Abstract
The skeletal morphology of the arm spine joint of the brittlestar Ophiocomina nigra was examined by scanning electron microscopy and the associated epidermis, connective tissue structures, juxtaligamental system and muscle by optical and transmission electron microscopy. The behaviour of spines in living animals was observed and two experiments were conducted to establish if the spine ligament is mutable collagenous tissue: these determined (1) if animals could detach spines to which plastic tags had been attached and (2) if the extension under constant load of isolated joint preparations was affected by high potassium stimulation. The articulation normally operates as a flexible joint in which the articular surfaces are separated by compliant connective tissue. The articular surfaces comprise a reniform apposition and peg-in-socket mechanical stop, and function primarily to stabilise spines in the erect position. Erect spines can be completely immobilised, which depends on the ligament having mutable tensile properties, as was inferred from the ability of animals to detach tagged spines and the responsiveness of isolated joint preparations to high potassium. The epidermis surrounding the joint has circumferential constrictions that facilitate compression folding and unfolding when the spine is inclined. The interarticular connective tissue is an acellular meshwork of collagen fibril bundles and may serve to reduce frictional forces between the articular surfaces. The ligament consists of parallel bundles of collagen fibrils and 7–14 nm microfibrils. Its passive elastic recoil contributes to the re-erection of inclined spines. The ligament is permeated by cell processes containing large dense-core vesicles, which belong to two types of juxtaligamental cells, one of which is probably peptidergic. The spine muscle consists of obliquely striated myocytes that are linked to the skeleton by extensions of their basement membranes. Muscle contraction may serve mainly to complete the process of spine erection by ensuring close contact between the articular surfaces.
Highlights
The spininess of sea-urchins and their relations is celebrated in the name of their parent phylum– Echinodermata (‘hedgehog-skinned’)
Whilst the general structure of ophiuroid spine joints had previously attracted only sporadic attention [14,15,16], there has been in recent years a proliferation of descriptive data on specific aspects of their skeletal morphology, which has been provoked by their potential taxonomic and palaeontological significance, and which has revealed a remarkable diversity across the class [17,18,19,20,21,22,23,24,25]
The present paper focuses on the arm spine joint of the common NE Atlantic and Mediterranean ophiuroid Ophiocomina nigra
Summary
The spininess of sea-urchins and their relations is celebrated in the name of their parent phylum– Echinodermata (‘hedgehog-skinned’). The functional importance of ophiuroid arm spines has been recognised [18], there is still a dearth of information on (1) the soft tissue components of the spine joints and (2) the functional significance of their skeletal morphology, which impedes a full understanding of the biological roles of the spines and depreciates the information content of ophiuroid fossils [20,21]. This stands in contrast to the considerable attention that has been paid to the functional morphology of the vertebral joints of ophiuroid arms [26,27,28,29,30,31]
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