Sensory organs and rhinophores

Sea slugs live in an environment where chemical cues and touch play a fundamental role in interpreting their surroundings. To locate food, recognise potential reproductive partners or detect danger, they possess several sensory organs, particularly well developed in the anterior region of the body. Their shape and arrangement vary among groups, reflecting differences in their biology and lifestyle.

Detail of anaspidean (Aplysia punctata) rhinophores
with the shape of rabbit ears

Rhinophores: the sense of smell underwater

Rhinophores are two sensory appendages usually located on the dorsal surface of the head, behind the oral tentacles when these are present. They are particularly conspicuous in nudibranchs and are one of the most characteristic features of this group. Despite their appearance, they are not respiratory organs: their primary role is chemoreception, the detection of chemical substances dissolved in the water.

Through their rhinophores, many sea slugs can detect chemical cues associated with food and recognise substances released by other individuals, including potential reproductive partners. In certain species, these cues also help them locate specific organisms on which they feed. For example, experiments have demonstrated that the tropical nudibranch Phestilla sibogae uses chemical stimuli to locate and recognise corals of the genus Porites, on which it feeds (Murphy & Hadfield, 1997).

Rhinophores vary considerably in shape. They may be smooth, bear small tubercles or have transverse folds and lamellae that increase the surface area of the sensory epithelium. In many dorid nudibranchs, these structures are particularly conspicuous and may have a distinctly lamellate appearance. In some species, the rhinophores can retract into protective pockets, whereas in others this mechanism is absent. The number, shape and arrangement of the lamellae, together with the presence of retractile pockets, are useful characters for distinguishing species and taxonomic groups.

Oral tentacles and the sense of touch

Many sea slugs also have a pair of oral tentacles at the front of the head. In addition to detecting physical contact, these structures may detect chemical stimuli at close range or directly on the substrate. In several species, rhinophores and oral tentacles have complementary sensory functions: rhinophores are particularly important for detecting chemical cues at a distance, while oral tentacles are more involved in contact perception. However, this functional distinction is not identical across all groups.

Other regions of the body may also contain sensory cells capable of detecting contact and changes in the surrounding environment. Therefore, although rhinophores are the most readily identifiable sensory organs, they are not the only structures involved in sensory perception.

Eyes and orientation

The eyes of nudibranchs are small and are generally located beneath the tissues of the head region, close to the nervous centres. Their visual capabilities are limited compared with those of many other animals, but they are not necessarily restricted to distinguishing light from darkness. An experimental study of Berghia stephanieae demonstrated that this species can respond to visual stimuli containing some spatial information and orient its movements accordingly. These responses also vary with the animal’s nutritional state and the presence of food-related odours. Further research is needed to determine how widely these capabilities are shared among other species (Quinlan & Katz, 2023).

As in other molluscs, statocysts are small internal organs involved in balance and body orientation. Together with tactile and chemical receptors, they contribute to coordinating movements and responding to environmental conditions.

Overall, the sensory organs of sea slugs form a diverse and specialised system. Studying their anatomy helps us better understand how these animals move, locate food and interact with other organisms and with their environment.

References

• Göbbeler, K., & Klussmann-Kolb, A. (2007). A comparative ultrastructural investigation of the cephalic sensory organs in Opisthobranchia (Mollusca, Gastropoda). Tissue and Cell, 39(6), 399–414. https://doi.org/10.1016/j.tice.2007.07.002
• Murphy, B. F., & Hadfield, M. G. (1997). Chemoreception in the nudibranch gastropod Phestilla sibogae. Comparative Biochemistry and Physiology Part A: Physiology, 118(3), 727–735. https://doi.org/10.1016/S0300-9629(97)00014-5
• Quinlan, P. D., & Katz, P. S. (2023). State-dependent, visually guided behaviors in the nudibranch Berghia stephanieae. Journal of Experimental Biology, 226(18), jeb245213. https://doi.org/10.1242/jeb.245213

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