Rhabdopleura is a genus of colonial sessilehemichordates belonging to the Pterobranchia class. They are exclusively marine, benthic organisms whose species occur within all major oceans and range in habitat from intertidal to c. 900 m.[1] As one of the oldest living genera with a fossil record dating back to the Middle Cambrian, it is also considered to be the only living genus of graptolites.[2]
Rhabdopleura is the best studied pterobranch in developmental biology.[3] Research in the 2010s by Jörg Maletz and other paleontologists and biologists have demonstrated that Rhabdopleura is an extant graptolite.[4][5]
History
The first evidence for extant species of Rhabdopleura (Rhabdopleura normani) was identified in the Shetland Islands in 1869. Since then, a total of at least nine extant species have been identified.[6]
Species
List of species from WoRMS Editorial Board (2025):[6]
Living species
The genus Rhabdopleura contains at least nine living species.
Rhabdopleura Zooid: Sketch based on image from Beli et al. (2018).
Individual Rhabdopleura larvae are lecithotrophic with planula-like features and a uniform arrangement of cilia.[4] Those larva metamorphosize into individual zooids with a bilaterally symmetrical body plan and a single pair of tentacled arms used in feeding.[9] The zooid body is tripartite, being divided into a protostoma, a mesostoma and a metastoma. Generally speaking, the zooids appear to remain integrated with the larger stolon systems of their colonies throughout their lives.[1]
Rhabdopleura colonies are thought to begin when an individual larva secretes an initial dome structure known as a prosiculum. Inside the prosiculum, the individual larva then metamorphosizes into a foundational zooid. This zooid emerges from the prosiculum and proceeds to secrete an initial tube. From within this tube, an individual bud forms that produces a second zooid. The second zooid may then produce its own tube, along with a corresponding bud.[10]
The collective aggregation of tubes within the colony is known as a coenicium and the zooids that inhabit it are themselves connected to one another via living tissue referred to as stalks or stolon.[9] Although most species of Rhabdopleura appear to form the prosiculum directly within the water column, one species, Rhabdopleura recondita, is known to form colonies inside the calcareous skeletons of dead bryozoan colonies.[10]
Feeding
Like other pterobranchs, Rhabdopleura are considered ciliary mucus suspension feeders that predominantly consume either suspended particles and/or diatoms. While feeding, individual zooids from a colony project their arms out from openings in the coenicium and proceed to remove particulate matter from the water column.[9]
1 2 Gordon, D.P., Quek, Z.B.R., Orr, R.J.S. et al. (2023). Morphological diversity and a ribosomal phylogeny of Rhabdopleura (Hemichordata: Graptolithina) from the Western Pacific (Singapore and New Zealand), with implications for a re-evaluation of rhabdopleurid species diversity. Mar. Biodivers. 53, 4 (2023). https://doi.org/10.1007/s12526-022-01310-3
↑ Mitchell, Charles E.; Michael J. Melchin; Chris B. Cameron; Jörg Maletz (2012). "Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite". Lethaia. 46: 34–56. doi:10.1111/j.1502-3931.2012.00319.x. ISSN0024-1164.
1 2 3 4 Gordon, D.P., Quek, Z.B.R., Huang, D. (2024). “Four new species and a ribosomal phylogeny of Rhabdopleura (Hemichordata: Graptolithina) from New Zealand, with a review and key to all described extant taxa”. Zootaxa. 5424(3):323-357. http://dx.doi.org/10.11646/zootaxa.5424.3.3
↑ Beli, E., Aglieri, G., Strano, F., Maggioni, D. Telford, M.J., Piraino, S., Cameron, C.B. (2018). The zoogeography of extant rhabdopleurid hemichordates (Pterobranchia: Graptolithina), with a new species from the Mediterranean Sea. Invertebrate Systematics, 32(1) 100-110. https://doi.org/10.1071/IS17021
1 2 3 Brusca, R. C. (2023). Invertebrates (W. (Entomologist) Moore & G. Giribet (Eds.); Fourth edition.). Oxford University Press.
1 2 Beli E, De Castro Mendonça LM, Piraino S, Cameron CB. (2022). “Development and Phenotypic Plasticity of Tubes and Tubaria of the Living Graptolite Rhabdopleura recondita (Pterobranchia, Hemichordata)”. Diversity. 14(12), 1080. https://doi.org/10.3390/d14121080
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