Interactive identification key to all brittle star families (Echinodermata; Ophiuroidea) leads to revised morphological descriptions

Abstract

Ophiuroidea is the largest class among extant echinoderms, with over 2000 described species assigned to 33 families. Here, the first identification key to the recently revised classification was developed, and revised morphological descriptions were derived from it, expanding the previous short diagnoses. The key was built by analyzing internal and external skeletal characters of predominantly the type species of each family, including at least two mutually exclusive attributes per family. Various numeric and multistate characters were used to create a traditional as well as an interactive key using the DELTA and Xper software programs­. Illustrations (SEM and digital photos) are included in the key to facilitate the assessment of character states by users. Not only is it the first identification key to the families, according to the recently proposed new classification and the examined species, but this interactive key also assists users in understanding the family level taxonomy of brittle stars. The interactive key allows new characters and states to be added, when more species will have been analyzed, without the need to reconfigure the complete key (as may be necessary with conventional keys).

The evolutionary relationship between arm vertebrae shape and ecological lifestyle in brittle stars (Echinodermata: Ophiuroidea)

“…In other words, the evolutionary transformation sequence of vertebrae is from a zygopondylous type to a semi-streptospondylous type, and then to a streptospondylous type, or from zygospondylous to combshaped (Litvinova, 1989a(Litvinova, ,b, 1994. Ophiuroid clades with streptospondylous vertebrae include Hemieuryalidae, Euryalidae, Gorgonocephalidae, Asteronychidae, Ophiobyrsidae, Ophioleucidae, and Ophiocamacidae (Goharimanesh et al, 2021).…”

“…In other words, the evolutionary transformation sequence of vertebrae is from a zygopondylous type to a semi‐streptospondylous type, and then to a streptospondylous type, or from zygospondylous to comb‐shaped (Litvinova, 1989a,b, 1994; Thuy & Stöhr, 2018). Ophiuroid clades with streptospondylous vertebrae include Hemieuryalidae, Euryalidae, Gorgonocephalidae, Asteronychidae, Ophiobyrsidae, Ophioleucidae, and Ophiocamacidae (Goharimanesh et al, 2021). Streptospondylous and semi‐streptospondylous shapes have been hypothesized to allow the arm to bend in every direction and to twist spirally (Alitto et al, 2020; Litvinova, 1994).…”

“…Also, the dorso‐distal extension in the vertebrae (extended keel according to Goharimanesh et al [2021]) may play a role in the function of arm movements, as proposed by LeClair and LaBarbera (1997) and Clark et al (2018). The keeled species in the current research (belonging to Ophionereidae and Ophiotrichidae) are separated from non‐keeled species on PC2 (Figure 4), a distinction also observed by LeClair (1996).…”

“…The Euryalida and Ophiurida now belong to the superorder Euryophiurida (O'Hara et al, 2018). The Euryalida currently includes three families, where all species possess streptospondylous vertebrae, whereas the Ophiurida consists of five families (Goharimanesh et al, 2021; O'Hara et al, 2017) with zygospondylous vertebrae. Within the superorder Ophintegrida, a variety of streptospondylous, zygospondylous and intermediate types can be found (O'Hara et al, 2017, 2018).…”

“…Moreover, species could be grouped mainly based on the absence/presence of the keel structure (i.e. “extended keel” as defined in Goharimanesh, et al [2021]) on the distal dorsal face of the vertebrae, a structure of which the functional role is not known (LeClair, 1996). Besides, despite the important role of prehensility (grasping an object partly or wholly with combined ventral and lateral flexion) for balance, feeding, and locomotion (Organ, 2010), the morphological differences between prehensile and non‐prehensile ophiuroid arms are not yet fully understood.…”