The tendons of Ophiocomina nigra and their role in autotomy (Echinodermata, Ophiuroida)

Abstract

In the arm of the ophiuroid Ophiocomina nigra the intervertebral muscles are linked to the vertebral ossicles by tendinous connective tissue fibres. When an arm autotomizes, rupture of the tendons at one end (the autotomy insertion) permits each muscle in the autotomizing segment to separate cleanly from an ossicle while its other attachment (the non-autotomy insertion) remains intact. The anatomical relations, composition and function of the tendons were investigated by histochemical, electron microscopical and experimental methods. The tendons consist of a carbohydrate-rich secreted collagen derived from the basal lamina of the muscles. At autotomy their rupture is preceded and facilitated by an increase in extensibility, which represents the first evidence for variable tensility in an echinoderm connective tissue not composed of interstitial collagen. Granule-containing juxtaligamental cell processes are associated with the tendons of the autotomy insertions but are absent from the non-autotomy insertions. There appears to be widespread release of granules from these processes at autotomy. The results of a simple experiment implicate the juxtaligamental cells in the control of tendon extensibility and a possible mechanism for this control is presented.