Malacology is the study of mollusks. This includes animals like octopus, snails, slugs, and clams. It is the second largest phylum of animals, making them one of the most successful groups on the planet. There are over 80,000 described species of mollusks with many more left to be discovered. Mullusca is composed of 8 recognized classes including Cephalopoda, Gastropoda, Polyplacophora, Scaphapoda, Monoplacophorans, the Aplacophorans, Caudofoveata and Solenogastres, and Bivalvia.
Mollusca and the Environment
To understand the ecology of mollusks you should have some background on the environmental conditions affecting them. These include things like temperature, salinity and oxygen availability.
Temperature is the most important factor limiting the ranges of most marine species. This is especially true of ectothermic species like mollusks. An ectotherm is an organism that’s body temperature is regulated by the environment around them. Temperature is a major factor limiting the range of many species and can impact metabolism, shell formation, and behavior among other things. Each species has a Specific Heat Tolerance which is maximum and minimum temperature that it can tolerate. As the animal approaches its maximum heat tolerance, behavioral and physiological reactions will begin to limit damage caused by these high temperatures. This includes the release of heat shock proteins that help prevent protein degradation and misfolding at higher temperatures.
The intertidal zone and hydrothermal vents represent the most thermally stressful environment for mollusks. The intertidal zone is the area between the ocean’s low tide and hightide line. This area can exhibit dramatic temperature fluctuations as animals are forced to tolerate both the varying water temperature and air temperature. Organisms living in this environment can have 10-20° C (18-36° F) fluctuations in body temperature between tides! Even more extreme the seawater in hydrothermal vents can reach temperatures of 370°C (700°F)! The mollusks living around these vents must tolerate water temperatures that can change by over 13° C in just seconds!
Salinity can have important implications on the biology of mollusks. Marine mollusks are typically osmoconformers meaning the salinity in their body is the same as surrounding waters. However, mollusks living in brackish or freshwater environments must maintain proper salt levels in their body tissues via active processes. These are Osmoregulaters, which have different salinity levels in their body compared to the outside environment. Some shelled mollusks can seal their shells to protect their body from salinity fluctuations, including many bivalves and some gastropods that use their operculum to seal the shell opening.
Like with other animals, oxygen is necessary for mollusks to respirate and use energy to live. There are two strategies used by mollusks to uptake oxygen from the environment. Some change the amount of oxygen they consume based on the amount of oxygen available, these are called oxyconformers. Others called oxyregulators will maintain the same rate of oxygen uptake regardless of its environmental availability. Oxyregulation is like breathing more heavily when you’re out of breath vs oxyconformity which is like breathing the same rate regardless of activity. Mollusks can even be found in anoxic environments or places lacking oxygen. They use a wide range of adaptions to tolerate these hostile habitats.