. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 338 M. A. FARMER ET AL. postfixed in 3% osmium tetraoxide, dehydrated, and embed- ded in Spurr's medium as previously described (Trench et al., 1981). The tissues of Tridacna spp. were fixed, embed- ded, and observed as described in Trench et al. (1981). Thick sections (1 /im) were prepared for examination by light microscopy on an LKM Ultratome V. These were photographed with an Olympus Vanox microscope and a PM-10 camera. Ultrathin sections for electron microscopic examination were prepared on an RMC-6000 ultrami- crotome,
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 338 M. A. FARMER ET AL. postfixed in 3% osmium tetraoxide, dehydrated, and embed- ded in Spurr's medium as previously described (Trench et al., 1981). The tissues of Tridacna spp. were fixed, embed- ded, and observed as described in Trench et al. (1981). Thick sections (1 /im) were prepared for examination by light microscopy on an LKM Ultratome V. These were photographed with an Olympus Vanox microscope and a PM-10 camera. Ultrathin sections for electron microscopic examination were prepared on an RMC-6000 ultrami- crotome, stained with uranyl acetate and lead citrate in the standard manner, and observed and photographed with a Philips 400 transmission electron microscope (TEM). Results and Discussion Light microscopic examination of Corculitm cardissa revealed that, similar to observations made on symbionts living in mantle tissues of Tridacna (Fig. 1; Mansour, 1946a; Fitt and Trench, 1981; Norton et al., 1992; Norton and Jones, 1992), algal cells are arranged in rows in both the mantle (, Kawaguti, 1968; Figs. 1, 2) and the gills (Fig. 2). However light microscope observations could not re- solve the tertiary tubule structure in either genus of clam. Electron microscopic examination of the gills of Corcu- lum (Fig. 3) shows that the algae are indeed juxtaposed to animal blood cells. Kawaguti's (1968) early descriptions from C. cardissa note that algae are sometimes accompa- nied by "wandering cells," but he includes no figures. In contrast, TEM pictures of symbionts in Tridacna crocea and T. maxima clearly show nearby animal cells (Kawaguti, 1966; Fitt and Trench, 1981). Algal symbionts in C. car- dissa are not in direct contact with the animal's blood cells, but are separated from the hemolymph and the blood cells. Figure 3. Transmission electron micrograph of a portion of the gill of Corculum cardissa showing a portion of the tertiary tubule (t), and a blood cell (BC) close to c
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