Siphonal eyes of giant clams and their relationship to adjacent zooxanthellae
Giant clams of the genus Tridacna possess up to several thousand eyes upon the surface of their exposed hypertrophied siphons; this extraordinary number of eyes is unique in the animal kingdom. The siphonal eyes of Tridacna are simple photoreceptors that lack a lens, but possess a pigment cup made of several to many layers of olive-green zooxanthellae. Each eye contains 280 to 350 polygonal retinal cells (15 to 20 ..mu..m in diameter) which bear numerous ciliary blebs. Retinal cell blebs posses a ciliary axoneme composed of 9 x 2 + 0 microtubule doublets which splay outwards from the cilium's basal plate. The ciliary blebs give rise to many microvilli that are believed to be the photoreceptive portion of the retinal cell. Whereas tridacnid eyes possess directional sensitivity, because they lack a lens, they are individually incapable of image discrimination. However, both the ability of giant clams to detect nearby movements and the high density of eyes on the siphonal surface indicate that tridacnid eyes may function cooperatively in small numbers much like the elements of a primitive compound eye. Owing to the transparency of the retinal cells and the biconvex shape of the siphonal eye, it is apparent that Yonge's hypothesis that zooxanthellae surrounding the eye's capsule may derive more sunlight than other similar siphonal tissues is reasonable.
- Research Organization:
- Simon Fraser Univ., Burnaby, British Columbia
- OSTI ID:
- 6701276
- Journal Information:
- Veliger; (United States), Vol. 23:3
- Country of Publication:
- United States
- Language:
- English
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