Ringvold H and Moum T (2020), On the genus Crossaster (Echinodermata: Asteroi...

ECB-ART-48666

PLoS One 2020 Jan 07;151:e0227223. doi: 10.1371/journal.pone.0227223.

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On the genus Crossaster (Echinodermata: Asteroidea) and its distribution.

Ringvold H , Moum T .

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Several starfish (Echinodermata, Asteroidea) are keystone species of marine ecosystems, but some of the species are difficult to identify using morphological criteria only. The common sunstar, Crossaster papposus (Linnaeus, 1767), is a conspicuous species with a wide circumboreal distribution. In 1900, a closely similar species, C. squamatus (Döderlein, 1900) was described from the NE Atlantic Ocean, but subsequent authors have differed in their views on whether this is a valid taxon or rather an ecotype associated with temperature variations. We assessed the differentiating morphological characters of specimens from Norwegian and Greenland waters identified as C. papposus and C. squamatus and compared their distributions in the NE Atlantic as inferred from research cruises. The field data show that C. papposus is found mainly in temperate and shallow waters, whereas C. squamatus resides on the shelf-break in colder, mixed water masses. Intraspecific diversity and interspecific genetic differentiation of the two putative species, and their phylogenetic relationships to several Crossaster congeners worldwide, were explored using mitochondrial and nuclear DNA sequences. The molecular evidence suggests that C. papposus is the more diverse and geographically structured taxon, in line with its wide distribution. C. papposus and C. squamatus are closely related, yet clearly distinct taxa, while C. papposus and C. multispinus H.L. Clark, 1916, the latter from the South Pacific Ocean, are closely related, possibly sister taxa.

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	Fig 1. Sampling stations in the Norwegian Sea where Crossaster papposus (red dots) and C. squamatus (green dots) specimens were recorded, and MAREANO video recordings of Crossaster spp. (black dots). Specimens for DNA analysis were also collected from NW Greenland in the NW Atlantic, and North and South Pacific Ocean. Fig1 is constructed with Quantum GIS (version 12.2.3).
	Fig 2. Photographic representations of Crossaster worldwide.2a. C. borealis (dorsal side) from Alaska, Bering Sea. 2b. C. borealis (ventral side, same specimen as 2a). 2c. Live C. squamatus from West of Shetland, September 2009, at 1050 m depth. Identified from video image by Daniel Jones. 2d. C. japonicus (Fisher, 1911) from NW Westport, New Zealand. 2e. Live C. papposus from Gravdal, near the city of Bergen, Norway. 2f. Ethanol preserved C. campbellicus from South New Zealand. 2g. Ethanol preserved C. multispinus from East New Zealand. 2h. Live C. papposus from Tellnes, near Bergen, Norway. 2i. Frozen C. squamatus from Barents Sea, IMR/ Ecocruise, st. 737. (Photo credits: 2a and 2b by Roger Clark, 2c by Daniel Jones/ SERPENT Project, National Oceanography Centre, 2d by Geoff Lemmey, CC license/ South Australian Museum and 2e-2i by Halldis Ringvold/ Sea Snack Norway.).
	Fig 3. a (left). Whole, conserved specimens of small Crossaster papposus, and b. C. squamatus. They are recorded from MAREANO stations 1218â471 (R = 2 cm), and 1086â438 (R = 1,8 cm), respectively. (Photo credit: Arne Hassel/ Institute of Marine Research.).
	Fig 4. The dorsal skeleton of Crossaster papposus is formed by narrow bars with large membranaceous spaces.Specimen recorded at MAREANO station 1218â471 (R = 2 cm). The dorsal skeleton sample is cut out and photographed from below. The arrows show papulae within membranaceous space. (Photo credit: Halldis Ringvold/ Sea Snack Norway.).
	Fig 5. The dorsal skeleton of Crossaster squamatus is scale-like, with irregular shaped plates, and with little membranaceous space.Specimen recorded at MAREANO station 1086â438 (R = 1,8 cm). The dorsal skeleton sample is cut out and photographed from below. The arrows show papulae within membranaceous space. (Photo credit: Halldis Ringvold/ Sea Snack Norway.).
	Fig 6. Distribution of Crossaster papposus (red dots) and C. squamatus (green dots) recorded by the BIOFAR and BIOICE programs [22, Ringvold et al. In prep.].
	Fig 7. Distribution of Crossaster papposus and C. squamatus from the Faroe Island, Iceland and Norway (data collected by the BIOFAR, BIOICE and MAREANO programs, respectively), in relation to depth and sea floor temperature recorded.The vertical bars indicate the minimum and maximum depths; triangles abundance-weighted mean depth; red circles abundance-weighted mean sea floor temperatures.
	Fig 8. Phylogeny of Crossaster and Solaster species based on COI.The phylogeny was inferred from mitochondrial COI sequences using the Maximum Likelihood method and Lophaster furcilliger as the outgroup. The tree with the highest log likelihood (-3528,66) is shown. The percentage of bootstrap replicates in which the associated taxa clustered together is shown next to the branches. The tree is drawn to scale, with branch lengths measured by the number of substitutions per site.
	Fig 9. Phylogenetic relationships of the focal taxa based on COI.Relationships among Crossaster papposus and C. squamatus individuals, and C. multispinus, as inferred from mitochondrial COI sequences using the Maximum Likelihood method and C. borealis as the outgroup. The tree with the highest log likelihood (-1972,69) is shown. The percentage of bootstrap replicates in which the associated taxa or individuals clustered together is shown next to the branches. The tree is drawn to scale, with branch lengths measured by the number of substitutions per site.
	Fig 10. Crossaster relationships based on rDNA.Phylogenetic relationships among C. papposus and C. squamatus individuals, and representative C. multispinus, C. borealis, C. penicillatus, and C. campbellicus, was inferred from nuclear rDNA sequences using the Maximum Likelihood method. The tree with the highest log likelihood (-12678,68) is shown. The percentage of bootstrap replicates in which the associated taxa or individuals clustered together is shown next to the branches. The tree is unrooted and drawn to scale, with branch lengths measured by the number of substitutions per site.
	Fig 11. Recordings of Crossaster papposus (red dots) and C. squamatus (green dots), the former species sampled by the Olga expedition, and the latter from the North-Sea Expedition [12]. C. papposus is distributed close to the Svalbard shore, whereas the one recording of C. squamatus was at the shelf-break.

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