Muscle cells in the tiny marine Antarctic mite Halacarellus thomasi: an ultrastructural and immunocytochemical study

All musculature examined in the tiny, 0.3-mm, marine Antarctic mite Halacar ellus thomasi (i.e. body and appendages) appeared ultrastructurally to be o f the transversely striated type with continuous Z-lines. Tubules of sarcop lasmic reticulum lay among the myofibrils. The complexity of the sarcotubul ar system, sarcomere lengths of over 6 mu m, and the abundance of mitochond ria are interpreted as signs that the mite is slow moving, but capable of c onsiderable and sustained contraction forces, features deemed necessary in the strong currents of the frigid water prevailing in the mite's habitat. P resence and distribution of regulatory (troponin, tropomyosin, caldesmon an d calponin), contractile (actin, myosin, paramyosin and miniparamyosin) and structural (alpha-actinin, titin, minititin and nebulin) proteins were det ermined immunocytochemically. The results are consistent with the notion of a well-functioning contractile machinery but, furthermore, provide evidenc e for the great importance of the structural proteins alpha-actinin, miniti tin and nebulin in maintaining muscle-cell stability under the environmenta l conditions in which the mite has to function.