The Nature and Origin of the Epidermal Scales of Notodactylus handschini--an Unusual Temnocephalid Turbellarian Ectosymbiotic on Crayfish from Northern Queensland.

ABSTRACT

The temnocephalid Notodactylus handschini, ectosymbiotic on the crayfish Cherax quadricarinatus from northern Queensland, is unique among known turbellarians in having its dorsal epidermis covered by rows of closely adjacent scales. These are borne on epidermal plinths separated by arthrodial gutters and are up to 100 µm tall with rhombic bases 40-55 µm by 15-20 µm. Above the bases, the rhombic cross section gradually becomes oval so that the scales are essentially elongate conoids, the slender tips of which curve inwards towards the worm's mid-line. In mature worms, the more median scales may be reduced distally into squat truncated cones only 40-50 µm tall. The scales consist of glycoprotein; rhabdites discharged from cells in the dorsal parenchyma contribute the protein, whereas the carbohydrate component probably comes from the glycocalyxes of the epidermal microvilli. The latter act as templates around which the glycoprotein mixture coalesces, seemingly by a simple tanning process, into tightly packed tubes 180-200 nm in diameter with walls 40-45 nm thick. The scales lack any limiting wall or membrane other than a loose amorphous layer, 90-150 nm thick, formed by disintegration of the tubes distally and compensated for by continuous growth basally. Each scale is attached to its epidermal plinth by the bases of its constituent tubes ensheathing the microvilli; attachment is reinforced by cross-striated fibrils, probably collagen, embedded in the epidermis and inserted between the microvilli into tube bases near the scales' corners. Scale surfaces bear rich growths of microorganisms. The use of rhabdites to form permanent scales is probably an adaptation to the worm's unusual sedentary habit; it supports, paradoxically, an earlier hypothesis that the primary function of rhabdites in turbellarians other than temnocephalids is to provide a continuously renewable coating compatible with epidermal ciliation.