Anatomy of the sea slugs

Sea slugs exhibit a great diversity of forms and anatomical adaptations, but they share the basic body organisation of gastropod molluscs. The body comprises three main regions: the head, the muscular foot and the visceral mass, which contains many of the internal organs. The mantle, a tissue that covers the visceral mass and delimits the pallial cavity, can also take on a wide variety of forms and, depending on the group, may be associated with an external, reduced or internal shell.

Throughout the evolution of marine gastropods, this body plan has undergone considerable modifications. In some groups, the body is broad and flattened, with a highly developed mantle; in others, it is elongated and bears numerous dorsal projections. The arrangement of the gills, the shape of the visceral mass and the organisation of the internal organs also vary. These differences are particularly evident when comparing umbraculids, doridacean nudibranchs and aeolidacean nudibranchs.

It is important to note that the term Opisthobranchia, traditionally used to group several marine gastropods, does not correspond to a clade recognised in modern phylogenetic classification. For this reason, this section describes the anatomical characteristics of different lineages without treating them as a single evolutionary group.

Torsion and detorsion: a key transformation in gastropod evolution

One of the most important anatomical transformations in gastropod evolution is torsion, a developmental process that changes the relative position of the visceral mass, mantle and pallial cavity in relation to the head and foot. In the classical model, this process is described as an approximately 180° rotation of the visceropallial complex during larval development.

As a consequence of this reorganisation, the anus and pallial cavity, initially located in the posterior region of the body, come to occupy a more anterior position, close to the head. Torsion also affects the arrangement of other internal organs and the course of the nervous connections. The process is more complex than a simple mechanical rotation, and studies of gastropod development have revised several aspects of the classical model.

Throughout evolution, different lineages have undergone secondary modifications of this organisation. One of the best-known is detorsion, understood as a partial or more extensive reversal of certain anatomical changes associated with torsion. It may involve the reorganisation of the pallial cavity, anus and respiratory structures. However, it should not necessarily be interpreted as an exact reverse rotation of 180°, since development and subsequent anatomical changes follow different patterns in different lineages.

Torsion also had important consequences for the organisation of the nervous system. In some gastropods, the visceral nerve connections cross one another, an arrangement known as streptoneury. In other lineages, these connections have undergone reorganisation, resulting in a less extensively crossed arrangement known as euthyneury. These patterns also depend on the concentration, fusion and displacement of nerve ganglia, and not exclusively on detorsion.

Traditional classifications used the terms “Prosobranchia” and “Opisthobranchia” to group major assemblages of gastropods according to certain anatomical characteristics. Today, both terms are considered historical because the groups they designated do not adequately reflect evolutionary relationships. It is therefore incorrect to assume that all animals traditionally referred to as “opisthobranchs” underwent the same process of detorsion. Torsion and its subsequent modifications should be understood as developmental and evolutionary processes that have contributed to the anatomical diversity of modern gastropods.

Anatomical differences between the main groups

Umbraculids

Umbraculids (Umbraculida) have a distinctive body organisation, characterised by a highly developed dorsal mantle that covers much of the body. In the genera Umbraculum and Tylodina, a relatively reduced, thin and weakly calcified external shell is retained. Its shape resembles a small shield or cap. Some species also have microscopic calcareous spicules in the mantle.

The head bears a pair of rolled rhinophores and a flattened anterior region resembling a cephalic veil, which may have short oral tentacles. The foot is broad and muscular, adapted for movement across the substrate.

One of the group’s most distinctive features is a large, bipinnate gill located on the right side of the body, between the foot and the dorsal mantle. This gill is associated with the pallial cavity and participates in gas exchange with the surrounding water. In Umbraculum umbraculum, the gill extends along a considerable portion of the right side of the body, and the anus opens behind it.

The buccal apparatus also has distinctive features. The radula is well developed but lacks a central or rachidian tooth; the teeth are lateral and arranged in numerous rows. In addition, the lip of the buccal mass bears horny elements that participate in food processing.

Overall, umbraculids combine a highly developed dorsal mantle, a residual external shell and a well-differentiated lateral gill. This combination of characteristics distinguishes them from nudibranchs, which have a different respiratory organisation and external morphology.

Doridacean nudibranchs

Doridacean nudibranchs generally have a broad, oval and relatively flattened body, with a well-developed ventral foot and a dorsal mantle that extends beyond the margins of the foot. The mantle surface may be smooth, granular or covered with tubercles, depending on the species.

The head bears a pair of rhinophores, which in many species can retract into protective sheaths or cavities. On the posterior part of the dorsum, most doridaceans have a crown of branched gills arranged around the anus. This dorsal gill plume is one of the group’s most characteristic external features.

However, the gill arrangement is not universal. Members of the family Phyllidiidae, for example, lack the external perianal gill crown typical of many other doridaceans and have a different respiratory organisation. This illustrates how substantial anatomical modifications can occur even within a group with a readily recognisable general body form.

The visceral mass and digestive gland are located mainly beneath the mantle. Compared with aeolidaceans, doridaceans do not have numerous dorsal projections containing diverticula of the digestive gland. Their internal organisation is generally more compact, although the precise arrangement of the organs can vary among families.

Overall, the broad body, well-developed mantle and posterior gill crown make many doridaceans recognisable externally. However, these characteristics are not sufficient to identify every lineage within the group, and internal anatomy also exhibits differences that are relevant to taxonomy.

Aeolidacean nudibranchs

Aeolidacean nudibranchs generally have an elongated body, with a pair of oral tentacles and two rhinophores at the anterior end. Their most striking external feature is the presence of numerous dorsal and lateral projections called cerata, arranged in rows or clusters along the body.

The cerata are not merely superficial appendages. Diverticula of the digestive gland extend inside them, so digestive tissue penetrates these projections and broadly follows their external distribution. This arrangement distributes portions of the digestive gland across an extensive body surface and contributes to gas exchange with the surrounding water.

In most aeolidaceans, the tips of the cerata contain a specialised sac called a cnidosac. This structure can store functional nematocysts obtained from the cnidarian prey on which the animal feeds, such as hydroids. Nematocysts are microscopic capsules containing the characteristic stinging apparatus of cnidarians. After being ingested, some of these nematocysts can be selected, transported and stored in the cnidosacs, where they contribute to the animal’s defence.

The combination of cerata, digestive gland diverticula and, in most species, functional cnidosacs constitutes one of the most characteristic anatomical features of aeolidaceans. However, the organisation and development of these structures vary among different nudibranch lineages, and not all nudibranchs possess cerata.

The digestive system

The digestive system of sea slugs comprises the organs responsible for capturing food, processing and digesting it, absorbing nutrients and eliminating undigested waste. Its general organisation follows the basic gastropod body plan, but it varies in relation to diet and the evolutionary history of each group.

The mouth and buccal mass

The mouth is located at the anterior end of the body, usually on the ventral side of the head. It connects to a muscular buccal mass containing the main structures involved in food capture and processing.

In many species, this mass contains the radula, a flexible ribbon bearing numerous microscopic chitinous teeth that are replaced as they wear down. The radula is supported by a muscular and cartilaginous apparatus that facilitates its movement. Depending on the species, it may function as a scraping, cutting or piercing structure, or it may have specialised teeth for gripping or fragmenting particular prey.

The shape, arrangement and number of radular teeth vary considerably among species. These differences are related to feeding habits and constitute important characters in anatomical and taxonomic studies. Some gastropods, however, have reduced or lost the radula, demonstrating that this general pattern is not universal.

Depending on the group, jaws or other horny structures involved in handling food may also be present around the buccal mass. In umbraculids, for example, the buccal lips bear separate masticatory elements, whereas these structures may have a different shape and degree of development in other lineages.

The buccal mass connects to the oesophagus and may be associated with salivary glands that secrete substances capable of lubricating food and, depending on the group, contributing to digestion. The form and development of these glands are not identical in all sea slugs.

The oesophagus, stomach and intestine

From the buccal mass, food passes into the oesophagus, a tube that transports it towards the stomach and the posterior regions of the digestive tract. The length, diameter and organisation of the oesophagus can vary depending on the species and its diet. Some gastropods have specialised regions involved in food processing, but these are not a general feature of all sea slugs.

The stomach is one of the main sites of digestive processing. Its shape and organisation vary among lineages and are related to the characteristics of the food and the function of the digestive gland. Food continues to be processed in this region before passing into the intestine.

In the intestine, much of the digested material completes its passage through the digestive tract, and nutrients are absorbed. Unassimilated material moves towards the anus, through which it is eliminated. The position of the anus varies among groups and is related to the organisation of the pallial cavity and to the anatomical modifications characteristic of each lineage.

In many doridaceans, the anus opens on the posterior dorsal surface, surrounded by the gill crown. In aeolidaceans, by contrast, it may occupy different positions on the right side of the body, in relation to the distribution of the digestive gland diverticula. The position of the anus should therefore not be generalised to all sea slugs.

The digestive gland

The digestive gland, also called the hepatopancreas in part of the zoological literature, is an important organ in gastropod digestion and metabolism. It contributes to the secretion of digestive enzymes, the intracellular processing of food, nutrient absorption and storage, and other metabolic functions.

The shape and distribution of this gland are particularly useful for understanding anatomical differences among the main groups of sea slugs. In many doridaceans, it forms a relatively compact mass located mainly in the visceral region. In aeolidaceans, by contrast, it is highly branched, and its diverticula extend inside the cerata.

This branching partly explains why the cerata of aeolidaceans may show characteristic internal colouration: the tissues and contents of the digestive gland branches can be visible through the body wall. The tips of many cerata also contain cnidosacs, but these structures have a different function and should not be confused with the digestive diverticula.

The distribution of the digestive gland provides a clear example of how the same functional system can adopt different anatomical arrangements during evolution. The aeolidacean pattern is particularly distinctive, but it cannot be extrapolated to all sea slugs.

The circulatory system

Sea slugs have an open circulatory system or, in certain groups, one described as semi-open. This means that the circulatory fluid is not confined exclusively to a continuous network of vessels, but also flows through internal spaces and sinuses where it comes into contact with tissues. For this reason, the circulatory fluid of molluscs is called haemolymph, rather than blood in the strict sense commonly used for vertebrates.

Haemolymph transports nutrients, metabolic waste products and other substances required for the organism to function. It also participates in defence processes and, in many species, contains respiratory pigments that contribute to oxygen transport.

The heart and haemolymph circulation

The heart is a muscular organ located within the pericardial cavity. In many heterobranch gastropods, it consists of two main chambers: the atrium, which receives haemolymph from the gas-exchange regions, and the ventricle, which pumps it into the arterial vessels.

After leaving the heart, haemolymph is distributed through vessels and internal spaces of the body, including the sinuses of the haemocoelic system, where it bathes the tissues directly. It then returns towards the respiratory regions and from there to the heart. This pathway is not completely closed and varies according to the anatomical organisation of each lineage.

In many species, haemolymph contains haemocyanin, a copper-based respiratory pigment that transports oxygen and may give the fluid a bluish colour. However, the composition of haemolymph and the role of respiratory pigments can vary among groups.

Relationship with respiratory structures

Circulation is closely associated with respiratory surfaces because haemolymph transports respiratory gases between these surfaces and the tissues.

In umbraculids, the bipinnate gill on the right side of the body participates in gas exchange. In many doridaceans, this function is performed mainly by the dorsal gill crown. Aeolidaceans lack this gill plume; gas exchange occurs across the body surface, with the cerata contributing through their extensive surface area.

These differences demonstrate that respiratory structures can vary considerably while the general principle of the circulatory system remains the same: distributing substances required for metabolism and facilitating the removal of waste products.

The nervous system

The nervous system of sea slugs coordinates movement, feeding, responses to environmental stimuli and many internal functions. As in other gastropods, it is organised around a set of nerve ganglia connected by nerve cords, connectives and commissures.

Ganglia are clusters of neuronal cell bodies that process and transmit information. In gastropods, a major part of the central nervous system lies around the oesophagus, forming a circumoesophageal nerve ring. The degree of separation or fusion of the ganglia, as well as the length and arrangement of their connections, varies among lineages.

The main nerve ganglia

The main components include the cerebral ganglia, which are involved in processing information received from the head and coordinating various functions; the pedal ganglia, which innervate the foot and control movement; and the pleural ganglia, which are associated with the body wall and mantle. The visceral and parietal ganglia participate in the control and innervation of internal organs, although their arrangement and degree of differentiation vary considerably.

Buccal ganglia associated with the buccal mass and the anterior regions of the digestive system are also present. These ganglia help control the movements involved in food ingestion and processing.

Nervous connections between the different centres integrate sensory information with motor responses. Although this general organisation is retained in many gastropods, it does not always occur as a set of clearly differentiated ganglia. In some lineages, ganglia have partially fused, become concentrated in the anterior region or changed position during evolution.

Torsion and subsequent anatomical reorganisations have also influenced the course of the visceral nerve connectives. However, the final arrangement of the nervous system does not depend solely on torsion or detorsion, but also on evolutionary processes involving ganglion concentration and reorganisation.

The nervous system and behaviour

Peripheral nerves connect the ganglia to muscles, internal organs and sensory structures. In this way, the nervous system regulates locomotion, responses to contact with other organisms and movements of the digestive tract, among other functions.

Sea slugs have also played an important role in neurobiological research. The sea hare Aplysia, for example, has been widely used in studies of learning, memory and the cellular mechanisms of nerve transmission. The relatively large size of some of its neurons and the accessible organisation of certain neural circuits have facilitated research into the basic mechanisms of neuronal function.

Overall, the nervous system has a relatively compact organisation, yet it can coordinate movements, integrate stimuli and enable sea slugs to adapt their responses to a wide variety of environments.

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