10 Fijian Seashells by the Seashore

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Shells at Lawaki Resort, Fiji

What is a seashell? A seashell is the hard exoskeleton of marine mollusks such as snails, bivalves, and chitons that serves to protect and support their bodies. It is composed largely of calcium carbonate secreted by the mantle, a skinlike tissue in the mollusk’s body wall. (Rafferty, 2008) 

Table of Contents

What are Molluscs?

Molluscs are multi-cellular animals (cells with a nucleus), they live in and create the shell.

  1. Bilateral plane of symmetry that when observed, divides the animal into symmetrical halves
  2. Soft-bodied (invertebrates – lacking a backbone)
  3. Wholly or partially enclosed in a calcium carbonate shell
  4. They are an extremely diverse group in the animal kingdom ranging from snails to squid) (Table below: 7 Classes of Molluscs)
  5. Sizes range from 1mm (millimeter) to 1.3 meters)
  6. Varied means of reproduction – (separate sexes) a) eggs and sperm are released in the water and fertilized; b) water currents may cause internal fertilization; c) eggs are fertilized by using a muscular penis; d) both sexual organs present in one organism.
  7. Feeding Patterns – a) burrows in muddy sediments consuming microorganisms & detritus filter-feeding; b) scrape algae from rock surfaces, c) catching algae from water currents.
  8. Defense – a) withdraw into its shell; a snail has the added advantage of having a hardened plate on the foot that blocks the shell opening. once the animal has withdrawn. b) Poisons and fluid excreted to confuse the predator.
  9. External features – the mantle, the foot, the head, and the mantle cavity.
ClassExampleApprox. SpeciesDistributionLifespan
Bivalvia (Mainly filter feeders)Clams, mussels, oysters, and scallops20 000Marine and freshwaterUpto 40 years
Cephalopoda (Carnivores)Octopi, squid, cuttlefish, and nautili900Marine only1–15 years
Gastropoda (Carnivore or Herbivore)Snails and slugs70 000Marine, freshwater, land20–50 years
Polyplacophora(Carnivore & Grazers)Chitons940Seabed and rocky tidal zone
Scaphopoda (Deposit feeders)Tusk shells500Marine Only
MonoplacophoraCap like shells31Seabed from 200 meters to 7000 meters
Aplacophora
Table: Classes of Molluscs

How do Molluscs create their shells? and how do they grow?

The shell is made of a lattice of different minerals and organic molecules.  Like the Mammalian bones in humans (a matrix of minerals of calcium and protein of collagen). the molluscs shell is an incredibly strong substance able to survive a variety of stresses   The molluscs shell though, is different from the mammalian bone by the type and proportions of minerals in their shell, they have a mineral content of 95 to 99 percent per weight to 1-5 percent organic, making the shell a biological rock that is alive.  Interwoven between the minerals is the organic protein giving strength and flexibility, and most important the light weight.

The shell begins to develop in the tiny larval stage, the process begins with the shell gland, a small invagination from which the ingredients to make the shell are secreted and structured.  The gland is located in the mantle a hard region where all their organs are located.  First, an organic molecule layer is secreted, which will become the future periostracum (a thin, organic coating that is the outermost layer of the shell), this layer becomes the base in which the snails can perform biomineralization (the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues).

The minerals are deposited on the organic matrix and slowly built upon, some molluscs deposit aragonite on the inside of the shells, while the outside is calcite, as the snail begins to grow the shells begin to spiral around creating the dome inside, in which they will reside (The spiraling can happen in two directions, right-handed spiraling and left-handed spirling.)  The molluscs absorb the minerals, carbon, and hydrogen from the water and food they eat, converting these elements into the required compounds and enabling them to start building their shells.

IN DEPTH SUPPORTING VIDEO: You might want to watch these videos as well: How Seashells Are Made, Life Cycle of the Conch, The Mathematics of Sea Shells

Video: Tate Talk: How do Seashells Grow
Common NameDescription
Caltrop MurexThe shell of Murex tribulus can reach a length of 65–160 millimetres (2.6–6.3 in). This quite common snail has a shell with a very long siphonal canal and numerous very long, fragile and acute spines, providing protection against predators. It feeds on other mollusks. (“Murex tribulus”, 2021)
Purple Pacific DrupeThick, globose shell, up to 5 cm, with low spire, large body whorl, and flat base. Colour white with dark brown nodules. Dark violet, narrow aperture with conspicuous groups of denticles. Columella with three strong, plicate ridges. (“Drupa morum”, 2021)
Ramose MurexA large, solid, very rugged, and heavy shell, of up to 330 mm in length. It has a relatively globose outline, possessing a short spire, a slightly inflated body whorl, and a moderately long siphonal canal. One of its most striking ornamentations is the conspicuous, leaf-like, recurved hollow digitations. It also presents three spinose axial varices per whorl, with two elongated nodes between them. The shell is colored white to light brown externally, with a white aperture, generally pink towards the inner edge, the outer lip, and the columella. (“Chicoreus ramosus”, 2021)
Sauls MurexThe shell size varies between 60 mm and 142 mm
Table: Murex Shells of Fiji – Murex is a genus of medium to large-sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly called “murexes” or “rock snails”
Cone ShellsDescription
Admiral ConeThe size of the shell varies between 35 mm and 109 mm. The color is the shell is chestnut with darker revolving lines, and upper, basal and one or two approximate bands, finely reticulated with yellow on a white ground. This pattern is overlaid with large, irregularly disposed triangular white spots (“Conus ammiralis”, 2022)
Geography ConeHas a broad, thin shell, cylindrically inflated. Geography cones grow to about 10 to 15 cm (4 to 6 in) in length. The size of an adult shell varies between 43 and 166 mm (1.7 and 6.5 in). The ground color of the shell is pink or violaceous white, occasionally reddish. It has a mottled appearance, clouded and coarsely reticulated with chestnut or chocolate, usually forming two very irregular bands. The geography cone has a wide, violaceous white or pink aperture and numerous shoulder ridges or spines. (“Conus geographus”, 2021)
Imperial ConeThe size of an adult shell varies between 40 mm and 110 mm. The color of the thick shell is yellowish-white or cream, with numerous interrupted revolving lines and spots of dark brown and two irregular and wider light brown bands. In the synonym Conus fuscatus, the light brown coloring extends in clouds and irregular markings over the surface, so that the bands can scarcely be defined. The shell has a flat but nodular spire and shoulders. (“Conus imperialis”, 2021)
Leaden ConeThe thin shell is striated throughout. The color of the shell is yellowish or violaceous white, clouded. with chestnut, with distant revolving series of chestnut spots and short lines, most conspicuous on two irregular lighter bands. (“Conus circumcisus”, 2021)
ConeThe thick shell is obconic, with the whorls enrolled upon themselves. The spire is short, smooth or tuberculated. The narrow aperture is elongated with parallel margins and is truncated at the base. The operculum is very small relative to the size of the shell. It is corneous, narrowly elongated, with an apical nucleus, and the impression of the muscular attachment varies from one-half to two-thirds of the inner surface. The outer lip shows a slight sutural sinus. (“Conus”, 2021)
Weasel ConeThe size of an adult shell varies between 40 mm and 107 mm. The low spire is striate, flamed with chocolate and white. The body whorl is yellowish, or orange-brown, encircled by rows of chestnut dots, usually stained chocolate at the base. There is a central white band, with chocolate hieroglyphic markings on either side, and a shoulder band, crossed by chocolate smaller longitudinal markings. The border markings of the bands are reduced to spots. The aperture has a chocolate color with a white band. (“Conus mustelinus”, 2021)
Table: Cone Shell In Fiji – All cone snails are venomous and capable of “stinging” humans; if live ones are handled their venomous sting will occur without warning and can be fatal. The species most dangerous to humans are the larger cones, which prey on small bottom-dwelling fish; the smaller species mostly hunt and eat marine worms.
ShellsDescription
Clithon diadema (Broderip, 1832)This species is known from South East Asia, New Guinea, Solomon Islands, Vanuatu, Samoa and Fiji. It lives on stones near the mouth of streams, often in brackish water. The shells have rows of sharp long spines. The spines are thought to protect the snails from predators which are more common near the mouth of streams than further inland shorter spines as the spines get worn as the snail ages.
Neritina variegata (Lesson, 1831)This species is found in South-East Asia, Solomon Islands, Vanuatu, Fiji, Samoa, Guam, Truk and Ponepe. It lives in streams on stones and boulders from a few metres from the sea to about 5 km inland. The shell is yellow or olive with black markings but it is often completely covered with a black encrustation. The operculum is black and lacks the orange-red horn border found in other Neritina species. It is abundant in fast flowing streams.
Fijidoma maculata (Mousson, 1865)This species is endemic to Fiji and has been found only in inland Viti Levu in the Wailoa, Wainimala and upper Wainibuka and Ba rivers. It is not tall like other thiarids but is round like a neritid due to an enlarged body whorl. Its shell is adapted to living in swift flowing rivers. It reproduces by parthenogenesis and no male have been found. When the young are released from the brood pouch, they are 1.0 -1.6 mm high and are entirely brown with no dark red markings. They grow to about 14 mm high.
Neritina squamaepictaThis shell has a sharp point at the corner of its columella area made by an elevated fold. The columella has a bright red splash at its upper edge. It is found on stones not far from the sea in South-East Asia, Solomon Islands, Vanuatu, New Caledonia and Fiji. Only a few are present at any one site in Fijian streams, but quite a large population was found in slow flowing water in a shallow pool on Efate, Vanuatu.
Septaria bougainvillei (Recluz, 1841)This limpet species is endemic to Fiji, New Caledonia and the island of Tanna (Vanuatu).
Table: Fiji Freshwater Shells
National Geographic Video: Killer Cone Snails

Visitor Advisory 1: (Environmental Ask) Strolling along the beach collecting shells and little beach trinkets to take home as souvenirs, although perceived as a relatively harmless pastime, is extremely environmentally unfriendly. The shells, crab carapaces, and skeletons of other marine invertebrates all contribute to the process of building our reefs and beaches. These organisms gradually break down, adding limestone to the sandy beach and lagoon areas (MES, 2018). Please leave only footprints and take your photos as memories. It is important to allow natural processes to run their course. Below, we dip our toes into the fascinating world of these small invertebrates.

Footnotes

  • Animal – (kingdom Animalia), any of a group of multicellular eukaryotic organisms. They are thought to have evolved independently from the unicellular eukaryotes. Animals differ from members of the two other kingdoms of multicellular eukaryotes, the plants (Plantae) and the fungi (Mycota), in fundamental variations in morphology and physiology. This is largely because animals have developed muscles and hence mobility, a characteristic that has stimulated the further development of tissues and organ systems.
  • Bilateral symmetry – one plane of symmetry will divide a bilateral animal into symmetrical halves, the median longitudinal, or sagittal, plane. Bilateral symmetry is characteristic of the vast majority of animals, including insects, fishes, amphibians, reptiles, birds, mammals, and most crustaceans.
  • Eukarya Category – Organisms whose cells have a nucleus, a sort of sack that holds the cell’s DNA. Animals, plants, protists, and fungi are all eukaryotes because they all have a DNA-holding nuclear membrane within their cells. (Abe,2002)
  • Exoskeleton – An exoskeleton is an external skeleton that supports and protects an animal’s body,
  • Invertebrate – Lacking a backbone or spinal column; not vertebrate
  • Morphology – A branch of biology that deals with the form and structure of animals and plants
  • Multicellular – A multicellular organism is an organism that consists of more than one cell, in contrast to a unicellular organism.
  • Physiology – A branch of biology that deals with the functions and activities of life or of living matter (such as organs, tissues, or cells) and of the physical and chemical phenomena involved.
  • Symmetry – is the repetition of the parts in an animal or plant in an orderly fashion. Specifically, symmetry refers to the correspondence of body parts, in size, shape, and relative position, on opposite sides of a dividing line or distributed around a central point or axis. With the exception of radial symmetry, external form has little relation to internal anatomy, since animals of very different anatomical construction may have the same type of symmetry.
  • Tree of Life Diagram
Tree life three domain system scaled

References

  • Abe, S. (2022). The Three Domains of Life. NASA Astrobiology. Link
  • Adam B. Johnson, Nina S. Fogel, J. David Lambert (2019). Growth and morphogenesis of the gastropod shell. PNAS. Link
  • Barwick & Cadien (2017). Aplacophora [Photograph].
  • Barrett & MacKay (2016). Moon Snails [Photograph]
  • Chicoreus ramosus. (2021, November 7). In Wikipedia. Link
  • Conus ammiralis. (2022, February 8). In Wikipedia. Link
  • Conus. (2022, November 13). In Wikipedia. Link
  • Conus geographus. (2021, November 2). In Wikipedia.
  • Conus mustelinus. (2020, December 23). In Wikipedia. Link
  • Conus imperialis. (2021, March 2). In Wikipedia. Link
  • Encyclopædia Britannica, Inc. (2012). Tree of life illustrating the three-domain classification of life forms [Photograph].
  • Facts Factory | Animals. (2021, April 21). How Do Snails (And Other Molluscs) Create Their Shells? [Video]. YouTube. Link
  • Frink, S. (2008). Bayscallopeyes [Photograph].
  • Fleetham, D. (2019). Perspectives [Photograph].
  • Fleetham, D. (2016). Nautili [Photograph].
  • Ggor NZ (2013). Chiton [Photograph].
  • Kennedy, J. (2019). Cephalopod Class: Species, Habitats, and Diets. Thoughtco. Link
  • Kennedy, J. (2018). What Is a Bivalve? Thoughtco. Link
  • Kennedy, J. (2019). Gastropoda Facts. Thoughtco. Link
  • MAA (2014). Seashells: the plainness and beauty of their mathematical description. Mathematical Association of America. Link
  • MES (2018). Sensible shopping tips for visitors. Mamanuca Environment Society. Link
  • Murex tribulus. (2021, May 15). In Wikipedia. Link
  • National Geographic. (2012, January 12). Killer Cone Snails [Video]. YouTube. Link
  • Oxford Mathematics. (2014, October 14). The Mathematics of Sea Shells [Video]. YouTube. Link
  • Plawen, L. S. (2022). mollusk. Britannica. Link
  • Rafferty, J. P. (2008). Seashell. Britannica. Link
  • Tate Geological Museum. (2020, October 20). Tate Talk- How do Seashells Grow- part 1 [Video]. YouTube. Link
  • Tate Geological Museum. (2020, October 20). Tate Talk- How do Seashells Grow- part 2 [Video]. YouTube. Link
  • Unkown (2017). Monoplacophora [Photograph].