ABSTRACT
Individuals of the sand dollar Sinaechinocyamus mai (Wang) were collected from the low intertidal zone at Tunghsiao, western Taiwan, from July 1990 to July 1991 at monthly interval, except January to April and June. The mean body length was 7.3 ± 1.011 mm. A total of 146 individuals were examined, including juveniles and adults and they all contained sand grains in their intestinal diverticula. The percentage of sand weight in diverticulum to the wet body weight is reversely correlated to the body length, revealing that juveniles contain higher proportion of sand than adults. The ratio of sand weight to the body length does not change significantly during the study period. Diverticulum sand consists mainly (74%) of the size ranging from 0.075 to 0.125 mm in diameter, whereas this range occurs only 0.1% in the substrate. In addition, the mineral types of sand grains in the diverticulum are feldspars and quartz (88.4%) and Iron oxide (11.6%) and have same proportion to the substrate. The sand grains are eliminated from the diverticulum into intestine, rectum and then out via the anus. These results indicate that sand dollars, based mainly on grain size, ingest sand grains as food particles from the substrate and store them in the diverticulum as weight belt to maintain their position in a sand, movable substrate ground.
