The structure of the sea lilyCalamocrinus diomedae, with special reference to the articulations, skeletal microstructure, symbiotic bacteria, axial organs, and stalk tissues (Crinoida, Millericrinida)

Summary

Calamocrinus diomedae was collected in deep water in the eastern Pacific. The skeleton and soft tissues were studied by light microscopy and by scanning and transmission electron microscopy. The skeleton of the arms and pinnules was unusual in lacking any galleried stereom; thus the ligaments as well as the muscles inserted on labyrinthic stereom. Therefore, the type of skeletal microstructure may not always be a sure guide to where the ligaments and muscles insert, and crinoid paleontologists should use caution when reconstructing soft parts from fossil skeletons. Contrary to the original species description, the skeleton of the theca apparently includes no infrabasal ossicles, andC. diomedae is monocyclic like other millericrinid sea lilies. Although many of the soft tissues resemble those of other crinoids, there are several exceptional features. The visceral mass within the theca includes not only a glandular axial organ typical of crinoids generally, but also what appears to be a second axial organ of the eleutherozoan type; dual axial organs have been seen before only in isocrinid sea lilies. The gut contents include partially digested crustacean prey, including some calanoid copepods. The soft tissues of the axial cord of the stalk comprise a haemal vessel, an aboral extension of the glandular axial organ, aboral coelomic extensions of the chambered organ, coelomic nerves, stalk nerves, an inconspicuous central through-going ligament, and aggregations of cells in each radial sector. Spherical bodies filled with rodshaped symbiotic bacteria are embedded in the soft connective tissues of the arms and pinnules. Possible chemosynthetic and bioluminescent roles for these bacteria are discussed.