Ophiothrix fragilis (Abildgaard, in Müller, 1789)

Madeira, Patrícia, Kroh, Andreas, Cordeiro, Ricardo, De, António M., Martins, Frias & Ávila, Sérgio P., 2019, The Echinoderm Fauna of the Azores (NE Atlantic Ocean), Zootaxa 4639 (1), pp. 1-231 : 29-31

Ophiothrix fragilis (Abildgaard, in Müller, 1789)

( Fig. 7 )

Reports for the Azores:

Ophiothrix rubra $ Ljungman, 1872: 624–625 ; $ Lyman 1874: 248; Barrois 1888: 32; Ophiothrix lusitanica Ljungman, 1872 — $ Barrois 1888: 32, 72; Ophiothrix fragilis (Abildgaard, in M̹ller, 1789)— $ Koehler 1898: 58–59, 1909: 200; $ Nobre 1924: 89; $ Mortensen 1927a: 174–175, fig. 98; $ Nobre 1930: 69; $ Nobre 1938: 65–68, figs. 34–35; $ Chapman 1955: 400; Marques 1980: 103; $ Marques 1983: 3, fig. 3; Pereira 1997: 332; $ Morton et al. 1998: 76, 169, fig. 4.2L; $ Wirtz & Debelius 2003: 271; García-Diez et al. 2005: 48; Micael & Costa 2010: 322; Micael et al. 2012: 3.

See: Koehler (1921b: 74–77, figs. 48–49); Mortensen (1927a); St̂hr (2005: 269, figs. 16A-N).

Occurrence: Mediterranean Sea and East Atlantic, from Iceland to South Africa ( Tortonese 1965, A.M. Clark 1974), including the Azores, Madeira ( Nobre 1938), Canaries and Cape Verde ( Madsen 1970).

Depth: 0–509 m ( Tortonese 1965, Cherbonnier 1970); AZO: 0–150 m (herein).

Habitat: on hard bottom, among algae, shells, serpulids, under stones, etc. ( Mortensen 1927a, Tortonese 1965); predated specially by Luidia ciliaris ( Philippi, 1837) ( Mortensen 1927a) .

Larval stage: planktotrophic (26 days; MacBride 1907).

Material examined: DBUA-ECH 064 (Islet of Vila Franca do Campo, SMG, AZO, c. 37°42’21”N, 25°26’34”W, 1997.03.03; 1 spm, DD = 8 mm); DBUA-ECH 121 (S„o Roque, SMG, AZO, c. 37°44’37”N, 25°38’19”W, 2012.11.16, intertidal; 9 spms, DD = 2–4 mm); DBUA-ECH 125 (Poços de S„o Vicente, SMG, AZO, c. 37°50’06”N, 25°40’10”W, 1996.07. 17, 8 m; 2 spms, DD = 3–6 mm); DBUA-ECH 126 (Poços de S„o Vicente, SMG, AZO, c. 37°50’06”N, 25°40’10”W, 1996.07.03, 10 m; 2 spms, DD = 3–6 mm); DBUA-ECH 127 (Poços de S„o Vicente, SMG, AZO, c. 37°50’06”N, 25°40’10”W, 1996.07. 17, 10 m; 8 spms, DD = 4–8 mm); DBUA-ECH 133 (Baixa da Caloura, SMG, AZO, c. 37°42’20”N, 25°30’30”W, 1996.12.05, 15 m; 3 spms, DD = 2–5 mm); DBUA-ECH 195 (FRM, AZO, c. 37°16’14”N, 24°46’52”W, 1996–06–07; 2 spms, DD = 3–4 mm); DBUA-ECH 254 (PIX, AZO; 1 spm, DD = 5 mm); DBUA-ECH 342 (Sabrina Bank, SMG, AZO, 37°52’09”N, 25°54’08”W, 2011.07.07, 150 m; 1 spm, DD = 5 mm); DBUA-ECH 395 (Horta harbour, FAY, AZO, c. 38°31’51”N, 28°37’23”W, 2011.03. 10, 6 m; 1 spm, DD = 3 mm); DBUA 416 (Sabrina seamount, SMG, AZO, c. 37°52’35”N, 25°53’50”W, 2011.07.07, 34 m; 1 spm, DD = 4 mm).

Description: disc subpentagonal covered more or less evenly by short stumps, terminating in a crown of very small sharp and thin spines, typical three in number but occasionally more (this is particularly true in the largest specimen, DD = 8 mm, DBUA-ECH 064). Radial shields about 19–24% DD, naked with a few scattered stumps (most individuals) to almost entirely covered. No spines present on the dorsal surface of the disc. Stumps also present on the ventral surface of the disc. Mouth shields somewhat triangular (length <breadth). A hole is present at the base of each jaw. No oral papilla. Numerous crowded tooth papillae. One small tentacle scale; arms about three to four times the DD; the largest specimen (DD = 8 mm, DBUA-ECH 064) with four of the five arms showing evidence of predation with varying degrees of regeneration. Dorsal plates of arms naked with longitudinal keel, protruding distally to form a faint elevated knob. Ventral arm plates almost rectangular with a concave outer edge. Seven thorny arm spines, increasing gradually in length from very small lowermost to the fifth or sixth arm spine, then decreasing in size again; distally lowermost arm spine hooked-shaped. Colour: disc uniform light brown, green, greyish-green to purple or pink; the disc colour was frequently lighter towards the centre to almost white or light pink; some specimens had a deep green circular spot in the centre of the disc; others presented as well small brown to dark green specks; radial shields sometimes almost totally white or having a white distal part (also the base of the arms), mid-way deep green, proximally pink as the rest of the disc arms green to greyish green with stripes (darker brown, greyish green or pink); some individuals had a conspicuous white spot on the dorsal plates; arm spines as the arms; exceptionally, oral surface of the disc orange becoming somewhat darker pinkish in the centre and arms banded yellow and orange with the dorsal plates presenting dark lines forming an irregular pattern (DBUA-ECH 342). Colour (in ethanol): many individuals had lost all colour, presenting a uniform cream colour, though some showed a conspicuous white circular spot in the centre of the dorsal surface of the disc.

Remarks: Ophiothrix fragilis is a highly polymorphic species, a fact reflected by the numerous synonyms and varieties described ( Tortonese 1965), which is mirrored by the history of this species in the Azores. Ljungman (1872) based Ophiothrix rubra an animal collected in the Azores. Lyman (1874) re-examined Ljungman’s type specimen and concluded that O. rubra was conspecific with Ophiothrix lusitanica , whereas Ljungman believed that it was closely related to the Ophiothrix echinata . Barrois (1888) agreed with Lyman and commented that Ophiothrix lusitanica seemed rather common on the shores of all Azorean islands. Conversely, Koehler (1898, 1909) identified material collected in the Azores by Hirondelle at 130 m depth and by Princesse Alice between 54 and 98 m depth as ‘ pentaphyllum ’ and ‘common’ forms, respectively. Later, Koehler (1921b) reduced the forms previously described as independent species or varieties to four basic subspecies: ‘ echinata ’, ‘ lusitanica ’, ‘ pentaphyllum ’ and ‘ abildgaardi ’. Mortensen (1927a) found Ljungman’s type material to be identical with O. fragilis , confirming unambiguously the synonymy (though without commenting which variety it was most closely related to). Among later authors working with material from the Azores, only Chapman (1955) ventured to identify a specimen collected in archipelago as O. fragilis var. lusitanica .

Historically, many authors commented that diagnosis of O. fragilis subspecies was quite difficult since they are linked by intermediary forms (e.g., Mortensen 1927a; Nobre 1938; Tortonese 1965; A.M. Clark 1974). Among the littoral subspecies, ‘ lusitanica ’ is viewed as the southern form from the Atlantic French coasts south to NW Africa, ‘ echinata ’ is viewed as a typical Mediterranean form, the form ‘ triglochis ’ is thought to be restricted to Southern Africa and ‘ nuda ’ to Cape Verde ( Koehler 1921b; Cadenat 1938; Nobre 1938; Tortonese 1965; Madsen 1970; A.M. Clark 1974). Nevertheless, it is known that these subspecies can co-occur sympatrically (e.g., Cadenat 1938, Alva & Vadon 1989). Comparing the description of O. rubra type specimen by Ljungman (1872) and Lyman (1874) with our own specimens, the most striking commonality is the presence of thorny spinelets on the radial shields. This morphological feature is normally associated with O. alopecurus , which Koehler (1921b) synonymised as a rare form of ‘ echinata ’. However, in this subspecies the disc stumps have only up to three terminal spines (‘trifid stumps’). This contrasts with the occasional presence of stumps terminating in up to five thorns in our material, which is more characteristic of the subspecies ’ lusitanica ’. Another typical feature of this subspecies, shared with our specimens, is the absence of disc spines, and even and regular distribution and size of disc spinelets. Interestingly, A.M. Clark (1974) observed the same intermediary characteristics between ‘ echinata ’ and ‘ lusitanica ’ in specimens from South Africa previously identified as Ophiothrix triglochis (= Ophiothrix fragilis ). The choice, in which subspecies the Azorean specimens should be placed is even less clear considering the colour pattern alone (see Figs. 7 A–E). For example, some specimens had a conspicuous white or lighter coloured spot on the dorsal plates, which is characteristic of ‘ echinata ’. In contrast, other specimens presented a uniform green colour which places them in the range of ‘ lusitanica ’ variety. The specimen from deeper waters (150 m; DBUA-ECH 342) presented an overall structure similar to the shallow-water material. However, the animal presented a brightly yellow/orange colour pattern typical of the deeper water variety ‘ pentaphyllum ’ ( Fig. 7F ). Overall, the material of O. fragilis housed in the DBUA-ECH collection presented features intermediary to the varieties ‘ echinata ’ and ‘ lusitanica ’. Whether these subspecies reflect environmental differences (e.g., differences in temperature and depth) or underlying genetic differences between populations is not clear, but recent genetic studies on the European varieties by Muths and coauthors (2009) suggest that O. fragilis represents a single, but plastic, species.

Madsen (1970) synonymized Ophiothrix indigna Koehler, 1906a (from Madeira and Gulf of Cadiz) with O. cotteaui ( de Loriol, 1900) as it merely represented adults of the latter species. However, Madsen erroneously placed O. indigna ’s type locality in the Azores. Ophiothrix fragilis can be easily distinguished from O. cotteaui or even from O. luetkeni Thomson, 1873 (see below) by its naked dorsal arm plates with a more or less produced distal edge.

Figures

FIGURE 7. Ophiothrix fragilis (Abildgaard, in M̹ller, 1789) (juveniles; DBUA-ECH 121: A–C; DBUA-ECH 342: F). Dorsal view (A); ventral view (B); detail of the disc, ventral view (C); dorsal view (D–F).