Arachnocorallium Haeckel, 1887

Trubovitz, Sarah, Renaudie, Johan, Lazarus, David & Noble, Paula, 2022, Late Neogene Lophophaenidae (Nassellaria, Radiolaria) from the eastern equatorial Pacific, Zootaxa 5160 (1), pp. 1-158 : 16-17

Genus Arachnocorallium Haeckel, 1887 , emend. Petrushevskaya, 1971

Type species: Arachnocorys (Arachnocorallium) hexaptera Haeckel, 1887

(= Psilomelissa calvata Haeckel, 1887 )

Description. Lophophaenid genus often comprised of only a cephalis, with inconsistent and usually minimal development of a thoracic segment. The base of the cephalis is narrow, and clearly exhibits the median bar, dorsal, and lateral spines extending out from the base of the cephalis, usually with minimal obstruction from any thorax development. Apical and ventral spines are not usually as well expressed as in other Lophophaenidae .

Remarks. Arachnocorallium was originally described as a subgenus of Arachnocorys by Haeckel (1887). He described it somewhat vaguely, as having a “thorax with six divergent ribs or radial beams.” Arachnocorys hexaptera was the first species Haeckel (1887) listed under this subgenus, and it was designated as the type species of Arachnocorys (Arachnocorallium) by Campbell (1954, p. D126). However, this species was not illustrated by Haeckel (1887) nor Campbell (1954). In 1971, Petrushevskaya emended Arachnocorallium , elevating it to the genus level and synonymizing Arachnocorys (Arachnocorallium) hexaptera Haeckel, 1887 with Psilomelissa calvata Haeckel, 1887 to revise the concept of the type species ( Arachnocorallium calvata ; the only species of this genus described at that time). Petrushevskaya elevated Arachnocorallium to the genus level because she considered the type species of Arachnocorys ( Arachnocorys circumtexta ) to be sufficiently different from Arachnocorallium calvata . Since Arachnocorallium calvata (Haeckel) Petrushevskaya, 1971 has triradial symmetry, rather than the six divergent ribs mentioned in the subgenus description by Haeckel (1887), it seems that Petrushevskaya significantly changed the original meaning of Arachnocorallium when she elevated it to the genus level. For many decades, the species Arachnocorallium calvata Haeckel was the only taxon belonging to this genus, but recently three new species of Arachnocorallium have been described by Renaudie and Lazarus (2015) from the Southern Ocean. Furthermore, the high level of variation in Arachnocorallium calvata could potentially justify breaking this group into multiple species (see species discussion below).

The apical spine is not always easy to determine, particularly in Arachnocorallium calvata , which either has no clearly visible spines on the cephalis, or otherwise has three thin, conical spines of equal strength. Petrushevskaya (1971) noted that Arachnocorallium calvata does not have an apparent ventral spine. In the annotated specimen below, we mark the probable location of the ventral spine, although it is not overtly distinct from the wall of the cephalis.

Here we observed the following species of Arachnocorallium : Arachnocorallium calvata ( Haeckel 1887) Petrushevskaya, 1971 and Arachnocorallium stilla Renaudie and Lazarus, 2015 .

Range.?Miocene—Recent in the tropical world oceans, and Miocene—Recent in the Southern Ocean, so far as known.