SCAPHOPODS: CHARACTERISTICS, MORPHOLOGY, REPRODUCTION, EXAMPLES - BIOLOGY - 2024

The scaphopods are a group of animals belonging to the phylum Mollusca. They are also known as tusk shells due to the resemblance they have to the tusks of elephants. Etymologically, its name comes from the words skaphe (boat) and podos (foot), so its meaning is that of an animal that has a boat-shaped foot. And nothing is more true, since they present a foot with that shape.

They were first described by naturalist Heinrich Bronn in 1862. Scaphopods are very ancient mollusks, believed to have originated during the Devonian period, as the oldest known fossils date from that period. Approximately a little more than 850 species distributed throughout the world have been described.

Specimen of scaphopod. Source: © Hans Hillewaert

characteristics

Scaphopods are classified within multicellular eukaryotic organisms. This is so because their cells have a cell nucleus inside which is the DNA packed forming chromosomes. These cells, which are specialized in various functions, are in turn grouped together, forming the different tissues that make up the animal.

These animals are triblastic, since they present the three germ embryonic layers known as endoderm, ectoderm and mesoderm. They are also coelominated and deuterostomized.

Likewise, scaphopods reproduce sexually, with external fertilization and indirect development.

These animals present bilateral symmetry, since they are made up of two exactly equal halves. This can be seen by drawing an imaginary line along the longitudinal axis of your body.

Regarding their habitat, scaphopods are purely marine animals. They are widely distributed throughout the seas of the planet. They are located mainly semi-buried in the substrate, with a very small surface of their body outside the seabed.

Taxonomy

The taxonomic classification of scaphopods is as follows:

-Domain: Eukarya

-Animalia Kingdom

-Filo: Mollusca

-Class: Scaphopoda

Morphology

- External anatomy

In general, the body of these animals has a tubular shape, although they are of variable size. Depending on the species, there are some that barely measure less than 5mm, while there are others that can exceed 20 cm.

Like most mollusks, they have an external shell. Inside it is housed the fleshy body of the animal, made up of the head, the foot and the visceral mass.

Shell

It can be whitish or yellowish in color, as well as smooth or streaked. Its shape resembles that of an elephant tusk. It has two openings, one with a small posterior location and a slightly larger anterior one.

Head

It is the smallest part of the animal's body. It is rather rudimentary, since it does not have any type of sense organs.

Its most striking elements are two protrusions that are on both sides and from which thin extensions emerge, which have cilia on their surface and are known by the name of captacles. Those captacles are found surrounding the animal's mouth.

Visceral mass

It occupies practically the entire interior of the animal's shell. Inside all the organs are contained. It is surrounded by the mantle.

Foot

This structure protrudes through the anterior opening of the animal's shell. It is cylindrical in shape and has the ability to be contractile. This is of great help, since the foot is in charge of digging in the substrate in which the scaphopod is buried. Thanks to this foot it is that the animal can stay anchored to the substrate in which it lives.

- Internal anatomy

Scaphopods have digestive, excretory, nervous, circulatory and reproductive systems. They do not have a respiratory system as such, since it does not have specialized structures in this function, so gas exchange occurs through the mantle.

Digestive system

The digestive system of scaphopods is complete. It has an entrance hole, which is the mouth, and an exit hole, the anus.

The mouth is surrounded by captacles that help capture possible prey. Inside the oral cavity is the radula, which is a structure typical of mollusks.

Immediately after the oral cavity is the esophagus, which is short and narrow in diameter. This communicates with the stomach. The duct of a gastric gland attached to the digestive system empties into this.

Later is the intestine, which is where nutrients are absorbed and the duct finally empties into the anus, through which digestion waste is released.

Nervous system

The nervous system is ganglionic type, made up of numerous neuronal clusters called ganglia.

Presents the so-called brain ganglia, from which nerve fibers emerge that innervate some structures such as statocysts. Very close to these nodes are the pleural nodes.

Internal anatomy of a scaphopod. Source: Maulucioni. Original by KDS4444

Likewise, the buccal and stomach ganglia that are responsible for the innervation of the digestive tract are also appreciated. From these ganglia emerge nerves that communicate with the brain ganglia. There are also nerves that run throughout the anatomy of the animal.

Circulatory system

These animals do not have specialized organs for circulation such as a heart or blood vessels. Instead, the circulatory system is made up of only the so-called sinuses, such as the peri-renal sinus.

Excretory system

It is quite simple. It is made up of two structures analogous to the kidneys, known as nephridiums. These have ducts, which lead to a hole (nephridiopore) that open into the paleal cavity.

Reproductive system

Scaphopods are dioecious organisms, which means that the sexes are separated. Each individual has a gonad (testicle or ovary) that is located in the posterior part of the interior of the visceral mass. They present a conduit that leads to the nephridipore, specifically the right one.

Reproduction

The type of reproduction observed in scaphopods is sexual. This involves the fusion of male and female sex cells (gametes).

The scaphopods present external fertilization, since the gametes unite outside the body of the female. When the moment of reproduction arrives, both the female and the male release the gametes (sperm and ovules) to the outside through the nephridiopore.

In water, both cells unite and fuse to form an egg. Taking into account embryonic development, scaphopod eggs are made up of three regions: upper region, lower region and central region.

The upper region, from which the ectoderm is formed, the lower region, which will give rise to the mesoderm, and the central region, from which the endoderm originates. From these embryonic layers the cells and tissues of the adult animal will develop.

These organisms present an indirect development, since when the eggs hatch, a larva emerges from them. This larva has the shape of a top and is of the trochophore type, free-living. This means that it moves freely through water currents.

Later, that larva begins to undergo metamorphosis and transforms into a belligerent larva, which is typical of some groups of mollusks. This presents a kind of shell and its characteristic element, the veil. Later, when it disappears, the larva falls to the seabed and metamorphosis culminates to become an adult individual.

Nutrition

Scaphopods are heterotrophic, carnivorous organisms, although they can also feed on some algae. Due to the small size of their digestive system, they feed on very small particles of food. The type of animals on which they feed includes foraminifera and ostracods.

They capture their prey through the extensions that they present, the captacles, specifically thanks to a substance of gelatinous consistency that they secrete and that allow their prey to adhere to them. Once in the oral cavity, the food is subjected to a process of degradation by the radula, then through the esophagus it is transported to the stomach where it undergoes the action of various digestive enzymes.

It then passes into the intestine, where the absorption process occurs. Finally, the food particles that are not absorbed and used by the animal's body are expelled through the anal orifice.

Examples of species

Antalis vulgaris

It is the most representative species of scaphopods. Measures approximately 35mm in length. It has the typical morphology, similar to a white horn, with a hole at each end and a resistant protective shell. It is located mainly on the coasts of Western Europe, specifically in the North Sea and the Mediterranean Sea.

Specimens of Antalis vulgaris. Source: Georges Jansoone (JoJan)

Antalis enthalis

In appearance, it is very similar to Antalis vulgaris. However, it has some differences such as size (it measures approximately 50 mm) and also has grooves on the surface of its shell. It is found mainly in the North Sea and to a much lesser extent in the Mediterranean Sea.

Cadulus jeffreysi

It is short in length and has a completely smooth white shell. Its body is characterized because it is a little bulky in the center, it is not as stylized as that of other species. It is found off the coast of Norway, the Azores Islands and the Mediterranean Sea.

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Buchsbaum, R., Buchsbaum, J. Pearse, and V. Pearse. (1987) Animals without backbones.. University of Chicago Press.
3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Steiner, G. (1997). Scaphopoda from the Spanish coasts. Iberus. fifteen
6. Trigo, J. and Souza, J. (2017). Phylum Mollusca. Class Scaphopoda. Chapter of the book: Inventory of the marine biodiversity of Galicia. LEMGAL project