This is a very complex genus to ID as nearly all ID’s require a macro shot of the poly area, or a microscope.
I have added one ID I believe to be accurate based on the description of the original document.
The below information is based on the original description, found here.
There are 3 “types” of Dendronephthya; Glomerate, Divaricate and Umbellate as described below by the original description.
“Kükenthal has divided this difficult genus into three main groups: (I) Glomerate; (II) Divaricate; (III) Umbellate, giving precision to similar suggestions by previous workers such as Holm.
I. The Glomerate are characterized by:
- the comparatively slight branching of the polyparium;
- the grouping of numerous bundles of polyps into roundish bunches which make the surface of the polyparium entirely irregular.
There is a marked definiteness about the Glomerate division which suggests “naturalness” and makes it easy to refer to a species to the group. Text-fig. 1 is a diagrammatic illustration of what is meant by the Glomerate habit of growth.
II. The Divaricate are characterized by:
- the profuse branching of the polyparium;
- the length and slenderness of the twigs;
- the divergent separateness of the polyp bundles;
- the absence of anything that can be called bunches of the Glomerate type or umbels of the Umbellate type.
It should be noted that a Divaricate polyparium may have a continuous contour like that of a well-pruned tree (see diagram).
III. The Umbellate are characterized by:
- the umbel like or sometimes corymb like aggregates formed by the terminal twigs, the heads of the umbels being bundles of polyps;
- the disposition of all or most of the polyp heads on the surface of the colony.”
Identifying which is a Glomerate, Divaricate or Umbellate form is relatively simple. However, if one goes deeper into a single form, it will prove to be very difficult.
Here is an example of a specie I am reasonably certain is accurate, thanks to a great photo provided by Cornelis Opstal, via Dreamstime.com.
The Diagnosis of Dendronephthya suensoni, found on page 52 is as follows:
“Divaricate; outline irregular; not obviously flattened; polyps in little groups (4 – 10 ), distinctly scattered; polyps stalks medium; supporting bundle medium; point spicules one pair only, of which one member is a long projecting curved spindle associated with a much smaller one at its base; crown of some 6 rows of horizontally disposed spindles; grade VI; spicules: canal-walls show numerous forms with greatly developed thorns; colour: cortex and polyps have deep red spicules, polyps grey-yellow.”