Mollusca is the largest marine phylum, comprising about 23% of all the named marine organisms. With approximately 85,000 extant species, members of this group are some of the most well-known organisms in the ocean. Mollusca is generally broken down into seven distinct classes: Aplacophora, Polyplacophora, Monoplacophora, Gastrapoda, Cephalopoda, Bivalvia, and Scaphopoda. More generally, molluscs include marine snails, chitons, limpets, shellfish, squid, octopus, and cuttlefish. This list hints at their vast diversification, and descriptions of body plans are often contextualized as deviations from a simplified “Hypothetical Ancestral Mollusc” (HAM) in scientific literature. Scores of books have been written to describe the proliferation of this group to capitalize a wide variety of habitats and ecological niches, but there are a few unifying features or morphological characteristics that are generally used to differentiate between groups.
The most universal feature of the body structure of molluscs is a mantle, the dorsal body wall which covers the visceral mass of the animal. It is the mantle which secretes the shell, a feature which has been lost or modified in the various groupings. Within the Cepholapoda, for example, there is a trend towards decreasing shell mass as you move from nautilus to cuttlefish to squid to octopus. All bivalves, by contrast, possess two shells attached by a hinge. The most abundant metallic element in molluscs is calcium, which can be identified via EDX analysis. Generally, the shell consists of three layers: the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite, and an inner layer consisting of laminated calcite. SEM has been used to track the formation of the shell from larva (Weiss et al. 2002) to adult (Wise & Hay 1968a, b, Wilmot et al. 1992, Nudelman et al. 2008), showing that mineral formation does not occur in an aqueous solution, but in a hydrated gel-like medium which ultimately forms a cross-lamellar structure. Current research is analyzing structural damages to shells in the face of ocean acidification (Welladsen et al. 2010).
Another example of a unifying feature in Mollusca is the radula, which is a feeding apparatus present (in some form) in almost all molluscs. In most classes, the radula functions like a belt sander and is used to grind bacteria and microalgae off of rocks or to break down drift algae or tissue. In Cephalopods, the radula has been modified into a beak to help in the ripping apart of live prey. Only bivalves lack a radula, as most have evolved to adopt a filter-feeding strategy. Radular morphology and tooth structure is often indicative of diet specialization, and many studies have classified this variation (Padilla 1998, Runham & Thornton 2009, Medina et al. 2016, Venkatesan et al. 2016). Specifically, the advent of SEM technology has provided a direct way to monitor small-scale variations. This has led to the discovery of entirely novel feeding strategies, such as the Plant-Sucking Slug (Elysia viridis), a shelless gastropod who uses a single radular tooth to pierce the cells of particular algae (Trench et al. 1973a, b, 1974, Wägele & Martin 2014). It then absorbs the algal chloroplasts into its tissues in a process known as kleptoplasty and harnesses their photosynthetic ability to generate energy.
The amount of work dedicated to this group is sizable, in reflection of its diversity as a phylum. While soft tissues require thorough treatment and preparation to be usable with an SEM, there are techniques available which make it possible. One study proposed a simple method of preparation which allowed for the imaging of the sensory epithelia in the statocyst of a cephalopod mollusc (Barber & Boyde 1967). A statocyst is a simple cell lined with ciliated epithelia which contains magnetic particles. As the particles move due to gravity, the cilia are triggered, helping to orient the organism. This application has the potential to be very useful, as cilia are utilized for many purposes across taxa.
Ultimately, Mollusca stands as an example of the versatility not just of a group of organisms under evolutionary pressures, but of what SEM can accomplish as a powerful analytical tool. As the technology has advanced, so too have the methods developed to explore with ever-increasing precision. Due to their numbers and diversification, molluscs will continue to be useful study subjects in an uncertain future.