Physical Description
Species of the genus Protopalythoa consist of colonies of small, thick-walled polyps (Erhardt & Knop 2005). This thick wall has incorporated sand grains and other debris, which provide support and mechanical defence, as species in this genus do not produce a calcareous skeleton (Erhardt & Knop 2005; Mather & Bennett 1994).
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| Photographic illustration of alternating tentacles--every second tentacle is slightly angled toward the oral disc. |
They are usually no bigger than 1.5 centimetres tall x 0.5 cm diameter, and are usually a dull brown to dark green on the oral disc. Because the oral disc is so large in comparison to the rest of the polyp, they are unable to fully retract, with only the tentacles and oral disk being able to fold slightly inward (Erhardt & Knop 2005).
This species, since it is in the taxonomic order Zoantharia, has tentacles surrounding the oral disc in multiples of six (Erhardt & Knop 2005; Mather & Bennett 1994; Ruppert et al. 2004)—on an individual polyp, a total of 42 tentacles was counted, with every second tentacle being at an alternating angle.
The tentacles alternate angles due to the sectioning within the organism (the mesenteries)—the tentacles that are held in a more upward position emerge from between the internal walls of the sections, while those that angle away from the oral disc are connected to the mesentery walls (Ryland 1997).
The tentacles of species in this genus are generally short, usually one half to a quarter of the diameter of the oral disc (Davey 1998). Like all Cnidarians, these polyps have radial symmetry, with an oral-aboral axis, as well as the presence of cnidocytes in the body wall and tentacles, and are used in prey capture and defence (Mather & Bennett 1994; Ruppert et al. 2004). Because this species has shown evidence of symbiotic zooxanthellae in their body wall (outlined on the pages 'Ecology' and 'Anatomy and Physiology'), then they potentially gain energy from the process of photosynthesis (Erhardt & Knop 2005).
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| Polyps in their 'complete' closed state. Only the tentacles and oral disc can be partially retracted, unlike other corals. This photo also shows sand sticking to the polyps, and the presence of the stolon at the base of the polyps, connecting them to each other. |
Because this species only makes small colonies, individuals are connected to each other by a stolon (Erhardt & Knop 2005; Gosliner et al. 1996; Mather & Bennett 1994), which is a small connection that allows exchange of gases and nutrients between polyps.
In the past, distinctions between the two genera Palythoa and Protopalythoa have been based purely on morphological differences, such as the colour, shape and size of polyps. In recent years however, it has become evident that this method of distinction is redundant, since colonies can have enormous amounts of variation in these features based purely on environmental factors (Reimer et al. 2006). Another, slightly less variable method of distinction between the two genera is based on the presence or absence of a coenenchyme.
Palythoa species consist of colonies with polyps completely immersed in a coenenchyme, while Protopalythoa polyps stand free, being connected to each other by a stolon (Davey 1998; Erhardt & Knop 2005; Reimer et al. 2006).
The lack of a skeletal structure in these two genera has made distinction between the two, and of the species within them, especially difficult (Davey 1998; Mather & Bennett 1994; Reimer et al. 2006).
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Diagram illustrating different colony morphotypes. "A" indicates a colony completely embedded in coenenchyme, "B" indicates a colony partially embedded, and "C" illustrates a stolonate colony. The Protopalythoa species discussed here is a stolonate colony.
Diagram modified from Reimer et al. (2006). |
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