Read on if you want to know more about bivalves with burglar alarms…

Molluscs show an amazing morphological diversity in eye type. Best known is the vertebrate-like camera eye of coleoid cephalopods such as octopuses, one of the most-cited examples of evolutionary convergence. Another group of molluscs that has received far less attention has independently acquired the type of eye arthropods are famous for – compound eyes.

The ark clams (Arcidae) are a family of marine bivalves with about 200 species worldwide. Species inhabiting shallow waters (e.g. the Turkey wing Arca zebra) possess numerous compound eyes on the edge of their mantle (pallium), an arrangement reminiscent of the array of reflector eyes of scallops such as Pecten and pinhole eyes of giant clams in the genus Tridacna. Similar to the compound eyes of arthropods, arcid pallial eyes are composed of dozens of ommatidia, but the cellular architecture is rather different. In ark clams, each ommatidium is composed of one or two photosensitive ciliated receptor cells and several layers of pigment cells (which presumably increase the efficiency of the eye). In contrast, the ommatidia of arthropod compound eyes, which are paired cephalic structures, consist of several rhabdomeric receptor cells lying beneath a lens. These differences provide strong evidence for the independent origin of compound eyes in clams.

Ark clams are “equipped with an absurd number of eyes” (Nilsson 1994, Philosophical Transactions of the Royal Society of London Series B, vol. 346, p. 206), for example up to 300 compound eyes with more than 100 ommatidia each in the red-brown ark (Barbatia cancellaria). This indicates that vision is very important, but as the ommatidia lack a lens, the eyes have only poor resolving capacities. They do not form an image, but only perceive simple spatial movement. So what are they used for? Evidently they trigger the closure of the clam’s shell in response to visual motion, hence a protective reflex. This closure always happens in the same way, so information about the direction of a stimulus is irrelevant. The nervous system is thus relieved of the costly task of processing spatial information, which helps to understand why ark clams can afford to develop so many eyes. The compound eyes have been compared to burglar alarms designed to detect predators reliably, while avoiding false alarms. Similar alarm systems have evolved in other sedentary animals. Giant clams employ pinhole eyes, but two groups of annelid fan worms, the tube-dwelling sabellids and serpulids, also possess compound eyes. The compound eyes of serpulids seem to be highly effective, and their performance might rival that of arthropod compound eyes.

Still, the lack of focussing structures in ark clams is not so easy to explain. Poor resolution means that the clams are unable to detect predators at a distance, which surely should be advantageous? Not necessarily. Ark clams are filter feeders and need to stay open to feed. They should thus avoid unnecessary closing, and the main threat seems to come from rather slow-moving predators. Thus, the optical properties of the eye could reflect a compromise between predator detection and feeding efficiency.

In addition to the compound eyes, ark clams have pigmented cup eyes on the mantle edge. These are also lensless but apparently quite sensitive. They might be used for other tasks and are also found in many other bivalves that do not possess compound eyes.