Sonia J. Rowley

Octocorals are the most diverse of all the coral groups (see Williams & Cairns, 2018), with gorgonian corals being of significant conservation and ecological importance. Originally classified under the order Gorgonacea Lamouroux, 1816 (now taxonomically obsolete), gorgonian octocorals, which are within the order Alcyonacea Lamouroux, 1812 (subclass Octocorallia Haeckel, 1866; see Bayer, 1981), currently comprise the suborders Holaxonia, Calcaxonia, and the group Scleraxonia. However, such groups are not phylogenetically supported (e.g., McFadden et al. 2010), with the Alcyonacea in need of thorough taxonomic revision. Fortunately, the subclass Octocorallia is a monophyletic group - all taxa within the Octocorallia are descended from a common ancestor - it's just all the chaos within!

Figure 1: Rumphius (1741) Het Amboinsche Kruidboek, Amsterdam: Changuion, with a line drawing of Isis hippuris Linnaeus, 1758, thought to be a 'marine plant' by Rumphius.

Gorgonian taxonomy began almost four centuries ago, originally described and classified as 'marine plants' (Rumphius 1741). Eventually, gorgonians were classified based on their colony morphology and the shape and arrangement of their calcareous skeletal elements, the sclerites (Kükenthal 1919). 

Figure 2: Diversity of gorgonian colonies and branching structures (left). Middle image shows a variety of different sclerites (microscopic skeletal elements), and right, micro-CT scan reconstructions by Drs A. Summers & SJ. Rowley of different polyps. Arrows highlight sclerites and octomerous structure at the base of the polyp.

Sclerites are found within the polyps and coenenchyme (colonial soft tissue between the polyps), and in some taxa, the axis itself. By 1961 Bayer delineated gorgonians from the remainder of the octocorals on the basis of axis mineralogy. Even though these groups are not phylogenetically supported, they are still in use mainly to assist with field identification. 

Figure 3: The cross-section of each gorgonian group axis. a) Scleraxonian axis cross-section showing the coenenchyme, polyp, and the central medulla (which is a composite of sclerites; Annella reticulata Ellis & Solander, 1786, 6 mm). b) Holaxonian axis cross-section showing loculus, central core, and coenenchyme (Astrogorgia sp., 7 mm). c) Calcaxonian axis cross-section showing coenenchyme, axis, and polyps (Viminella sp., 4 mm). Ax = Axis, Cc = Central core, Ce = Coenenchyme, Lo = Loculus, Me = Medulla, P = Polyp. Images by SJ. Rowley.

This highlights that comparative morphology alone does not delineate species and species groups. The advent of molecular techniques has added phylogenetic support for some groups, but confusion for others. Interestingly, certain groups have what I call 'bathymetric polyphyly', whereby they have all the diagnostic morphological traits of a particular genus or family but genetically they are very different when compared across depth. This demarcation across depth with little studied in between (i.e., at mesophotic depths) suggests that with greater sampling effort particularly at mesophotic depths (see Collections) may begin to unravel some of these taxonomic conundrums. This is especially intriguing as these groups are subject to different environmental regimes. Herein lies my personal fascination with these animals; how they are related and why?