Arbacia lixula

Arbacia lixula (Linnaeus, 1758)

(Fig. 23)

Echinus aequituberculatus — $ Drouët 1861: 93;

Echinocidaris aequituberculatus Desmoulins— $ Drouët 1861: 210–211; Barrois 1888: 31;

Arbacia aequituberculata Gray, 1835 — $ Agassiz 1863: 20;

Arbacia pustulosa Gray, 1835 — Agassiz 1872: 232–234, pl. 1g, fig. 5, pl. 2a, figs. 15–33, pl. 5, figs. 19–21, pl. 28, fig. 6, pl. 38, figs. 10a–c; $ Barrois 1888: 74–75; $ John 1889: 285;

Arbacia pustulosa (Leske, 1778) — $ Simroth 1888: 231; $ Koehler 1895a: 224, 1898: 9;

Arbacia lixula (Linnaeus, 1758) — Jackson 1912: 158; Mortensen 1927a: 290, 1935: 566–572, pl. 70, fig. 13, pl. 87, figs. 11, 12; Harvey 1956: 51, 63; $ Marques 1983: 4–5; $ Marques 1984: 103–108, fig. 1; Pereira 1997: 333–334; $ Morton et al. 1998: 92–93, figs. 3–3H1, 3– 4X, 5–1R, 5–2T1, 8–1M; Pérez-Ruzafa et al. 2002: 284–285; $ Wirtz & Debelius 2003: 259; Cardigos et al. 2005: 165; García-Diez et al. 2005: 50; Schultz 2006: 118–119, figs. 211–215; $ Haddad & Barreiros 2008: 9, fig. 3c; Micael & Costa 2010: 322; $ Micael et al. 2010: 329; $ Kroh et al. 2011: 99–105, figs. 2–4; Madeira et al. 2011: 248–249, figs. 4, 5A, 6A; Micael et al. 2012: 3; $ Wangensteen et al. 2012: 1–16;

Arbacia aequituberculata (Blainville, 1825) — Koehler 1921b: 113–114, fig. 75; $ Nobre 1924: 89; $ Nobre 1930: 30, 69; $ Cadenat 1938: 366, 373; $ Chapman 1955: 399;

non Arbaciella elegans Mortensen, 1910 — $ Marques 1983: 5 [juvenile of A. lixula]; Pereira 1997: 334 [based on Marques 1983]; Haddad & Barreiros 2008: 9; Micael & Costa 2010: 322 [based on Marques 1983]; Micael et al. 2012: 3 [based on Marques 1983].

See: Mortensen (1935); Schultz (2006); Wangensteen et al. (2012).

Occurrence: East Atlantic and Mediterranean Sea; present along the Atlantic warmer waters of Africa to the Gulf of Guinea and Angola (Mortensen 1927 a, Cadenat 1938, Cherbonnier 1959), including the Azores, Madeira, Selvagens, Canaries and Cape Verde (Pérez-Ruzafa et al. 2002, Wangensteen et al. 2012); it is present as well in Brazil (Wangensteen et al. 2012).

Depth: 0–55 m, common on the first few meters in the Azores (<15–18 m), but can occur at depths as great as 55 m (herein).

Habitat: hard substrata, particularly dominant on exposed rocky shores of the Azores (Morton et al. 1998); nocturnal omnivorous grazer (Schultz 2006).

Larval stage: planktotrophic (c. 26 days; George 1990).

Fossil record: spines and test fragments were reported from Pleistocene sediments of Santa Maria Island (Madeira et al. 2011).

Material examined: DBUA-ECH 013 (Rosto do C„o, SMG, AZO, c. 37°44’37”N, 25°38’19”W, 1997.02.07, 13 m; 1 broken bt); DBUA-ECH 014 [Capelas (Morro), SMG, AZO, c. 37°50’37”N, 25°41’18”W, 1996.05. 26, 9 m; 1 spm, D = 36 mm]; DBUA-ECH 015 (S„o Roque, SMG, AZO, c. 37°44’37”N, 25°38’19”W, 2006.07.11, intertidal; 1 bt, D = 38 mm); DBUA-ECH 016 (Vila Franca do Campo, SMG, AZO, c. 37°42’50”N, 25°25’58”W, 2006.07.20, intertidal; 1 bt, D = 38 mm); DBUA-ECH 107 (FRM, AZO, c. 37°16’14”N, 24°46’52”W, 1990.06; 3 spms, D = 36–46 mm); DBUA-ECH 235 (Horta harbour, FAY, AZO, c. 38°31’51”N, 28°37’23”W, 2009.12. 4, 5 m; 1 bt, D = 3 mm); DOP 3014 (Channel PIX–FAY, AZO, 38°34’16”N, 28°32’31”W, 2008.06. 16, 55 m; 1 spm, D = 10 mm); DOP 3024 (Channel PIX–FAY, AZO, 38°34’16”N 28°32’31”W, 2008.06. 16, 50 m; 1 spm, D = 4 mm); MB-NMHN 367–20743 (Ponta Delgada harbour, SMG, AZO, c. 37°44’12”N, 25°39’26”W, 1982, collected and identified by Vasco Marques as Arbaciella elegans; 11 spms, D = 5–8 mm); MB-NMHN 372–20748 (Castelo Branco, FAY, AZO, c. 38°31’05”N, 28°43’23”W, collected and identified by Vasco Marques as Arbaciella elegans; 3 spms, D = 5–13 mm); MB-NMHN 385–20761 (SMG, AZO, 1982; 7 spms, D = 24–45 mm); MB-NMHN 385–2076 (SMG, AZO, 1988; 7 spms, D = 24–45 mm); MB-NMHN 401–20777 (SMG, AZO, 1982; 3 spms, D = 38–40 mm); MB-NMHN 403–20779 (Castelo Branco, FAY, AZO, c. 38°31’05”N, 28°43’23”W, 1979; 4 spms, D = 30–50 mm); MB-NMHN 412–20788 (SMG, AZO, 1988; 7 spms, D = 35–50 mm); MB-NMHN 415–20791 (SMG, AZO, 1982; 7 spms, D = 27–52 mm); MB-NMHN 416–20792 (SMG, AZO, 1982; 5 spms, D = 43–52 mm); MB-NMHN 548–20924 (Baixinha, GRA, AZO, c. 39°05’13”N, 27°59’09”W, 1982.08. 10, 15 m; 5 spms, D = 50–58 mm).

Description: test circular, hemispherical, flattened on the oral side with a height about of 40–50%D. Epistroma well developed, particularly evident on the aboral side. Apical disc about 50%D in smaller specimens changing gradually to about 21%D in larger animals, naked and in most dicyclic, though the Oc V tends to be wedged in between the genital plates. Madreporite enlarged. Periproct oval shaped, with four naked anal plates; Gonopores open at sizes greater than 6 mm, though in some individuals as large as 10 mm are still not developed (e.g., DOP 3014). Ambulacra narrow, about 30–40% of the inter-ambulacra at the ambitus, but widening to twice the size of the interambulacra at the peristomal edge; plates trigeminate, bearing a single primary tubercle each. These are arranged in alternating fashion near the apical disc but changing to a double vertical series just above the ambitus. Primary tubercles in the interambulacra reduced to one per plate in the area surrounding the apical disc, becoming numerous towards the ambitus, reaching up to four to five per plate in the larger specimens, the most interradial of which is usually smaller; in smaller specimens (<13 mm D), the spines only develop at the ambitus, about the third or fourth plate from the apical disc, giving a rather naked appearance of apical side. Peristome relatively large, about half of the D, ambulacral margins protruding and interambulacra terminating in well-defined buccal notches, giving the peristome an overall sinuous pentagonal appearance. Primary spines about 60–70%D; the primary spines in the small individuals (<5 mm D) are dorsoventrally flattened, sword like shaped, changing to the typical tip-pointed spines seen in larger specimens through ontogeny. Colour: solid black to dark brown; in smaller specimens (<5 mm), the spines are rather translucid with a black hue. Colour (naked test): interambulacra and ambulacra pink or reddish, particularly so in pore zones; apical disc can be darker, with a greyish hue.

Remarks: according to the text, Drouët (1861) only found Arbacia lixula (= Echinocidaris aequituberculatus) in Terceira Island and concluded that it was a rare species in the Azores. Conversely, Barrois (1888, as Arbacia pustulosa) commented that this species was quite common in rocky shores of the Azores, where it occurs in association with Paracentrotus lividus (as Strongylocentrotus lividus). Marques (1983, 1984) reported densities of Arbacia lixula up to 15 individuals/m 2 on S„o Miguel and Graciosa rocky shores, between 2 and 15 m depth. Marques’ observations agree with our observations though no quantitative studies have been made recently. Marques also noted that P. lividus and A. lixula seldom co-occur in the Azores. Though both species are frequently observed in same low intertidal waters of the archipelago (<2 m), we agree with Marques in the sense that Arbacia lixula tends to be more numerous in relatively more exposed shores, such as vertical walls in ports, whereas P. lividus seems to prefer areas of low slope and with less direct exposure to the wave action, living inside bore-holes (personal observation). Cardigos et al. (2005) recorded this species in the area of Don Jo„o de Castro Seamount (between Terceira and S„o Miguel islands), one of the rare examples in Azores of a shallow-water hydrothermal-active volcanic seamount (the top of the seamount lies 13 m deep).

Additionally, small specimens of this species have been misidentified as Arbaciella elegans Mortensen 1910, including the ones re-examined here from Museu Bocage (MB-NMHN 367–20743 and MB-NMHN 372–20748), collected and identified by Marques (1983). A recent revision showed that records of Arbaciella in North Atlantic and Mediterranean waters were misidentifications of juveniles of Arbacia lixula, reducing the distribution of this species to the original tropical West African coasts, south of Cap Blanc (for discussion see Kroh et al. 2011).