Let’s take a closer look at the external body parts of the Variable Thorny Oyster (Spondylus varians).
Divers seldom get a good look at the beautiful undulated orange mantle of this bivalve due to the fact that when this creature detects light fluctuations or motion, it closes its shells rapidly. The detection of light and movement is done by a row of primitive eyes (or eye-spots), called pallial eyes, situated between the mantle and the shell. These "eyes" are primarily used for detecting potential predators, functioning more like motion detectors than organs for real visual perception.
The row of pallial eyes of the Variable Thorny Oyster are situated along the ventral margin of their mantle, which lines the shell. These blue eyes are among the best-known molluscan eyes. Inside they contain a thin sheet of guanine crystals, creating an inward curved mirror at the back of the eye. This mirror forms an image of the environment, which is reflected onto a double retina located directly behind the lens. The retina is composed of different photoreceptor cells that transfer the detected information to the Variable Thorny Oyster's well-developed sensory nervous system. This allows the animal to detect changes in light and motion, such as the approach of a potential predator, and therefor the Variable Thorny Oyster can respond accordingly by closing its shell.
The Variable Thorny Oyster is the largest of the spiny oysters (family Spondylidae) and can grow to a maximum diameter of about 20cm/8 inches or so. Spiny oysters are not true oysters but rather scallops. Unlike most scallops that spend their lives buried in the substrate, these bivalves cement themselves to a hard substrate like rocks or sunken ship wrecks.
The Variable Thorny Oyster's undulated orange or sometimes blue mantle with white marbled blotches and patches is truly beautiful and makes for a very interesting subject for both underwater photographers and filmmakers alike.
Filming this subject might be somewhat tricky due to the fact that any rapid or sudden movement by the underwater videographer or photographer might trigger a defensive response from this mollusk. The scallop might simply close its shell, and there goes your subject.
Before filming, turn on your lights and adjust all camera settings. Choose a spot at a short distance with matching light conditions to do so. Once you are ready to film, approach the oyster from a distance of about 3m/10ft, and now, very, very slowly without making sudden movements. When you are close enough, simply press the record button of your underwater camera and start filming, using the zoom if necessary. If you are unlucky and the oyster closes before you had the chance to film it, just move away because as long as you stay in front of this mollusc with your lights on, it won't open soon. You might have to try a few times before you succeed or just simply move to another subject in the vicinity to avoid wasting your valuable gas supply return at a later moment in time.
This short underwater videoclip has been filmed in Mauritius 🇲🇺
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