When you don’t possess true eyes to recognise the shape of things, to see where you’re going, or what’s coming, then you must certainly have some other highly developed sense to survive. Living in a medium that has thousands of different compounds dissolved or suspended in it, a virtual soup of chemicals, released either intentionally or just as a byproduct of existence by every living thing, has led the nudibranchs to evolve extremely sensitive organs of “smell”, chemical detectors – the rhinophores. The rhinophores enable the nudibranch to navigate life by locating food, a mate and avoiding predators.
Take a close look at the above image. Here we can see the detail of the rhinophores of Hypselodoris obscura. Six things are immediately obvious in this image:
– The rhinophores are located right at the anterior or leading end of the animal, in an upright posture to immediately detect approaching signals in the water flowing over them.
– By being on top of the head they are situated right over the main cerebral ganglions thus improving reaction time by reducing length of neural transmission.
– There are two of them, so the incoming chemical cues can be read in “stereo” to assist in direction-finding.
– They can be individually rotated to better aid that direction-finding; notice in this photo, at that moment in time, the closer has rotated 90 degrees to the right of the nudibranch’s axis while the other has only rotated 45 degrees.
– They consist of many leaflets or lamellae up their length. This greatly increases efficiency/sensitivity by providing a larger surface area for the testing of the water flowing over them. Additionally, this arrangement serve to slow the flow of water across the lamellae the better to enable the chemical compounds to bond with the receptors.
– Not all nudibranchs have retractile rhinophores but this example – Hypselodoris obscura – is a cryptobranch all species of which have pockets for the rhinophores to retract into beneath the mantle surface for protection.
Rhinophores come in all shapes, sizes and surface presentations. Read the chapter on “The Senses” in Understanding Nudibranchs – The Book, on this website, to learn more not only about the rhinophores of the true nudibranchs but also the head tentacles of the rest of the sea slugs that perform the same function.
For an animal like the nudibranch, that is slow moving in all types of water clarity, highly functioning eyes would be nowhere near as much of an asset as long range, at a distance, chemical detectors like the rhinophores.
David A Mullins – January 2021