Stone pen in new genus Lithoptilum and sea pen in Anthoptilum, Calibelemnon removed

NEWS - Researchers report new analysis results that support the establishment of a new genus, Lithoptilum, to accommodate rock pens as a taxon close to Anthoptilum within the Anthoptilidae (Pennatuloidea) and require the deletion of the species Calibelemnon francei and the genus Calibelemnon within the Scleroptilidae.

Upasana Ganguly and Scott France of the University of Louisiana at Lafayette used a phylogenetic tree based on multilocus data and a single mitochondrial marker that showed all rock pens form a monophyletic clade within a larger clade representing the Anthoptilidae, deleting Calibelemnon (Scleroptilidae) and erecting the genus Lithoptilum.

Sea pens (Pennatuloidea) are a specialized group of octocorals that evolved to live anchored to the seafloor on soft sediments using their stalks as anchors, whereas rock-dwelling sea pens or rock pens use sucker-like stalks to attach to hard substrate surfaces, an adaptation previously unknown to sea pens.

Four species have now been identified as rock pens based on stalk morphology, three species within Anthoptilum and one within Calibelemnon. But this study found that the type specimen of Calibelemnon symmetricum (Nutting, 1908) is a colony with an elongated stalk, which is typical of soft-sediment sea pens, making the genus homogeneous.

Ganguly and France explored the geographic distribution and habitat depths using the NOAA Ship Okeanos Explorer and operated ROVs to record high-definition video to investigate the evolutionary origins of rock pens. They found thousands of colonies in the central Pacific Ocean at a depth of 450 meters. No such rock pen fields were observed in deeper waters.

Phylogenomic analyses were based on DNA sequences of ultraconserved elements (UCEs) and compared with trees constructed using mitochondrial genes. The ancestors of sea pens evolved along a single lineage that is sister to the lineage comprising the genus Anthoptilum. All rock pen species should be grouped into a new genus within Anthoptilidae.

Mitochondrial gene sequence analysis shows that the rock pen has the same gene sequence as Anthoptilum grandiflorum and Anthoptilum murrayi in the Octocoral B gene sequence. The arrangement is different from all other sea pens but matches that of bamboo corals (Keratoisididae). Among the 7 species that have been described in Anthoptilidae, 6 species have the same gene sequence.

Original research

Upasana Ganguly & Scott C. France (2024). Expanded distribution and a new genus for rock-inhabiting sea pens (Cnidaria, Anthozoa, Octocorallia, Pennatuloidea). Zootaxa, 5507 (1): 123-139, DOI:10.11646/zootaxa.5507.1.5

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