Spiralians mollusks

Key points

• The Phylum Mollusca can be traced back to at least the Late Precambrian, when Kimberella probably fed on algae in Ediacaran communities.

• Early mollusks were characterized by some short-lived, unusual forms but with the molluskan features of a mantle, mineralized shell and radula; these were members of the small shelly fauna.

• Mollusk shell shape and even ornament can be modeled by a variety of microcomputer-based software packages; only a small percentage of theoretical morphospace is occupied by living and fossil mollusks.

• Bivalves are characterized by a huge variety of shell shapes, dentitions and muscle scars, adapted for a wide range of life strategies in marine and some freshwater environments.

• Most gastropods undergo torsion in early life; they have a single shell, often coiled. The group adapted to a wide range of environments from marine to terrestrial.

• Cephalopods are the most advanced mollusks, with a well-developed head, senses and a nervous system; they include the nautiloids, ammonoids and the coleoids. The group is carnivorous.

• During the Mesozoic many mollusks developed a number of protective strategies such as robust armor or deep infaunal life modes. The group may also have relied on multiformity of shape and color to confuse predator search images.

• Annelid worms were a sister group to the mollusks; their jaws, the scolecodonts, are relatively common in Paleozoic faunas.

She sells seashells on the seashore; The shells that she sells are seashells I'm sure. So if she sells seashells on the seashore, I'm sure that the shells are seashore shells.

Old nursery rhyme

This famous tongue twister was first recited over 200 years ago in England, and it is very likely based on the exploits of Mary Anning, the most famous fossil collector of her time. She is best known for her spectacular discoveries of marine reptiles in the Lower Jurassic rocks of her native Lyme Regis in southern England; but she made most of her regular income from selling fossil ammonites and other mollusk fossils to visitors. Most of us have found seashells while playing or walking on the beach and have been amazed by their colors, shapes and ornaments. Not only are clams, oysters and scallops good to eat, but their shells, throughout historic times, have featured as ornaments, tools and even currency. The Mollusca is the second largest animal phylum after the Arthropoda, with records of over 130,000 living species and a history extending back into the Precambrian.

MOLLUSKS: INTRODUCTION

The Phylum Mollusca includes the slugs, snails, squids, cuttlefish and octopuses in addition to all manner of marine shellfish such as clams, mussels and oysters (Box 13.1). Although some mollusks are the size of sand grains, the giant squid Architeuthis can grow to over 20 m in length, the largest and possibly the most frightening genus of all living invertebrates. Mollusks are probably the most common marine animals today, occupying a very wide range of habitats, from the abyssal depths of the oceans across the continental shelves and intertidal mudflats to forests, lakes and rivers. Mollusks are usually unseg-mented, soft-bodied animals with a body plan based on four features:

1 The head contains the sensory organs, and a rasping feeding organ, the radula, composed of chitin and designed to scrape and in some cases drill.

2 The foot is primitively a sole-like structure on which the animal crawls, but is considerably modified in many mollusks.

3 The visceral mass of the digestive, excretory, reproductive and circulatory organs is enclosed in the celomic cavity.

4 The mantle is a sheet of tissue lying dor-sally over the visceral mass that is responsible for secreting the shell.

Molluskan shells are secreted as calcium carbonate, mainly aragonite, with an organic matrix and an outer organic layer. In the case of the bivalves, a range of shell fabrics have evolved from simple prismatic structures, through nacreous and prismatic, to crossed-lamellar aragonitic and prismatic and foliated calcite fabrics. Shell structure has been used in the higher classification of the group (as in the brachiopods). Crossed-lamellar structures evolved independently in some gastropods. Beneath the mantle, the mantle cavity lies behind the visceral mass and is the respiratory chamber that houses the molluskan gills (ctenidia); the openings of the excretory and reproductive ducts and the anus open into the mantle cavity and their products are carried out on the exhalant current.

From simple beginnings as a limpet-like crawler back in the Precambrian, mollusks have evolved a spectacular range of shapes and sizes, and their hard, calcareous shells are readily fossilized.

A simple method of visualizing molluskan evolution is to consider the hypothetical ancestor, or archemollusk, with a minimal molluskan morphology; this approach has been modified and merged with a recent cladogram for the phylum (Fig. 13.1). There is still a great deal of uncertainty about the identity of the first mollusks, and new finds constantly change the picture (Boxes 13.2, 13.3). The most recent common ancestor of the mollusks probably had seven- to eightfold serial repetition, the presence of valves and a foot, and had a crawling mode of life (Sigwart & Sutton 2007).

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