Shared Skeletal Support in a Coral-Hydroid Symbiosis

Hydroids form symbiotic relationships with a range of invertebrate hosts. Where they live with colonial invertebrates such as corals or bryozoans the hydroids may benefit from the physical support and protection of their host's hard exoskeleton, but how they interact with them is unknown. Electron microscopy was used to investigate the physical interactions between the colonial hydroid Zanclea margaritae and its reef-building coral host Acropora muricata. The hydroid tissues extend below the coral tissue surface sitting in direct contact with the host's skeleton. Although this arrangement provides the hydroid with protective support, it also presents problems of potential interference with the coral's growth processes and exposes the hydroid to overgrowth and smothering. Desmocytes located within the epidermal layer of the hydroid's perisarc-free hydrorhizae fasten it to the coral skeleton. The large apical surface area of the desmocyte and high bifurcation of the distal end within the mesoglea, as well as the clustering of desmocytes suggests that a very strong attachment between the hydroid and the coral skeleton. This is the first study to provide a detailed description of how symbiotic hydroids attach to their host's skeleton, utilising it for physical support. Results suggest that the loss of perisarc, a characteristic commonly associated with symbiosis, allows the hydroid to utilise desmocytes for attachment. The use of these anchoring structures provides a dynamic method of attachment, facilitating detachment from the coral skeleton during extension, thereby avoiding overgrowth and smothering enabling the hydroid to remain within the host colony for prolonged periods of time

Marine Structures as Templates for Biomaterials

During the last two decades, “learning from nature” has given us new directions for the use of natural organic and inorganic skeletons, drug delivery devices, new medical treatment methods initiating unique designs and devices ranging from nano- to macroscale. These materials and designs have been instrumental to introduce the simplest remedies to vital problems in regenerative medicine, providing frameworks and highly accessible sources of osteopromotive analogues, scaffolds and drug delivery device proteins. This is exemplified by the biological effectiveness of marine structures such as corals and shells and sponge skeletons, extracts of spongin and nacre, sea urchin, sea snails and Foraminifera. Organic matrix and inorganic marine skeletons possess a habitat suitable for proliferating added mesenchymal stem cell populations and promoting clinically acceptable bone formation. A wide range of applications of these marine structures and their conversion methods are covered by excellent review papers and chapters. In this chapter based on our research, published work and book chapters, we aim to cover the nature, morphology and the use of some of these structures for tissue engineering, bone grafts, drug delivery and specific extracts such as proteins for regenerative medicine