heavy harvesting in one area can deplete local populations easily.
The Black Katy reaches maturity at two years of age and lives for another three years as an adult. It reproduces by external fertilization, meaning that eggs and sperm are released into the water. Each individual is either male or female (this is not always so in snails!). The eggs develop into a small planktonic larva, but settle out very quickly. In a laboratory study it was found that if a certain red corraline alga was in the test tank, the larval stage only lasted two and a half hours, but if this alga was not present it lasted longer.
Remembering those tiny chitons on the beach that were no larger than a baby’s fingernail makes me wonder that something so small has all the pieces and parts just like the adult animal, only all in nano-format. Then again, whether it is a blue whale, the largest creature on earth, a human, or a black chiton, we all start out as a single cell with all the information it takes. It just goes to show that size really doesn’t matter.