Malacology - An introduction to Mollusca, its characteristics, morphology, family, history, ecology and fossil record

Malacology

Chitons, clams, mussels, snails, sea slugs, tusk shells, octopus, and squid are among the mollusks studied in malacology. Mollusks can be found practically anywhere, including on the rocky shore, in freshwater settings, and in your own backyard.

Characteristics

Molluscs have the following characteristics: a soft, unsegmented body with a muscular foot or tentacles and a mantle capable of secreting a shell. Mollusks in general, but not all, have: a radula is an interior or external shell (tongue with teeth).

Introduction

With at least 50,000 live species, mollusks are one of the most diverse animal groupings on the world (and more likely around 200,000). Snails, octopuses, squid, clams, scallops, oysters, and chitons are among the species found there. Mollusca also includes several lesser-known families, such as the monoplacophorans, which were assumed to be extinct for millions of years until a specimen was discovered in the deep water off the coast of Costa Rica in 1952.

Molluscs are a group of creatures with soft bodies that usually feature a "head" and "foot" area. The shells of snails and clams, as well as the plates of chitons, are often covered by a hard exoskeleton.

Molluscs are incredibly essential members of many biological communities, as they are found in practically every habitat on the planet. They can be found anywhere from terrestrial mountain summits to deep oceanic hot vents and cold seeps, and they range in size from 20-meter-long giant squid to microscopic aplacophorans that dwell between sand grains.

Figure 1 The body fins of a cuttlefish, a coleoid cephalopod, move mostly through undulating.

Throughout history, these species have been essential to humans as a source of food, jewellery, tools, and even pets. On the Pacific coast of California, for example, Native Americans ate a lot of abalone, especially owl limpets. The impact of Native Americans on these molluscan communities, however, pales in comparison to the United States' overharvesting of select molluscan taxa in the 1960s and 1970s. Species that once numbered in the millions are now on the verge of becoming extinct. After many million white abalones were kidnapped and sold as meat in the 1970s, there are now only about 100 left. Mollusks have tasty soft parts as well as desired hard sections. Some mollusk shells are regarded to be quite attractive and costly. Molluscs, such as the common garden snail, can be a nuisance, and they are a prominent component of fouling populations on docks and on ship hulls.

Figure 2 The California Trivia is a marine snail on the left (Trivia californiana). Much of the shell is covered by the mantle here. The syphon is formed by rolling a part of the mantle into a tube shape slightly above the skull. A restoration of the Giant Squid, one of the largest of all mollusks, may be seen on the right (Architeuthis).

Systematics

The systematics of mollusks is still in change. As you can see from the cladogram below, some of the key relationships are still up for debate. The polytomies displayed reveal that the subject of which mollusks are the most closely related is still a point of contention.

New sorts of data, as well as increasingly larger and more complex analysis, are still being conducted. Cephalopods, scaphopods, and gastropods are examples of recently discovered resolved relationships.

Figure 3 Family tree of mollusca.

Morphology

Despite their incredible diversity, all mollusks have several distinguishing traits that identify their body plans. A head, a foot, and a visceral mass make up the human body. A mantle (also known as a pallium) covers everything and usually secretes the shell. The mantle is lost secondarily in some animals, such as slugs and octopuses, while it is used for different purposes, such as respiration, in others.

A radula (lost in bivalves) – a ribbon of teeth supported by an odontophore, a muscular structure is found in the buccal cavity of the mollusk. Typically, the radula is utilized for feeding. The ventral foot is used to propel the animal forward. The mollusk is propelled by this foot, which uses muscle waves and/or cilia in conjunction with mucus to move the mollusk forward. One or more pairs of gills (called ctenidia) are usually found in a posterior chamber (the pallial cavity) or a posterolateral groove enclosing the foot, at least in the more primitive members of each order. The pallial cavity comprises two sensory osphradia (for smelling) and is the compartment into which the kidneys, gonads, and anus is opened. Mollusks have a decreased coelom, which is represented by the kidneys, gonads, and pericardium, the primary body cavity that surrounds the heart.

Figure 4 The radula, the two "toothy" arcs you can see bordering the mouth of the freshwater Sinistral Pond Snail (Physella sp.), scrapes algae from the glass.

History and Ecology

Molluscs can be found in practically every ecosystem on the planet, and they are often the most visible organisms. While the majority of gastropods dwell in marine environments ranging from the intertidal to the deepest oceans, several important clades exist primarily in freshwater or terrestrial environments. Surprisingly, at a coral reef in New Caledonia, one study discovered nearly 3000 species in a single spot. Gastropods can achieve quite high diversity and richness in terrestrial communities, with as many as 60-70 species coexisting in a single habitat and abundance in leaf litter exceeding 500 individuals per four litres.

Marine mollusks can be found on rocky shoreline, coral reefs, mud flats, and sandy beaches, among other places. These firm surfaces are dominated by gastropods and chitons, while softer substrates are dominated by bivalves that dig into the silt. There are a few exceptions: Tridacna gigas, the world's largest bivalve, lives on coral reefs, and many bivalves (such as mussels and oysters) attach themselves to hard substrates. Some minuscule gastropods even live between the grains of sand.

Figure 5 Sinistral Pond Snails (Physella sp.) hatch from their eggs as juvenile snails, unlike many other marine mollusks that hatch as planktonic trochophore larvae. Ostracodes are the whitish, jellybean-shaped creatures (crustaceans).

At deep sea hydrothermal vents, large densities of gastropods and bivalves can be found. For the Mollusca and other outgroups, living in these or other dysoxic settings appears to be a plesiomorphic condition. For example, the molluscan groups Bivalvia, Monoplacophora, and Gastropoda, as well as the outgroups Brachiopoda and Annelida, make up the fauna of Palaeozoic hydrothermal vent communities.

Molluscan evolution appears to have been profoundly influenced by the adoption of various eating habits. One of the most prominent elements of the group's radiation is the shift from grazing to other types of food acquisition. The first molluscs grazed on encrusting creatures and detritus, according to our current understanding of relationships. Feeding on algal, diatom, or cyanobacterial films and mats, as well as encrusting colonial creatures, could have been selective or indiscriminate.

Fossil Record

Some of the oldest metazoans are found in the Mollusca. Late Precambrian rocks from southern Australia and northern Russia include bilaterally symmetrical benthic creatures with a single-valved shell (Kimberella) that looks like mollusk shells. Helcionelloid molluscs from Late Ediacaran (Vendian) rocks are the earliest unequivocal molluscs. Coeloscleritophora can also be found in the Early Cambrian.

Most well-known groups, such as gastropods, bivalves, monoplacophorans, and rostroconchs, are Early Cambrian, but cephalopods, polyplacophorans, and the Scaphopoda are Middle Cambrian, Late Cambrian, and Middle Ordovician, respectively. The majority of these early taxa are tiny (less than 10 mm in length). The Cambrian-Ordovician taxa have little relation to the Late Vendian-Early Cambrian taxa (most of which remain extant today).

Figure 6 Inoceramus sp., a bivalve from the Cretaceous of Alameda County, CA, appears on the left. Turritella andersoni, a gastropod from the Eocene of Ventura County, CA, is seen at right.

Molluscan taxonomic diversity remained modest from their initial emergence until the Ordovician, when gastropods, bivalves, and cephalopods showed huge increases in diversity. This diversification rises for bivalves and gastropods throughout the Phanerozoic, with very minor losses at the end-Permian and end-Cretaceous extinction events. Throughout the Phanerozoic, cephalopod diversity is significantly more variable, whereas the remaining taxa (monoplacophorans, rostroconchs, polyplacophorans, and scaphopods) have low diversity or have gone extinct.