The shells you find on the seashore are normally empty, and they represent the end of the story as they sit on your shelf after you’ve gathered them, or are simply left on the beach to gradually break down into tiny pieces until they become sand.
But what of the start of the story? How are they grown, and what once lived inside them?
Molluscs are one of the largest groups in the animal kingdom, with over 100,000 species. Most of them live in the sea, but some have taken to the land (garden snails and slugs) and others to freshwater (mussels and snails).
Others have their brightest colours of their shells hidden away permanently as their mantle or undersurface. In both those cases, camouflage as a reason for attractive colours makes no sense at all.
The truth? We do not yet understand why the shells often take the bright or beautiful forms that they do – but researches are continuing to try to discover the reason.
So perhaps as far as the story of seashells go, there isn’t an ending after all…
In the Gastropods and Bivalves the mantle is folded to enclose a cavity (the mantle cavity) inside which lie the gills and the outlets for the excretory and reproductive organs. This mantle cavity takes up a lot of space as it encloses the greater part of the animal’s body.
Some shells are lined with mother of pearl (as formerly used in high quality buttons) possibly to protect soft organs of the body. If a grain of sand finds its way between the mantle and the shell, the
mantle develops a small pocket around it. The lining layers then isolate the source of the irritation, producing what we know as a pearl.
IT'S A SHORE THING
Next time you’re collecting seashells on the beach spare
a thought for the hard work
that went into creating them,
and the incredible, vulnerable occupant they once housed
The 'skeletal remains' of a Gastropod, the Common Whelk. Beside it is a Baltic Tellin, which is a Bivalve
Of those which are marine dwellers, the majority are Gastropods and Bivalves. Although their body structure varies, they nearly all possess a shell. In Gastropods this is a protective coiled tube, while the Bivalves each have two rigid shells (valves) which enfold the soft body.
The outside of the body of all Molluscs (when Gastropod or Bivalve) is covered with skin, part of which – the mantle – covers the region containing the internal organs. The outer surface of the mantle secretes hard crystals of calcium carbonate, not unlike a form of hard chalk, to make the shell.
The concentric rings and lines, which you find on all shells, indicate the stages of growth. The rate of shell growth is uneven since, when food is scarce, the mollusc may use up all its energy to survive rather than develop.
If the shell is damaged the occupant makes its own repair by secreting calcium carbonate at the problem area. Amazingly, a crack can be made as good as
new in as little as twelve hours!
Special glands at the edge of the mantle produce colour in the outermost layers of the shell. Incredibly, the colours vary not only according to the particular mollusc’s genes, but also according to water temperature, available sunlight and each mollusc’s diet.
Some of the shells are absolutely beautiful, which leads to an interesting question: why, when most mulluscs do not really ‘see’?
Camouflage may be one reason. But many molluscs spend their lives hidden away, buried under sand or mud.
Waves will break shells down into sand
The mantle of a Gastropod develops in such a way that the single shell grows as a coiled tube, housing the body of the animal. In many species the mantle is able to produce special features in the shell, such as complicated patterns or changes in surface ornamentation. Surface ornaments on the shell include points, lines and ribs.
The coiling process of the Gastropod’s shell is usually such that the shell appears to be built around a central column (columella), which may be solid or hollow.
In many species such as the periwinkles, a hard shelly or horny stopper (operculum) is carried on the animal’s tail. When the tail is withdrawn into the shell, the stopper closes the hole up, protecting the occupant against predators, and also preventing the snail’s body drying up when exposed.
A Dogwhelk peeks from beneath the sand
In Bivalves, the two valves (which may be equal in size or shape, as with Prickly Cockle Shells, or unequal as with Scallops) are hinged together over the animal’s back by a leathery, elastic ligament. A toothed hinge prevents one valve sliding sideways across the other. Many Bivalves burrow in a vertical position, with its hinge at the top.
When a Bivalve is feeding, the shells gape open; this allows the food-collecting siphon to protrude, and inhale a current of water bearing food and oxygen into the mantle cavity. Here the gills remove the oxygen and filter off the suspended food particles. ‘Cleaned’ water is removed by the exhalent siphon. When a mollusc is disturbed, it uses its powerful adductor muscles to pull the two valves tightly shut.
The European Native Oyster's shell can become impressively large. The periods of growth make lines which are clearly visible, inset.
A MUSSEL’S INCREDIBLE MUSCLES!
No matter how uneven the rock surface, or how hard the waves lash them, mussels cling on. But how?
It’s all down to fine, sticky threads it creates…with its foot!
A mussel extends part of its foot and rolls it into a tube, then fills it with a mixture of proteins and other mystery substances which are produced by the byssal gland, also located in its foot. The substances then harden into a slender thread, which is not only incredibly strong, but also flexible enough to allow the mussel to move about should it want to, still safely attached to the rock.
In fact, the sticky substance is so incredible that scientists are now studying in the hope of creating new adhesives!
Want to learn more about the jellyfish you spot on the beach this summer? Check out the Wildlife Trusts's wonderful guide, below.