Category: Molluscs: other than Cephalopods

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Molluscs comprise a phylum of invertebrates that inhabit mainly marine but also freshwater and terrestrial environments. In spite of displaying striking diversity in size and form, molluscs as a rule all possess some form of chitin-based shell (often mineralised with calcium carbonate), the shell being secreted by a soft mantle with a respiratory cavity within and beneath this a muscular 'foot' for locomotion. The fossil record of molluscs extends back to the Cambrian, and if the Ediacaran animal Kimberella is a mollusc perhaps as far back as about 550Ma. Living molluscs are generally accommodated in one of seven classes. The most diverse mollusc class is the Gastropoda (e.g. snail, slugs, limpets and nudibranchs), with Bivalvia (e.g. mussels, oysters, scallops) and Cephalopoda (octopuses, squid, cuttlefish and Nautilus) being the other two major classes. Less well known are the tusk shells (Scaphopoda), chitons (Polyplacophora), spicule worms (Aplacophora) and deep-sea "limpets" termed Monoplacophora. Whilst cephalopods are exemplars of evolutionary convergence because of their many striking similarities to the vertebrates, gastropods, bivalves and the other remaining molluscs offer the biologist many compelling examples of convergence, ranging from shell form to sexual behaviour. Just a few of these are outlined below...

Molluscs can often be defined by their shell geometry (e.g. spiral in snails, planispiral in cephalopods, two valves in bivalves), and yet when discussing shell shape variation in terms of 'skeleton space' we find the possibilities are not infinite, and many striking examples of convergence are known. For example, one group of snails has independently evolved bivalved shells and several groups of molluscs (as well as the cheilostome bryozoan Lunularia patelliformis) have evolved limpet-like form.

Some molluscs possess eyes that are highly convergent. The giant clam Tridaca has pin-hole eyes along its mantle margin that are similar in form to those of Nautilus. Arca bivalves also have eyes along their mantle margin, but this time they are compound eyes, similar to those of the arthropods. Camera eyes are notable in heteropods, strombids (conches), littorinids, pulmonates and the snail Viviparus, all resembling the camera eyes of many cephalopods and vertebrates.

In terms of reproduction, several fresh-water snails, including pachylids and Viviparus, have independently evolved viviparity (live birth), and brooding and parental care have both evolved independently within the snails, converging with many other invertebrate and vertebrate groups. Yet another spectacular example of convergence is the evolution of love darts, not only within the snails and slugs, but more impressively in the earthworms.

One of the most remarkable convergences recognized among non-cephalopod molluscs is 'agriculture', whereby limpets deliberately encourage (or cultivate) algal farms, and littorinid snails cultivate a parasitic fungus through a mutualistic association. Fungal farming is also known in insects such as the attine ants, termites and ambrosia beetles.

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Topic Title Teaser text Availablity
Snail eating: an asymmetric diet

Snails may not be everyone's first choice on the menu but several distinct colubrid snakes have evolved expert techniques for gorging on these nutritious gastropods.

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Productid brachiopods and rudist bivalves n/a Not Available
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Agriculture in aquatic snails

Termites and ants are famous for tending fungal gardens, but did you know that also a marine snail farms a fungus? And this is not the only example of agriculture in this group…

Feeding in snakes and lizards

The Turtle-headed sea snake feeds on small eggs and its feeding shows intriguing similarities to the way lizards forage, and herbivorous mammals graze and browse.

Ink production in cephalopods and gastropods

A series of striking convergences can be found in the sea-hares (Aplysia), a group of gastropods and only remotely related to the cephalopods.  Not only do they emit ink clouds (the colour is derived from ingested red algae), but they also employ chemical cues that assist in defense.

Pinhole eyes in Nautilus and giant clam

The pinhole eye has evolved not only in the Pearly Nautilus, but also in another group of molluscs, the bivalves and specifically the giant clams (Tridacna).

Compound eyes in ark clams

Read on if you want to know more about bivalves with burglar alarms…

Camera eyes in gastropod molluscs

The fast-moving cephalopod molluscs are famous for their camera eyes, but why on earth have gastropod snails, which are not exactly known for their speed, evolved this superb visual organ at least four times?

Scanning eyes in molluscs and arthropods

Some sea snails have a linear retina. What a hopeless arrangement, to see the world through just a narrow slit! Not quite, because they have come up with a rather intriguing trick to extend their visual field - and it's a trick too good to use only once.

Silk production and use in arthropods

Remarkably, fossil silk is known, especially from amber of Cretaceous age. Material includes both silk with trapped insects, possibly from an orb-web, and strands with the characteristic viscid droplets that are the key in trapping prey.

Mussel attachment and the Pinna byssus

It is clear that the Pinna byssus has unusual properties in comparison to its equivalent in the bivalve mussel, and is conspicuously different in terms of crystallinity.

Agriculture: from ants to dugongs

Human farmers tending their fields are a familiar sight. But don't forget about those fungus-farming termites or the fish with a garden of algae…

Parental care in vertebrates, echinoids, molluscs and brachiopods

The independent evolution of parental care is far more widespread than birds and mammals, extending as far as molluscs and echinoderms!

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Bivalve molluscs: convergent shells and symbioses

Despite their range of shell types, there is evidence of extensive convergence, notably amongst the fresh water swan-mussels (unionids), mytilids and anomalodesmatans.

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Molluscan radulas and boring organs

Most likely the radula is primitive to the molluscs, but not surprisingly the various functional and feeding requirements have led to significant convergence.

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Gastropod molluscs: snail shell anatomy

Snail shells typically form a helical spiral, but within this geometry there is a considerable degree of convergence.

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Scaphopod molluscs: convergence and homeomorphy

The scaphopods, or elephant-tusk shells, are a relatively obscure class of the Mollusca. They provide instances of convergence both within the group itself and as homeomorphs with the cephalopods.

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Haemocyanin in arthropods and molluscs

The degree of similarity between the active sites in arthropod and molluscan haemocyanin has been called “remarkable” and “startling”, but actually suggests that wherever in the universe life employs copper for aerobic respiration it will call upon haemocyanin.

Transparent tissues: eyes, bodies and reflective surfaces

Read on if you want to know about the numerous animal equivalents to the invisible man...

Love darts in slugs, snails and annelid worms

The curious habit of stabbing their partners with sharp calcareous (or chitinous) darts during courtship and prior to actual copulation has understandably attracted considerable attention.

Viviparity (live birth) in animals

Viviparity is rampantly convergent, with famous examples in the reptiles, notably the lizards and snakes.

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Camera eyes in vertebrates, cephalopods and other animals

Camera eyes are superb optical devices, so it is not surprising that they have evolved several times. But why, of all animals, in the brainless jellyfish? Or for that matter in a slow-moving snail?