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Tusk Shell

Scaphopoda

Tusk shells or tooth shells are marine mollusks with shells that resemble an elephant’s tusk. As members of the class Scaphopoda (thus also called scaphopods), these animals inhabit the offshore areas and mostly bury themselves in sand or mud with their heads pointed downwards.

More than 350 species of tusk shells are broadly classified into two orders: the Gadilida and Dentaliida.

Description

Size

They are typically small, measuring around 0.5 to 18 cm (0.20 to 7.09 inches) in their shell length. The largest tusk shell, Fissidentalium metivieri, is 18 cm (7.09 inches) long.

Body Plan

Tusk shells have a tapering body covered by a hard shell. While the apex of the shell and the muscular mantle constitute the anatomically dorsal side of the animal, the concave side of the shell and viscera form the ventral surface. Similarly, the larger aperture of the shell, along with the foot, forms the anterior side, while the smaller apex represents the posterior.

Shell

All tusk shells possess an elongated, bilaterally symmetric shell that superficially resembles an elephant’s tooth or tusk.

In the order Gadilida, the shells are narrow and glassy-smooth, allowing them to flee their predators quickly. In contrast, members of the order Dentaliida possess rough and strongly ribbed shells.

Mantle

Within the shell lies the muscular mantle. An extensible foot extends from the broader end of the shell, leading to the head, which is almost always buried in sand. The apical or rear end of the animal’s body is sometimes exposed out of the substrate, and the apical aperture allows water to enter the body. A tusk shell’s interior is lined by thread-like cilia that facilitate water movement within the body cavity.

Radula

Being a mollusk, they possess a characteristically large feeding organ called the radula that breaks food into tiny particles. In members of Gadilida, the radulae and their accessory cartilaginous structures (bolsters) are modified into zippers to help the teeth crush the prey with the constant opening and closing of the radula. However, in Dentaliida, the feeding organs are ratchet-like, helping pull the prey into the esophagus.

Organ System

Circulatory1. The heart is either absent or reduced to a thin fold of the pericardium. Instead, blood is stored in the sinuses and circulated throughout the body.
2. One particular sinus, the perianal sinus, is often considered synonymous with the heart’s ventricle. 
Respiratory1. Since they lack lungs, the surface of the mantle cavity acts as a respiratory substitute.
Digestive1. The alimentary canal comprises a digestive diverticulum, esophagus, stomach, intestine, and anus.
2. An accessory digestive gland is connected to the stomach and adds digestive juices to the ingested food material.
Excretory1. Nitrogenous waste is excreted through the excretory organ, nephridia, located at the base of the anus.
2. They are the only mollusks that lack the characteristic reno-pericardial apertures. 
Nervous1. Their nervous system comprises a pair of cerebral and pleural ganglia that function as their brain.
2. The posterior part of the body contains a few other sets of ganglia, including the pedal, visceral, radular, and sub-radular ganglia.

Taxonomy

Although tusk shells are undisputed members of the subphylum Conchifera, their relationship with other members remains disputed.

According to the World Register of Marine Species, tusk shells are grouped into two orders: Gadilida and Dentaliida, with members of the latter being larger. Dentaliid shells taper uniformly from anterior (widest) to posterior (narrowest), whereas Gadilids have a shell whose widest part is slightly posterior to its aperture.

Fossil Records

They are the youngest molluskan class that originated in the Mississippian Subperiod. The  Rhytiodentalium kentuckyensis from the Ordovician Period is an early antecedent of the scaphopods, implying an evolutionary succession from rostroconch mollusks such as Pinnocaris

However, other hypotheses hint at a Devonian or Carboniferous origin from either a non-mineralized ancestor or a more derived, conocardioid rostroconch Arceodomus longirostris (of the Devonian Period).

Behavior

Feeding

They use the tentacles around their foot to sift food particles from the seafloor sediments, which they then pass to the radula for breakdown.

Habitat

They remain buried under seafloor sediments, like sand or mud.

Reproduction and Life Cycle

Tusk shells are gonochoric, with separate male and female sexes, each with a single gonad at the posterior end of their bodies. They release their gametes from their nephridia into the surrounding water for external fertilization.

The female lays her eggs one at a time. These eggs hatch into a free-swimming trochophore larva, which later transforms into a new form, the veliger. This new larva develops a three-lobed foot and metamorphoses into an adult carrying cephalic tentacles. 

Adaptations

References Article last updated on 6th June 2024
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