Crystallography in invertebrates: The gastropod radula

This post is part of a series in celebration of the United Nation’s International Year of Crystallography. Throughout the year, we will explore different types of invertebrates with crystalline or semi-crystalline structures, highlighting examples of Invertebrate Zoology (IZ) research at the National Museum of Natural History.

If you've ever hunted for shells at the beach, then you've probably collected tiny bivalves (for example, clams) with perfectly circular holes bored straight through near their hinges. Such a hole is the mark of a carnivorous marine gastropod snail. While the shells of gastropods are composites of calcium carbonate mineral and various proteins, it's not their shells that we're interested in for this post on crystallography in inverts. Instead, our focus is on the gastropod radula, which is a structure that snails and other mollusks use for feeding.

Meet Yolanda! Here, Yolanda is working at a dissecting microscope. (Credit: Yolanda Villacampa)

For an introduction to the radula, we turn to IZ museum specialist Yolanda Villacampa, whose research interests include scanning electron microscopy and digital camera imaging of the morphological characteristics of mollusks (among other duties). For these studies, Yolanda works with IZ research zoologists Jerry Harasewych, Bob Hershler, and Ellen Strong.

 

Yolanda introduces the Pseudotryonia pasajae

In my work in IZ, I provide technical, laboratory, and collection assistance for research zoologists specializing in freshwater and marine mollusks. I prepare research specimens (example pictured) and work with the scanning electron microscope (SEM; pictured), which is located in the SEM Lab at NMNH, to take high magnification images of unique, detailed structures of small mollusk shells, the snail operculum, which is the plate that can seal the opening of the shell in many gastropods, and tiny snail teeth, which comprise a feeding structure known as the radula.

The buccal material of tiny snails in a vial. Inset: SEM image of the shell of a rare species of freshwater snail, Pseudotryonia pasajae. (Credit: Yolanda Villacampa)

 

The SEM that Yolanda uses to collect high-magnification images. A gastropod radula is pictured on the monitors. (Credit: Yolanda Villacampa)

The teeth of the radula are formed of chitin, which is a type of polysaccharide (or biopolymer). Chitin also occurs in the mollusk shell, the crustacean and insect exoskeletons, and the beaks of cephalopods. In particular, the teeth of the radula can vary significantly in size, shape, and the number of cusps on each of the different kinds (pictured for Tryonia sp.), which include centrals, laterals, inner marginals, and outer marginals. These are important features when determining the differences among species. Additionally, these features reflect a gastropod’s diet, although some gastropods have evolved to lose the radula entirely.

The four radular tooth types of Tryonia pasajae, including the centrals, laterals, inner marginals, and outer marginals, captured using SEM. (Credit: Yolanda Villacampa)

I have had the opportunity to work on numerous new freshwater gastropod species that have yet to be described. One of my recent projects involved taking SEM images of the radular teeth of a rare freshwater snail species. This species, Pseudotryonia pasajae (described in Hershler et al., 2011) is known from only two locations in Durango, Mexico.  Bob Hershler has been studying gastropods for years and currently has a paper in press that documents the second recorded location of this species. This publication is part of a series of papers regarding the taxonomic clarification of this group of imperiled gastropods from the thermal springs of the Mexico/U.S. Chihuahuan Desert.

 

The crystallography connection

While the chitin in the snail radula is not a mineral, it does have crystalline properties, which means that crystallographic techniques can be used to explore the unit cell of packed chitin molecules as well as their degree of preferential arrangement (crystallinity). For example, in shells, the chitin molecules tend to vary in directionality relative to the shell surface (cross-sectional image of a shell pictured). Other biopolymers exhibit high degrees of arrangement, including collagen in bone and cellulose in plant cell walls.

In cross-section, the composite shell (calcium carbonate, various proteins, including chitin) of the mollusk exhibits a plywood architecture, as captured using SEM. (Credit: Jerry Harasewych)

The composition and structure of the radula also contribute to its mechanical properties, the most important of which is its toughness. High toughness helps the snail scrape off layers of food. Additionally, as Jerry points out, "limpets and chitons do have some iron in portions of the radular teeth," although these minerals tend to be poorly crystalline (imperfect atomic ordering) forms of goethite and magnetite. Fluorapatite, a calcium containing mineral, has also been found in some chitons. These mineral forms were documented with various spectroscopic and crystallographic techniques.

In general, minerals are incorporated into biopolymers and protein-based biological tissues for the purpose of increased strength.

 

For more technical reads, try the following:

Structure, morphology, composition and organization of biogenic minerals in limpet teeth by Mann et al. 1986 in the Proceedings of the Royal Society, Series B.

Form and function of radular teeth of herbivorous molluscs by Padilla 2003 in American Malacological Bulletin.

 

by Yolanda Villacampa and Liz Boatman

Comments

[Sophie Thompson]

I am doing a project on slugs in my biology class and I am wondering what the tissue that makes radula is? Also, what cells make up that tissue?