A new sponge-associated starfish, Astrolirus patricki sp. nov. (Asteroidea: Brisingida: Brisingidae), from the northwestern Pacific seamounts Author Zhang, Ruiyan Author Zhou, Yadong Author Xiao, Ning Author Wang, Chunsheng text PeerJ 2020 2020-05-27 8 e 9071 e 9071 journal article 10.7717/peerj.9071 ae249662-d6f6-4acf-ae69-420220f5034b 2167-8359 PMC7261123 32518717 4621673 Astrolirus patricki sp. nov. urn:lsid:zoobank.org:act: 0F238379-CD10-4E51-A74A-0FEF055D72A5 ( Figs. 14 ) Diagnosis. Arms 7, robust. Intercostal integument densely covered by irregular, abutting plates. No syzygy between proximal arm plates. The first pair of adambulacral plates separated byapair of marginal plates. Alargeinterradial plateabove thefirst marginal plates, visible from the abactinal side, covered by scattered spinelets. Mouth spines and proximal adambulacral spines robust, densely distributed. Suboral spines 3 4; subambulacral spines 1 2, proximal ones truncate, capitate. One lateral spine to each adambulacral plate, starting from about the 8th. Apair of gonads to each arm. Etymology. The name is originatedfrom the character ‘‘Patrick Star’’ inthe famous cartoon ‘‘SpongeBob Squarepants’’, who always spends time with his best friend ‘‘SpongeBob’’, a benthic sponge. Since all specimens of the new species were observed in situ living on sponges ( Fig. 1 ), it was name by Patrick to reflect this curious relationship. Material examined. Holotype , RSIOAS044 ( Fig. 1A ), 7 arms, of which 3 attached to the disk, others detached. r = 7.5 mm , the longest broken arm measures 153 mm without the missing tip. Length of genital area 35 mm , broadest part of arm measures 7 mm . Paratype RSIOAS028 ( Fig. 1B ), 7 arms, including 3 regenerating ones, all attaching to the disk; r = 7 mm , the longest arm measures 153 mm , R /r = 21.9; length of genital area 36 mm , broadest part of arm measures nine mm wide. Paratype RSIOAS003 ( Fig. 1C ), 7 arms, all arm detached from the disk; r = 10 mm , R = 190 200 mm , R /r = 19 20; length of genital area about 50 mm , broadest part of arm 9 10 mm , at 15 20 mm from the disk. Paratype RSIOAS052 ( Fig. 1D ), 7 arms, r = 7.5 mm , the longest arm measures 168 mm , R /r = 22.4. Paratype MBM 286625 ( Fig. 1E ), 7 arms, r = 7.5 mm , R = 171 mm , R /r = 22.8; length of genital area about 33 mm , broadest part of arm 7.5 mm . Description of the holotype . Arms 7. Disk thick, elevated from the plane of arms ( Figs. 2A 2C ). Abactinal surface of disk covered by small rounded plates ( Fig. 2G ) bearing multiple sharp, thin spinelets, with few scattered pedicellariae between and at the base of the spinelets ( Figs. 2C, 2F ). Madreporite body locates at the margin of the disk, elliptical with a curved rift in the center ( Fig. 2C ). Papulae absent. Abactinal surface of arm within genital area densely covered by abutting plates of irregular scale forms. Pedicellariae scattered in the proximal region connecting disk and the genital area, not forming costae ( Fig. 2C ). Costae present within the genital area, about 30 in number; costae thin, densely located (two to each adambulacral plate), composed of raised band of pedicellariae ( Figs. 2D 2E ). Beyond the genital area, pedicellariae forms bands wider than costae, two to each adambulacrals ( Fig. 2H ). Apair of gonads to each arm. Each gonad with 3 6 oval pedals ( Fig. 3G ). Adambulacral plate in proximal area subquadrate, elongated in middle and distal part of the arms ( Fig. 4 ). Proximally the ambulacral groove almost completely concealed by adambulacral spines ( Fig. 4 ). The first pair of adambulacral plates on adjacent arms entirely separated by a pair of marginal plates ( Figs. 3C 3D ). No syzygy between proximal plates. The first pair of marginal plates unit closely with an interradial plate ( Fig. 3D ). Interradial plate large, nearly naked, extending to the abactinal side of disk ( Fig. 2C ). Proximally Figure 2 Astrolirus patricki sp. nov. , abactinal view. (A) Paratype RSIOAS028. (B) Paratype RSIOAS003. (C), (D), (H), holotype RSIOAS044, (C) Abactinal surface of disk and proximal part of arms, with red arrow pointing at the madreporite body, white arrow at the interradial plate and yellow arrows at the marginal plates. The red frame indicates the proximal region of arm connecting the disk and genital region, where pedicellariae do no form regular costae. (D) Abactinal surface of arm genital area with mosaic plating, red arrows show the costae bands. (E) Paratype RSIOAS003, abactinal surface of arm genital area, red arrows show the costae bands. (F) Paratype RSIOAS052, zoom in view of the abactinal disk, showing the multiple sharp spinelets on disk plates. (G) Paratype RSIOAS052, a piece of dissected skin from abactinal disk, shot from the inner side of the skin, showing the small round disk plates. (H) Abactinal surface at the middle of arm, black arrows indicate the pedicellariae bands. Full-size DOI: 10.7717/peerj.9071/fig-2 Figure 3 Astrolirus . patricki sp. nov. actinal view. (A D), (G), holotype RSIOAS044. (A) Actinal surface of the disk. (B) oral plates and spines; (C) Interradial angle between arms, red arrow shows the first marginal plates, yellow arrow shows the second marginal plate. (D) Lateral view of the disk, showing the conjunction of plates in the interradii. Red arrows show the first marginal plates, yellow arrows show the first adambulacral plates, white arrow shoes the interradial plate. (E) Paratype RSIOAS003, lateral view of the disk, red arrows show the first marginal plates, yellow arrows show the first adambulacral plates, white arrow shoes the interradial plate. (F) Paratype RSIOAS052, adambulacral plates and spines at the middle of arm, yellow arrows show the subambulacral spines, white arrows show the furrow spines, red arrows show the lateral spines. (G) One of the paired gonads and digestive caeca in genital area. Full-size DOI: 10.7717/peerj.9071/fig-3 Figure 4 Astrolirus . patricki sp. nov. holotype RSIOAS044, mosaic image of abactinal arm. Yellow arrows show the subambulacral spines, white arrow shows the aboral furrow spine, black arrow shows the adoral furrow spine, red arrows show the lateral spines. Full-size DOI: 10.7717/peerj.9071/fig-4 2 3 marginal plates well-developed, present on the lateral side of the arm ( Fig. 2C ). The following marginal plates degrade to a line of protuberances on the side and corresponding to each adambulacral plates, bearing long lateral spine. Lateral spine present from about the 8th adambulacral plate, one to each plate ( Figs. 3F , 4 ). The longest measures 15 20 mm . Adambulacral plate armature includes: (1) 2 subambulacral spines and 1 aboral furrow spine, forming a diagonal row; the outer subambulacral spine usually the largest and most robust; (2) 1 adoral furrow spine, thinner than the aboral one ( Fig. 4 ). The first pair of adambulacral plate bear a diagonal line of 4 5 spines, two on the surface of the plate, and 2 3 smaller ones on the distal furrow corner ( Figs. 3B 3D ). At middle and distal part of arm, the number of subambulacral spine on each adambulacral plate reduces to 1 ( Fig. 4 ). Proximal subambulacral spinestruncate form, capitate ( Fig. 3C ). The furrow spines become shorter and thinner on distal plates. All adambulacral spines bear pedicellariae, leaving only the tip of the spines naked. Mouth plate large, bearing 3 4 robust suboral spines, forming a diagonal line, similar in form with the proximal subambulacral spines ( Figs. 3A 3B ); 2 small furrow spines present on the aboral corner of the plate, close to the first adambulacral plate; on the oral margin, the plate bears 2 very minute spines, one pointing to the actinostome and one pointing rather to the furrow, forming a sharp angle with the former. All oral spines bear pedicellariae. Variations in paratypes . In the paratypes , adambulacral plates at middle and distal part of arm bear equal number of spines as the proximal adambulacral plates ( Fig. 3F ), instead of having one less subambulacral spine as in the holotype . In several undeveloped arms in the paratypes , gonads were not spotted. The shape and size of the marginal and interradial plates vary slightly in paratype RSIOAS003 ( Fig. 3E ). Coloration. Color in life orange ( Figs. 1 , 2A 2B ). Distribution. Known from the northwestern Pacific Ocean, on seamounts, 1458 2125 m depth. Molecular and phylogenetic results. COI K2P distances between specimens of A. patricki sp. nov. are less than 0.003, which are considered to be intraspecific distances. A. patricki sp. nov. is closest to Brisinga species, with distances between 0.123 0.159 . The topology of the Bayes tree is shown in Fig. 5 and the nodes marked by black dots are support by both Bayes tree and ML tree. This tree is overall in line with results of the previous studies ( Mah & Foltz, 2011 ; Zhang et al., 2019 ). Figure 5 Phylogenetic tree of order Brisingida including Astrolirus patricki sp. nov. and 4 new specimens based on a concatenated dataset of COI , 16S, H3, 12S and 18S genes. Topology follows the result of Bayes tree, bootstrap values and posterior probabilities are shown for each node. Nodes marked by black dots are support by both Maximum Likelihood Tree and Bayes Tree. The new species and new data reported in this study are colored red in the tree. Full-size DOI: 10.7717/peerj.9071/fig-5 Remarks for A. patricki sp. nov. Types of A. patricki sp. nov. from different seamounts show little intraspecific divergences other than size and adambulacral spine number. The difference between A. patricki sp. nov. and the type species of the genus, A. panamensis , are listed in Table 2 . Although both species are characterized by the presence of abactinal plates, in A. panamensis the plates locate between the transverse costae, whereas in A. patricki sp. nov. a mosaic of abutting plates forms the abactinal arm skeleton. The two species further differ in arm number, adambulacral spine number and distribution, as well as the number of genital organs. It should be noticed that, in Ludwig’s original description of A. panamensis , ‘‘several, consecutive pairs of branched gonads on each arm (4 5 in one male specimen)’’ were recorded, each branch has their own gonopore ( Ludwig, 1905 ), but later when Fisher established the new genus, he described Astrolirus as having 2 4 gonads to each arm ( Fisher, 1917 ). Whether this new description resulted from a reexamination of the type specimens was not clearly indicated ( Fisher, 1917 ). Therefore, in this study we choose to follow Ludwig’s original descriptions when evaluate the differences between the new species and A. panamensis . Table 2 Major morphological differences between A. patricki sp. nov. and A. panamansis .

Diagnostic characteristics	A. patricki sp. nov.	A. panamansis
arm number	7	8 9
costae	raised bands of pedicellariae	a transverse line of up to 11 strong, thick plates
intercostal plates	abutting	usually isolated by small distances
adambulacral plate armature	1 2 subambulacral spines, 1 adoral and 1 aboral furrow spines	1 subambulacral spine, 1 adoral and 1 aboral furrow spines in most specimen
lateral spines	1 to each adambulacral plate	on every second or third adambulacral plate (in irregular change)
suboral spine	3 4	1
genital organ	1 pair to each arm	several, consecutive pairs on each arm (4 5 in one male specimen)

In addition, A. patricki sp. nov. ischaracterized bya largenumber of suboralspines (3 4), whereas in A. panamensis , only 1 suboral spine present on each oral plate. Interestingly, in A. panamensis , some large specimens ( r = 13 or 10.5 mm ) have the 2 suboral spines on a pair of oral plates covered in a same membrane ( Ludwig, 1905 ). This characteristic has only been reported in Brisinga andamanica Wood-Mason & Alcock, 1891 and Brisinga bengalensis Wood-Mason & Alcock, 1891 ( Alcock, 1893 ), whose biological function is not clear.