EYES AS OPTICAL ALARM SYSTEMS IN FAN WORMS AND ARK CLAMS

Eye structure and optics were investigated in two sabellid polychaetes (Sabella melanostigma, Dasychone conspersa) and three arcacean bivalv es (Area zebra, Barbatia cancellaria, Anadara notabilis). The polychae tes have numerous compound eyes arranged in pairs along the branchial tentacles. Each ommatidium is composed of three cells: one receptor ce ll forming a ciliary receptive segment, and two pigment cells forming an extracellular lens (crystalline cone). The ark clams Area and Barba tia possess large numbers of compound eyes arranged along the mantle e dge. The ommatidia of these eyes are composed of one or two ciliary re ceptor cells surrounded by several layers of pigment cells. There are no lenses in the ommatidia of the clam eyes. All three species of ark clam also have many pigment-cup eyes on the mantle edge. The cup eyes lack lenses, and the cavity of the cup is filled with rhabdomeric micr ovilli from the receptor cells. The crystalline cones in the sabellid compound eyes are powerful lenses that reduce the field of view of the receptor cells to slightly more than 10 degrees. The lensless ommatid ia of Barbatia have much larger fields of view (approximate to 30 degr ees). This difference correlates with a behavioural response to much f iner moving stripes in the fan worms. A comparison of compound eyes an d cup eyes in Barbatia reveals a poor resolution in both, but a much h igher sensitivity is estimated for the cup eyes. The tentacular eyes o f fan worms and the mantle eyes of ark clams trigger protective respon ses: retraction into the tube and shell closure, respectively. The res ponses are triggered by visual motion and the eyes work as burglar ala rms rather than imaging eyes. For this purpose, the compound eyes may seem to occur in affluent numbers: 240 eyes with a total of 12 000 omm atidia in Sabella and 300 eyes with a total of 39 000 ommatidia in Bar batia. The number of ommatidia that simultaneously monitors any direct ion in space is, on average, 43 in Sabella and 755 in Barbatia. The la rge number of eyes is explained as a visual strategy which provides a robust alarm system designed to reliably detect predators without caus ing false alarms. The literature on tentacular eyes of fan worms and m antle eyes of bivalves is reviewed, and the evolutionary origin of the se independently-acquired visual organs is discussed. I suggest the po ssibility that hyperpolarizing photoreceptor cells (shadow detectors) evolved from chemoreceptors that were inhibited by light.