Figure 1.
A schematic representation of the physical relationship between the coral host (Acropora muricata) and the colonial hydroid (Zanclea margaritae).
(i) At the surface of the coral colony the hydroid emerges through a pore surrounded by a collar of tissue formed from an extension of the coral epidermis. (ii) The hydroid stolon remains adjacent to the epidermal tissue collar as it passes deeper into the colony. The host epidermal tissue adjacent to the hydroid stolon invaginates and transforms into calicoblastic tissue deeper within the coral, preventing the hydroid from coming in contact with host gastrodermal tissues. (iii) At the position where it first comes into contact with the host skeleton the stolon follows the orientation of the skeletal element. (iv) Both the coral and hydroid employ desmocytes to attach the tissues to the coral skeleton. pr, pore rim; hm, hydroid mesoglea; hg, hydroid gastrovascular cavity; he, hydroid endoderm; hp, hydroid epiderm; hd, hydroid desmocyte; cp, coral epiderm; cm, coral mesoglea; cc, coral calicoblastic tissue; ce, coral endoderm; cg, coral gastrovascular cavity; cd, coral desmocyte; n, nematocyst; m, mucus cell; z, zooxanthellae; and sk, skeleton.
Figure 2.
Sequential histological sections showing the location of the partial endosymbiotic hydroid (Zanclea margaritae) within the coral host (Acropora muricata).
(A) The hydroid stolon lies within a cavity of the coral colony extending out to the surface through pores formed from extensions of the epidermal tissue layer. (B) The stolon extends through the coral, away from the stomal opening below the surface tissue layers, and remains in contact with either coral epidermal calicoblastic-like tissues or skeleton. Desmocytes (indicated by arrow heads) are present in both coral and hydroid epidermal tissues that are adjacent to coral skeletal material. Figure labels are described in the legend for Figure 1; ds, decalcified skeleton. Scale bars = 100 µm.
Figure 3.
Cross-section of the site of emergence of a hydroid (Zanclea margaritae) hydranth through the pore.
The surface epidermal layer extends down in to the cavity below the colony surface maintaining contact with the hydroid stolon and forming an inclusion of gastrodermal space. Labelling as in Figure 2. Scale bar = 50 µm.
Figure 4.
Transmission electron micrographs of the epidermal tissue layer of the surface pore at the coral surface.
(A) The single layered epidermal tissue that form the pore possess nematocysts and mucus cells in the upper areas similar to normal surface epidermal tissues (*) and the lower area of the tissues are characteristic of calicoblastic tissues, with highly vesiculated elongate cells. (B) The interface between the single-layered epidermal tissues making up the pore and the underlying hydroid tissues. A space may be seen between the tissues of the two organisms, but in some areas the two tissues may appear almost confluent. A layer of organic material lines the hydroid stolon. Adjacent calicoblastic-like coral cell layer are highly vesiculated and have a high density of mitochondria. Arrowheads indicate outer surface of the coral colony. cc, coral calicoblast cells; m, mitochondria; c, cavity; mc, mucus cell; ss, sub-epithelial space; om, organic material; hp, hydroid epidermal tissue. Scale bars: A = 5 µm; B = 2 µm.
Figure 5.
Desmocyte from the tissue of the coral Acropora muricata at the site of contact with the endosymbiotic hydroid, Zanclea margaritae.
(A) Coral desmocytes are present in the calicoblastic tissues facing hydroid tissues suggesting the presence of skeletal material, which may be very thin. (B) Close-up of a coral desmocyte showing the desmocyte tenons extending into the mesoglea, perpendicular to the interface with the skeleton. The matrix of long dense fibres that form the tenons terminate in electron dense plaques. Shorter fibres perpendicular to the tenon rod extend in to the collagen fibre-rich mesoglea. A band of organic material extends across the surface of the desmocyte, in-between the plaques and skeletal material. cd, coral desmocyte; om, organic material; hp, hydroid epidermal tissue; ss, sub-epithelial space; cc, coral calicoblast; df, desmocyte fibres; mf, mesogleal fibres; pq, plaque. Scale bars: A = 5 µm; B = 1 µm.
Figure 6.
Longitudinal section through the head region of a desmocyte from the hydroid, Zanclea margaritae.
Mushroom-shaped desmocyte with a broad base and top, and a constricted middle are found in areas of epidermal tissues associated with the coral calicoblastic tissues and skeletal material. Dense accumulation of electron dense filaments form membrane bound tonofibrillar rods (inset, arrowheads indicate membrane) which extend outwards in to the extracellular organic material. he, hydroid endoderm; hm, hydroid mesoglea; hp, hydroid epidermis; hd, hydroid desmocyte; om, organic material; cc, coral calicoblast; cp, coral epiderm; eom, extracellular organic material; f, fibres. Scale bar = 10 µm.
Figure 7.
Electron micrographs of a hydroid (Zanclea margaritae) desmocyte within the epidermal tissues adjacent to the skeletal material of the coral, Acropora muricata.
(A) The desmocyte is positioned within the epidermal cell layer, connected to the mesoglea by an extension of the collagenous layer. (B and C) Tonofibrillar rods at the apical end of the desmocytes are transversely striated, perpendicular to the fibres. The rods extend outwards from the apical surface in to an extracellular organic layer adjacent to the carbonate skeleton. (D) Fibrous mesogleal material extends through the interstices of the highly bifurcated distal portion of the desmocyte. he, hydroid endoderm; hm, hydroid mesoglea; hp, hydroid epidermis; eom, extracellular organic material; r, rod; dpm; desmocyte plasma membrane; mc, mesogleal channel. Scale bars: A = 5 µm; B–D = 500 nm.
Figure 8.
Ultrastructure of the coral skeleton (Acropora muricata) is influenced by the symbiotic hydroid, Zanclea margaritae.
The site of hydranths within the live coral colony corresponds to concave depressions and deformation of the radial sclerosepta. (A) A horizontal element (bar) has a circular depression, and the vertical ridge is flattened with a scooped-out appearance (B). Within the depressions the surfaces appeared smooth, with areas of parallel ridges (arrowheads). Fasciculated nodes lined the edges of the depressions (arrows). (C) Close-up of the boxed area in B. Crystalline surface structure is more finely granulated and does not exhibit the same scale-like clustering found in areas away from hydranths (D). Scale bars: A = 100 µm; B = 50 µm; C and D = 2 µm.