Isidoidae, Saucier & France & Watling, 2021

Saucier, Esprit Heestand, France, Scott C. & Watling, Les, 2021, Toward a revision of the bamboo corals: Part 3, deconstructing the Family Isididae, Zootaxa 5047 (3), pp. 247-272 : 268

Family Isidoidae , n. family

Type genus: Isidoides Nutting (1910)

Diagnosis. Colonies are planar with pseudo-dichotomous branching. Axis solid, calcareous, with concentric layers that can be white to dark golden-brown in color but not articulated. Polyps are mostly bi-serially or tri-serially arranged on the axis and are non-retractile, but in a preserved state the tentacles are tightly contracted and overlay the oral disk. The sclerites are smooth, flattened, finger biscuit-shaped rods of relatively uniform size that are abundant and tightly packed throughout the surface tissue of the colony, but rarely in the pharynx. Cross-shaped sclerites are present but few in number.

Included genera: Isidoides

Remarks. The Isidoides clade comprises a single species, Isidoides armata , that was described by Nutting (1910) based on specimens collected from the southwestern Pacific Ocean. There were no new reports of Isidoides until 2003, when independent collections on the Norfolk Ridge and off New Caledonia in 2008 and 2011 led to a re-description of the genus by Pante et al. (2013). That work also noted I. armata has a mitochondrial gene arrangement that is (so far as is known) unique among Octocorallia. In the original description of Isidoides, Nutting (1910) noted that the smooth sclerites resembled those of the bamboo coral genus Bathygorgia , although closer examination reveals the crystal arrangement in the sclerites of the two genera is quite different (unpublished observation). However, because Isidoides lacked the characteristic articulated skeleton of Isidae (= Isididae ), Nutting placed the genus in the Gorgonellidae (= Ellisellidae ). Bayer (1979) included Isidoides in a key to the Chrysogorgiidae , but with neither justification nor explanation of what characters suggested that relationship. Later, Bayer & Grasshoff (1994) stated that Isidoides “appears to be” a chrysogorgiid and not an ellisellid, but without further elaboration on the placement of the genus. Our phylogenetic results and those of Pante et al. (2013) do not support a close relationship of Isidoides with chrysogorgiids or ellisellids but instead find them to be sister to Keratoisididae . Bathygorgia , resurrected by Lapointe and Watling (2015), has “obliquely or longitudinally placed spicules [=sclerites], club- or biscuit-like in shape” (Wright 1885), which would seem to be similar to Isidoides sclerites as originally noted by Nutting (1910), but an examination of the type specimen of B. profunda shows the sclerites to be “biscuit-like” only in their elongate form but in fact are all small rods ( Lapointe and Watling, 2015). Thus, except for the lack of an articulated skeleton, Isidoides was thought to fit within the definition of the old Keratoisidinae . On the other hand, the axis of Isidoides is often a dark brown (see Pante et al. 2013) suggesting that the axis may be constructed more like that of primnoids, with concentric layers of gorgonin alternating with calcareous material. The structure of the axis has not yet been examined in further detail. Some consideration was given to including Isidoides in the family Keratoisididae , but to do so would have meant the latter family could not be characterized by any synapomorphy.