The Wonderful World of Mollusks: A Comprehensive Guide
Mollusks
Mollusks (Mollusca, from Latin molluscus, “soft”) are one of the major phyla of the animal kingdom. They are invertebrate protostomes, coelomate, triblásticos with bilateral symmetry (although some may have a high asymmetry) and non-segmented, soft-bodied, naked or protected by a shell. Mollusks are the most numerous invertebrates after arthropods, and include such well-known forms as clams, oysters, squid, octopus, snails, and a variety of other species, both marine and terrestrial. They are soft-bodied animals (divided into head, visceral mass, and foot) with three unique characteristics:
- A muscular foot.
- A calcareous shell secreted by an underlying integument called the mantle (sometimes absent).
- A toothed feeding organ called a radula (formed by rows of curved chitinous teeth).
External Anatomy
Mantle, Shell, and Mantle Cavity
The basic pattern of a mollusk is a soft-bodied organism, oval, bilaterally symmetric, and with a convex shell shaped like a Chinese hat (internal or absent in some groups). Instead of a shell, they may also possess spicules (which may be present at an embryonic stage but can merge to form a shell in adults) or plates, all with the same origin.
The epidermis of mollusks is covered with cilia and has numerous glands that produce mucus.
Foot
Another characteristic body part of mollusks, the foot, shows enormous evolutionary plasticity. It is equipped with complex and powerful musculature. It is assumed that, originally, it was used for crawling (like that of present gastropods), but it has experienced great diversification, resulting in the digging foot of bivalves, the foot divided into tentacles of cephalopods, or the swimming foot of some pelagic gastropods, among others.
Polyplacophora (Chitons)
Chitons are often found in coastal areas, generally rocky areas with strong surf. Therefore, it is advisable to take appropriate precautions to prevent accidents. It is recommended to wait until low tide for harvesting.
Gastropods
Terrestrial Gastropods
Terrestrial gastropods can usually be found where there is adequate protective cover. Good collection areas include shaded, damp areas covered with leaves, forest floors, and under the bark of trees and fallen logs. They are also found in mountainous areas under limestone slabs, especially in places with an abundance of mosses and litter.
Marine Gastropods and Bivalves
Marine gastropods and bivalves can be collected manually when conditions of access, waves, visibility, and depth permit. Otherwise, dredges or scuba diving must be used.
Freshwater Gastropods and Bivalves
For the collection of freshwater gastropods and bivalves, the most favorable water bodies are those found in areas with rocky beds. Harvesting is usually done during low water periods. Lentic environments (lakes) usually have fewer species than lotic environments (rivers). Bivalves are often found partially buried in gravel, sand, or silt. They have only one shell.
Features: Their eyes are located at the end of tentacles that they hide in case of danger.
Habitat: On land, like snails, they breathe through lungs. In the sea, like periwinkles, they breathe through gills.
Cephalopods
Cephalopods are exclusively marine animals with a wide geographical and bathymetric distribution. The collection of these animals can be done through the use of different fishing gears, and in the case of those species living in shallow water, they can be collected manually in the pits or rocky caves they inhabit. You can also collect shallow-water species by using objects (cans, old tires, pots, etc.) previously placed in the capture area.
Features: They have no outer shell. Some, such as squid, have an ink sac to cloud the water in case of danger. They have long tentacles on their head. They are oceanic and breathe through gills.
Cephalopods have highly developed vision and are believed to be able to determine the color and intensity of light. Using their excellent eyesight and chromatophores, cephalopods can blend in perfectly, creating color patterns very similar to the surrounding seabed.
Important Note: Cephalopods, like other sea creatures, generate light by mixing two substances that combine to form a compound that emits light, a mechanism similar to that used by fireflies or in “light sticks” used by some divers on night dives. To turn on a “light stick,” you generally need to bend it, breaking a glass capsule inside, which causes two chemicals to mix and react, generating a third substance that emits light.
This same process is essentially what happens in “photocytes” (light-generating cells) and “candles” (light-generating bodies) of living organisms. A substance called luciferin reacts with oxygen in the presence of an enzyme, luciferase. This chemical reaction results in a new molecule that, in oceanic animals, usually emits blue or green light.
Bivalves
These invertebrates are characterized by having a shell with two parts or valves. They are also known as pelecypods (having a foot shaped like a shovel or axe) and lamellibranchs (having laminar gills).
The mantle can be seen in the organism’s body. This is a folded sheet that wraps around the animal. Its cells precipitate CaCO3 (calcium carbonate) from the blood to grow the shell.
- The edge of the mantle may be smooth or have sensory extensions and even primitive eyes (mantle eyes).
Characteristics: They have two shells or valves, hence their name. They have no distinct head. They live in the sea, buried in the sand or attached to rocks. They breathe through gills.
Reproductive System: They have a pair of gonads located on the gastrointestinal tract, and during the breeding season, they occupy all available space. Saltwater species are dioecious (separate sexes in different organisms), while freshwater species are hermaphroditic (one body has both sexes). Fertilization is external, but the fact that the mother retains the eggs inside may lead one to mistakenly believe it is internal. Development is indirect, meaning the bivalve larvae go through several stages with a different appearance from the adult individual.