Arbacia nigra
Creators: name: Courville, Erwan
name: Poulin, Elie
name: Saucede, Thomas
name: Mooi, Rich
name: Lessios, Harilaos
name: Salinas, Andrea Martínez
name: Diaz, Angie
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Echinoidea
Arbacioida
Arbaciidae
Arbacia
Arbacia nigra

description: Arbacia nigra (Molina, 1782)
1782 Echinus niger Molina, 1782 p. 175.
1841 Tetrapygus niger L. Agassiz: p. 7.
1863 Arbacia nigra A. Agassiz: p. 20.
1910 Tetrapygus niger (Molina) Mortensen: p. 35–36; fig. XV 12, 15.
1935 Tetrapygus niger (Molina) Mortensen: p. 582–584; fig. LXX 10–12; LXXXVII 15–19.
Emended diagnosis. Arbacia with ocular plate V invariably insert; ambulacral plates quadrigeminate to quinquegeminate; additional secondary tubercles below and above primary tubercles for each interambulacral plate above the ambitus; cleaned test deep purple; valves of aboral ophicephalous pedicellariae bilobed; epistroma well-developed, present as punctation among tubercles in ambulacra and interambulacra; tips of oral spines with vestigial “enameled” tip only.
Type material. Type specimen was not established in the original description, which also lacks illustrations. Larraín (1975) designated a neotype, housed in the Museum of the Zoological Department of the Universidad de Concepción, Chile, under catalogue number MZUC-UCCC n° 7967. The neotype was collected in Bahía Concepción, Chile (36°40’S, 73°W) in 1973. It is preserved dry, retaining all its spines and soft tissues, except for a cleaned ambulacrum, extending from the apical system to the peristome (TD = 77.3 mm; PD = 29.5 mm; TH = 39.2 mm).
Description. Test circular to subpentagonal in outline at ambitus. Test large, reaching 80 to 90 mm in diameter. Living specimens deep purple. Denuded test and spines purple. As for other species in genus, test height highly variable. Test height relative to test diameter increasing with size, allometric relationship between test height and test diameter described by linear regression model ln(y) = 1.2112ln(x)—ln (1.6139); R² = 0.9612.
Apical system —Small, hemicyclic, with ocular plate V always insert (Figs. 3A, C, G, 4A, B) and plates I and IV also often insert in adult, large specimens (oculars I, V, IV insert 57%, according to Jackson 1912). Juvenile always with dicyclic apical system, ocular plates becoming insert with growth. Ocular and genitals free of tubercles in adults, but juveniles have vestigial tubercle on each ocular as do other species of genus. Ocular plates bearing ophicephalous pedicellariae. Genital plates and periproct surrounded by pedicellariae. Periproctal membrane with four anal plates, purple, with a white adoral band (Fig. 3A, C). Periproct oval, with maximum diameter in direction 1-IV.
Peristome —Relative peristome diameter less than in other Arbacia species, usually close to 35% of test diameter. Relative peristome size decreasing with growth. Buccal notches shallow. Tags long and narrow (spanning>2 tubercles). Perignathic girdle large, auricles in contact, sometimes fused with each other, forming arch over perradius. Allometric relationship between peristome diameter and test diameter described by linear regression model ln(y) = 0.8094ln(x) + ln(0.0714); R² = 0.968.
Ambulacra —Very narrow (one third width of interambulacra), each plate with one imperforate and non-crenulate primary tubercle of almost same size as interambulacral ones at the ambitus and progressively decreasing in size adapically (Figs. 3A, F, 4C). Above ambitus, ambulacral tubercles smaller than interambulacrals. Tubercles closely spaced, occupying most of each ambulacral plate, separated from each other by few small granules perradially (Fig. 3F). Tubercles small, forming two well-developed columns of tubercles reaching apical system (Fig. 3A). Ambulacral plates quadrigeminate to quinquegeminate, of arbaciid type aborally (Fig. 4C), forming more complex compound plating adorally, leading to wide phyllodes occupying entire oral side adorally (Fig. 3B, D). Close to peristome, single perradial pit for sphaeridium along perradius of each ambulacrum (Fig. 3B).
Ambulacral pores —As in all Arbacia species, oral and aboral pores differentiated. Aborally, pores round and deeply conjugate. These conjugate isopores correspond to Smith’s C1- type classification (Smith, 1978). Oral pores exhibit morphological features characteristic of Smith’s P 4 type (Smith 1978), including large circular interporal partition and broad circular attachment area. Tube feet, associated with small pores inside large periporal areas, ending in large adhesive discs with thick connective tissue, allowing for strong fixation to hard substrates.
Interambulacra —Wide, with five to six imperforate and non-crenulate primary tubercles per plate at ambitus (Figs. 3A, 4E). Smaller specimens (TD = 30 mm) bear three to four tubercles at ambitus (Figs. 3C, 4D). These tubercles with large mamelons, narrow platforms and broad areoles occupying most of plate width and deforming plate margins. Number of interambulacral plates low, usually between 14 (TD = 30 mm) and 17 (TD = 40 mm). Largest specimens reaching 89 mm TD bear up to 21 interambulacral plates. Height of plates increase quickly along aboral surface adapically (from 2 to more than 4 mm height). Above ambitus, size of tubercles reducing towards interradius and number of interambulacral primary tubercles decrease slowly towards apical system. Additional secondary tubercles developed aborally, forming second row of tubercles in adoral part of each plate (Figs. 3A, 4E). In largest specimens (TD> 45 mm), number of secondary tubercles higher, with another row of tubercles below primary tubercles (Figs. 3A, 4E). Otherwise, plates coarsely granulated in between tubercles. Epistroma well-developed, covering all plates and forming punctation or even crenulation, more easily visible in small specimens. In small specimens (TD <20 mm), naked interradial area restricted to interradius. In larger specimens, aboral interradial region covered with both primary and secondary tubercles.
Pedicellariae —Pedicellariae of three types, ophicephalous, tridactylous and triphyllous, all abundant over test (Fig. 5). All pedicellariae are purple in adult specimens.
Ophicephalous pedicellariae (Fig. 5A, B) strongly differentiated between oral and aboral surface, more than in other Arbacia species. Aboral surface covered with numerous ophicephalous pedicellariae, including area surrounding apical system. Valves of ophicephalous pedicellariae low, narrow, and strongly constricted in upper part, with indented and sharply serrated blade edges (Fig. 5A). These valves very typical, being flattened or even concave at tip, with broad and deeply grooved apophyses. Numerous lateral spines in medial region of inner valve surface (Fig. 5A, F). Buccal membrane covered by second type of ophicephalous pedicellariae (Fig. 5B), with higher valves, indented blade edges, and slight serration, without spines on inner surface. Second type of ophicephalous pedicellariae also observed sparsely distributed on oral surface.
Triphyllous pedicellariae (Fig. 5C) present but very scarce, resembling those found in other Arbacia species, as noted by A. Agassiz in Agassiz & Clark (1908). Suggested of absence of these made by Larraín (1975) likely erroneous, so absence of triphyllous pedicellariae not diagnostic for species, contrary to that author. Valves of triphyllous pedicellariae elongated and constricted in middle part. Blade edges finely serrated.
Tridentate pedicellariae (Fig. 5D) small, only one and half times as long as ophicephalous pedicellariae. Scarce and exclusively on oral surface, lacking on buccal membrane. Valves slender, elongated, and noticeably constricted in middle. Blade edges finely serrated. Near constriction, inner surface of valve covered by sharp, long thorns (Fig. 5D, G).
Spines —Primary spines longest at ambitus and decrease in size towards apical system. Close to apical system, spines short and large (Fig. 6A, B). Ambital and subambital spines with vestigial “enameled” tips, short relative to other Arbacia species (Fig. 6C, E).
description: Published as part of Courville, Erwan, Poulin, Elie, Saucede, Thomas, Mooi, Rich, Lessios, Harilaos, Salinas, Andrea Martínez & Diaz, Angie, 2023, Taxonomic reassessment of Tetrapygus niger (Arbacioida, Echinoidea): molecular and morphological evidence support its placement in Arbacia, pp. 249-261 in Zootaxa 5375 (2) on pages 255-258, DOI: 10.11646/zootaxa.5375.2.6, http://zenodo.org/record/10184302