Retractor septal muscle
Retractor septal muscle
lines the inside of the tentacles, pharynx, mesenteries, gastrovascular cavity and extends through the colony via the gastrodermal tubes. The gastrodermal cells are where zooxanthellae are located. Between these two layers lies the mesogloea consisting of a gelatinous matrix and this is where the sclerites (= spicules) occur. Within the polyps it forms a very thin layer, but forms the majority of coenenchyme of the colony outside of the polyp (fig. 1-2). Within the mesogloea exist the gastrovascular tubes of the polyps and a network of gastrodermis-lined tubes that connect the gastrovascular tubes of the various polyps, called gastrodermal tubes or solenia. Special cells called amoeboid cells are also found in the mesogloea. Some of them become sclerob-lasts responsible for the manufacturing sclerites; others are responsible for depositing gorgonin in gorgonians (Hyman, 1940).
Within the polyp itself, eight longitudinal partitions called septa (= mesenteries), extend from the body wall. In the upper portion of the polyp they extend completely across, and are joined to the oral disc and pharynx, forming eight separate compartments, each of which continues into a tentacle (Hyman, 1940). Below the level of the pharynx the inner edges of each septa are free. Along these free edges occur thickened, sinuous filaments (mesenterial filaments). The two septa directly across from the siplionoglyph (asulcal septa) bear very long filaments with lots of flagella, presumably to create upward currents. These two septa extend the furthest into the polyp. The other six septa and their filaments are shorter, contain digestive cells and do not extend as far; not all genera have these other six septa e.g. the family Xeniidae (Hyman, 1940).
The portion of the polyp that extends above the colony is know as the anthocodium. In the suborder Stolon if era, the lower portion of the anthocodium is thickened and may lie heavily spiculated, forming an anthostele or calyx. The upper, soft portion can then be withdrawn into this structure. In most other octocorals the anthocodium can be completely withdrawn into the coenenchyme leaving only a small hole behind. Some genera, however, cannot retract their polyps (e.g. Anthelia, Heteroxenia, Xenia). In sea pens, there is one main axial polyp (primary polyp) that runs the length of the stem and all other polyps (secondary polyps) extend from it (Hyman, 1940). In gorgonians, which have a shallow layer of coenenchyme due to the axial skeleton, the polyps tend to be short.
In many octocoral genera the polyp extends well below the surface of the colony and all or some, can reach all the way to the base of the colony. This brings up an interesting possibility. By extending so
A Clavularia polyp showing the anthocodium and anthostele
After Hyman, 1940
Anthocodium c^Sclerite (enlarged)
The pinnules are clearly evident on the tentacles of Clavularia spp. J. Sprung
The tentacles of "Green Star Polyp" Pachyclavularia (Briareum) spp. often have reduced pinnules. J. Yaiullo and F, Greco
These are the polyps of the common sea fan, Gorgonla ventalina. J. Yaiullo and F. Greco
The commonly imported "Clove Polyp" is an unidentified stoloniferan, possibly Acrossota sp. (see chapter seven). It has very distinctive polyps with fused pinnules. J. C. Delbeek
The polyps in this distinctive unidentified stoloniferan from Bali, Indonesia are completely devoid of pinnules. J. Sprung
The elongate polyps of Sarco-phytonspp. really seem to be like fibre-optic cables capable of piping light to the zooxanthellae contained in the interior. J. C. Delbeek
deeply into the body of a colony, these elongated tubular polyps may create a "fibre-optic" effect where they may actually "pipe" light into the deeper layers of the colony, especially in Sarcopbyton and Xenia species that have highly elongate polyps.
As mentioned above, the gastrovascular tube of each polyp in a colony is connected by a series of gastrodermal tubules called solenia. In forms whose polyps arise from an encrusting sheet or stolon (e.g. Antbelia, Pacbyclavularia, Clavularia, Briareum, Erythropodium, etc.), the solenia are limited to the base only. In most fleshy octocorals that grow above the substratum (e.g. Sarco phyton, Lobophytum, Xenia, etc.) however, the solenia extend throughout the colony often enclosing the bases of the gastrovas
cular tubes of the polyps (Hyman, 1940). The function of solenia is not clear. New polyps arise when solenia approach the epidermis. They form a small chamber just below the epidermis. A small swelling appears on the upper surface and this then develops tentacles becoming the anthocodium (this can clearly be seen on the stalks of AIcyonium species, aka "Colt Coral" from Indonesia). The epidermis then invaginates into the chamber below forming a mouth and pharynx. Solenia may also act to distribute food and nutrients through the colony; they may allow zooxanthellae to move around the colony and infect newly developed polyps (a function reminiscent of the zooxanthellal tubule system in tridacnid clams, see volume one); or they may be involved in maintaining hydrostatic pressures in more gelatinous genera such as Dendronepbtbya or Nephtbea.
Heteroxenia sp. exhibiting polyp dimorphism characteristic of the genus. The tentacle-bearing auto-zooids are large and run off the photograph. The siphonozooids are apparent on the capitulum in between the bases of the auto-zooids. J. Sprung
This fantastic red soft coral thriving in a behind-the-scenes display at the Monterey Bay Aquarium was collected in very deep water, It clearly shows autozooids and siphonozooids. To give an idea of scale, the autozooid polyps are 2.5 cm (1 in.) in diameter! J. Sprung
In some octocorals there are two distinctly different types of polyps present; these corals are said to have dimorphic polyps. Sarcophyton, Lobopbytum, Heteroxenia and sea pens are all dimorphic. However, Heteroxenia (the Latin name refers to the fact there are two different types of polyps) is only dimorphic beyond a certain size (see top of next page).
Autozooids are what one would normally consider polyps. Their structure is just as we described above but they may have reduced or absent siphonoglyphs. They are primarily used for feeding, respiration, light gathering and perhaps light "piping" to lower regions of the colony. They also bear the ovaries and testes.
Siphonozooids are reduced in size and either lack or have very rudimentary tentacles. They also have greatly reduced mesenteries
(usually only the two asulcal ones remain) and very well-developed siphonoglyphs. The siphonozooids do not feed and serve primarily to drive a strong water current through the coral. The fact that all branched, lobed and arborescent (tree-like) alcy-onaceans are monomorphic, whereas most of the large massive forms are dimorphic, suggests that dimorphism arose out of a need to transport water more efficiently through large colonies (Bayer, 1973). In support of this idea a colony of Heteroxenia fuscescens does not develop siphonozooids until the colony has reached a certain diameter; their numbers then increasing as the colony gets larger (Achituv and Benayahu, 1990). In some genera (e.g. Corallium and Paragorgid) the siphonozooids bear the gonads instead of the autozooids (Hyman, 1940).
There is a wide range of colony organization in octocorals. Ranging from very simple monomorphic polyps joined together by stolonic growths, to massive forms with polyps embedded in thick coenenchyme, to advanced forms with polymorphic, highly specialized polyps. We will not go into all the different colony growth forms in this section, but will merely point out the most commonly seen types in the marine aquarium trade.
Order Helioporacea (=Coenothecalia)
There is only one species within the family Helioporidae, Helio-pora coerulea and it is generally considered to be a relict. Although it is clearly an octocoral in anatomy, its skeleton is massive like a stony coral, and its colonial organization and growth form is similar too (Bayer, 1973)- Its skeleton is composed of crystalline fibers of aragonite fused into layers (Hyman, 1940). There are two cylinders within the skeleton, one larger than the other but both blind ended. The larger one houses the polyp while the smaller ones are the solenia, connecting the polyps together. There is a thin coenenchyme on the surface that is filled with a network of solenia (fig. 1-4). As the colony grows, a transverse plate forms which effectively cuts off the upper living layer. Thus only the outer thin layer is living tissue. Growth forms are usually lobes but can also be flattened vertical plates similar to Fire Coral, or thick stubby branches reminiscent of Porites. When the tissue is removed, or when a break occurs in the skeleton the underlying blue colour of the skeleton is revealed. This colour is thought to be caused by the infusion of iron salts into the
Cross-sectional view of the surface of a blue coral, showing a polyp, the coenenchyme and the network of solenia Modified from Hyman, 1940
skeleton. Heliopora is one of the few soft corals that can contribute to reel building, sometimes forming extensive reefs e.g. in areas off the southern coast of Okinawa, Japan.
Stoloniferans exhibit the simplest colony organization. Within this suborder there are also a variety of growth forms. There is no coenenchymal mass, but the polyps arise asexually from a creeping base, which may consist of separate stolons or a more or less continuous thin crust or mat (Hyman, 1940). The polyps are all interconnected by gastrodermal tubes running through the stolons or mats. The simplest growth form is found in Clavularia spp. (Waving Hand polyp). All the polyps are of a single type and
Creeping stolon of Clavularia, J. C. Delbeek arise singly from creeping stolons that emerge from the base of each polyp. The upper part of the polyp, the anthocodium, is thin-walled while the lower part, the anthostele, is hardened. The anthocodium can be retracted into the anthostele. A similar growth form can be found in the Xeniiclae genus Anthelia, but there is no anthostele and the polyps cannot be retracted (see chapter seven). Pachyclavularia spp. (Star polyp) spread by a creeping mat, with polyps arising from it. There can also be connections between the polyps at various levels, not just at the base (Bayer, 1973). Anthosteles are present, and the polyps can be fully retracted, giving the mat a bumpy appearance when the polyps are completely retracted.
The genus Tubipora is unique in its growth form. The polyps arise from a basal plate and are enclosed by skeletal tubes of fused spicules, bright red in colour. These tubes are themselves interconnected by lateral stolons that become fused, forming platforms, and from which new polyps can arise. As the tubes increase in length the lower portions are sealed off, so the polyp only exists in the portion of the tube above the preceding platform.
Another growth form is exemplified by the azooxanthellate genera Carijoa (=Telesto) and Coelogorgia, which has zooxan-thellae. In these genera, a single, very elongated polyp arises as a branch or stem, from a creeping base. Within this polyp lies an extensive network of solenia from wliich other lateral polyps arise. In Carijoa, lateral polyps arise along the length of a single stem. In Coelogorgia, the lateral polyps produce more polyps along their length, and these polyps produce polyps of their own, eventually resulting in large, tree-like growths.
Within the Alcyoniina we find the greatest variety in colonial organization and shape. Alcyoniina are characterized by the presence of an extensive coenenchyme. This is formed mainly by mesogloea containing calcareous spicules. The coenenchyme is filled with tubules (solenia) that connect all of the polyps together. The anthocodium of each polyp extends above the coenenchyme ancl may appear to be separate, but within the coral, they are all interconnected by the solenia. Newr polyps arise as outgrowths of the solenia. Therefore the oldest polyps tend to be the longest, often extending all the way to the base of the colony, while the younger polyps do not. All the members of this suborder conform to this basic organization whether they are massive forms such as
Sarcophyton and Lobophytum, with lobes and folds or more arborescent forms such as Nepbtbea or Litophyton. Even in the later form, the trunks are composed primarily of bundles of older polyps whose anthocodium emerge at the tips of the branches at various levels in the colony (Bayer, 1973).
Polyps within this suborder show a wide variety of size and shape. Some can be greatly elongated while others barely extend beyond the epidermis. Some have very small spicules while other the polyps in another genera can be heavily spiculated to the point where the spicules stick out around the polyp, offering protection and support in strong currents (see Nutrition). In some genera the polyps can be completely retracted, in some they can only be partially contracted into their protective spicule sheaths while in others then are non-retractile. Finally, it is within this suborder that dimorphic polyps first appear (see above, Auto-zoo ids and Siphonozooids). Many of the larger more massive species are dimorphic while most of the arborescent species are not. However, this is not a fast rule. Within the family Xeniidae, there are some forms that are dimorphic and some that are not. Heteroxenia spp. are dimorphic but may only be so during breeding season (Gosliner et al., 1996).
There are two main growth forms in the Alcyoniina: massive fleshy colonies and arborescent colonies with a thin coenenchyme. More massive colonies have a thick coenenchyme, heavily embedded with spicules. Colonies generally consist of a stalk topped by a rounded, mushroom-like cap that bears the polyps called the capitulum. The stalk can be elongated or short while the capitulum can be smooth and rounded (e.g. some Cladiella, Heteroxenia, Xenia), thrown into folds (e.g. Sarcophyton) or can have numerous polyp bearing lobes projecting from it (e.g. some forms of Cladiella, Lobophytum, and some forms of Sinularia). Arborescent forms fall into two groups. The Nephtheidae reach a large size but the trunk has very little coenenchyme. Instead these corals rely on abundant and spacious gastrovascular canals within their trunks to maintain their shape via hydrostatic pressure. As a result, members of the Nephtheidae can collapse and shrink to a small size when their gastrovascular canals expel water (Bayer, 1973)- The arrangement of the larger branches and the polyps on the branches are used to subdivide this family. The second group is typified by the Siphonogorgia. They have narrower, more rigid main trunks, and the branches are subdivided to a much greater extent. These soft corals look a lot
Growth forms of massive and arborescent soft corals
After Bayer et al., 1983
like gorgonians at first glance but lack the internal axial skeleton of gorgonians. Various terms are used to describe the colony morphologies of massive and arborescent forms (see table 1.2).
Term_Description_Examples bushy colonies with abundant Plexaurella branches arising Stereonephthya immediately above the holdtast and not forming an obvious main stem capitate unbranched colonies with a broad distal part (capitulum) on a distinctly narrower stalk
Heteroxenia Xenia dichotomously branching branched colonies in which the branching Dattern is a repeat Difurcation
Plexaurella digitate colonies consisting Alcyonium of several, finger-li
(colonial) that are profusely branched, with long slender branches bearing distinctly separated and diverging bundles of polyps encrusting thick fleshy layer Erythropodium covering the substrate Lobophytum
Sinularia glomerate (colonial) arborescent colonies Dendronephthya that are sparsely branched, with numerous bundles of polyps crowded to form roundish bunches lateral branching gorgonian colonies, Plexaura planar or nearly so, in which the branching originates irregularly, neither pinnate nor dichotomous lobate colonies consisting of Cladiella several stout lobes Sinularia pinnate branched colonies in Pseudopterogorgia which the branching pattern is feather-like, with the branches in one plane
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