Xylophaga alexisi, Voight & Segonzac, 2012

Voight, Janet R. & Segonzac, Michel, 2012, At the bottom of the deep blue sea: a new wood-boring bivalve (Mollusca, Pholadidae, Xylophaga) from the Cape Verde Abyssal Plain (subtropical Atlantic), Zoosystema 34 (1), pp. 171-180 : 173-176

Xylophaga alexisi n. sp.

( Figs 1-5 ; 6A )

“ X. sp. m Voight, in ms.” – Voight 2008: table 1.

TYPE MATERIAL. — Holotype ( MNHN 24465 ). — Paratypes: 5 ex. ( MNHN 24466 ) ; 5 ex. ( USNM 1132964 ) ; 18 ex. ( FMNH 318671 ) .

OTHER MATERIAL EXAMINED. — Xylophaga alexisi n. sp. ( FMNH 312486). — Xylophaga ricei Harvey, 1996 , type locality, 6 ex. ( FMNH 312285). — Xylophaga microchira Voight, 2007 , type locality, 14 ex. ( FMNH 308173 and 308174). — Xylophaga anselli Harvey, 1996 , type locality, 2 ex. ( FMNH 312286).

TYPE LOCALITY. — Cape Verde Abyssal Plain, 21°03.81’N, 31°12.21’W, 4626 m ( Fig. 1 ).

DISTRIBUTION. — Only known from the type locality. Wood block fixed to the benthic MAC 4, at least 50 cm above the bottom; deployed 17/02/1991, recovered 21/09/1991.

ETYMOLOGY. — The species is named in honour of Alexis Khripounoff for his accomplishments in the study of particulate flux and organism supply in numerous deep-sea fields.

DIAGNOSIS. — Shell small (<2.5 mm), delicate; posterior adductor scar composed of linear elements. Siphon incomplete; no cirri on either siphonal opening. Incurrent siphon 1.1 to 3.5 times shell length. Mesoplax shape changes from an erect sheet to an anteriorly directed shallow triangle without strong vertical extensions with growth.

DESCRIPTION Shells exceedingly fragile, either due to long storage in formalin or to the great depth at which they were collected; almost always crumple when dried. Posterior adductor scar, as seen through intact shell, made of short, linear elements that appear to extend from near ventral shell edge to posterior-dorsal edge. Slight posterior extension ( Fig.2A ). Prodissoconch II 412 Μm across, with regular growth lines ( Fig. 3 ). Pronounced fold between posterior adductor scar and umbonal-ventral sulcus.Umbonal-ventral sulcus weak, shell slightly flattened at condyle. Posterior ridge broadly inflated, shell narrows dramatically just posterior to the inflation ( Fig. 2A ). Umbonal reflection well developed, in larger individuals, supports the mesoplax. Gape between the right and left beaks notably broad. Beak with comparatively few toothed ridges parallel ventral margin. Ridges form narrow band at junction of beak and shell.

Mesoplax in small specimens, erect, comparatively thin, translucent, nearly invisible in dorsal view ( Fig. 2C ), becoming more anteriorly directed and calcified with growth; in larger specimens, forms a fairly broad triangle ( Fig. 2B ); with only minimal rounded vertical extensions.

Siphons, incomplete; excurrent siphon truncated ( Fig. 2A ). Excurrent opening a very small flap without detectable cirri, even in larger individuals ( Fig. 4 ). Very faint impressions of longitudinal walls detectable immediately distal to excurrent opening visible in limited number of specimens ( Fig. 4B ). Incurrent siphon long, 2.5 to 3.5 times shell length ( Fig. 2A ), opening lacks cirri ( Figs 2A ; 5 ). Growth of excurrent and incurrent siphons isometric relative to each other.

No small individuals found attached to larger ones.

Measurements of holotype Shell length 2.3 mm; width 2.5 mm; height 2.5 mm. Incurrent siphon length 6.3 mm; excurrent siphon length 2.5 mm.

REMARKS

Based on the specimens examined, all over 1.2 mm in length, the present species is unique in having an incomplete siphon with no cirri on either siphonal opening and having a linear rather than herringbone pattern to the posterior adductor scar. Table 1 compares this species to those previously named that are overtly similar. Xylophaga alexisi n. sp. is separable from X. ricei

A

B

from 31°N, 20°W, c. 5000 m depth ( Fig. 1 ) by the subterminal excurrent siphon in that species and the incomplete siphon in the present species. Xylophaga ricei is similar to X. pacifica Voight, 2009 , but lacks brooded young. The incomplete siphon of X. alexisi n. sp. separates it from X. anselli Harvey, 1996 with a complete siphon ( Figure 6 illustrates the difference between these character states); the latter species appears similar to X. heterosiphon Voight, 2007 . Its incomplete siphon also distinguishes it from X. atlantica Richards, 1942 ; this species, reported on the Mid-Atlantic Ridge ( Gaudron et al. 2010), has an excurrent siphon only slightly shorter than the incurrent. The linear elements in the posterior adductor scar distinguish this species from X. oregona Voight, 2007 , X. washingtona Bartsch, 1921 and other species in Turner’s (2002) group 5.

The incomplete siphon of the present species ( Fig. 6 ) could be argued to ally it with species in Turner’s groups 5 or 6; Turner understood this character to be unique to these two groups. The present species, however, appears to be more closely allied with the enigmatic X. microchira Voight, 2007 , a member of the off-shore northeast Pacific fauna ( Voight 2009). Both species share an incomplete siphon, a mesoplax that develops an anterior fold with growth and an excurrent siphon that grows isometrically relative to the incurrent siphon; the relationship between the siphon lengths in the present species is y = 1.0 x-0.99; r2 = 0.92, essentially parallel to that of X. microchira , reported by Voight (2007). The presence of conspicuous cirri at the excurrent opening in X. microchira , shown by Reft & Voight (2009), and the smaller cirri at the incurrent siphon readily separate the species. The present species is distinct from X. galatheae Knudsen, 1961 by its shell shape, less pronounced umbonal-ventral sulcus and lack of a pronounced posterior extension.Although specimens of X.alexisi n. sp. were examined by SEM, no apparent sensory structures such as those reported by Reft & Voight (2009) were found.

Habitat notes

As noted in the Introduction, the type locality is on a nutrient-limited abyssal plain with a fauna depauperate in both diversity and density. No major surface currents overlie the site. The bottom temperature is 2.4°C and its current speed is 3.3 cm /sec. Details of the site are reported by Auffret et al. (1992), Vangriesheim et al. (1993), Khripounoff et al. (1998) and Galéron et al. (2000).

Settlement and Growth

The wood, after 216 days at 4626 m depth, carries an estimated 170 boreholes/cm2. Borers reached such high densities that the wood is transparent between adjacent, non-overlapping boreholes. Young bivalves appear to still have been recruiting to the block of wood, as very small, nearly circular individuals were seen on the block’s surface.

Shell lengths of X. alexisi n. sp. are up to 2.4 mm; siphons lengths are up to 8.3 mm. The growth rate of the largest specimen extracted would have been 0.011mm per day,or 0.33mm per month, assuming it settled on the first day the wood was available at depth.

Figures

FIG. 1. — A, collection locality of Xylophaga alexisi n. sp. at 21°N, 30°W; R, collection locality of Xylophaga ricei Harvey, 1996 at 31°N, 20°W. (From Ryan et al. 2009, modified by Y. Lagabrielle.)

FIG. 2. — A, lateral view of an intact specimen of Xylophaga alexisi n. sp.; B, dorsal view of intact, larger specimen of X. alexisi n. sp. (note anteriorly directed mesoplax); C, dorsal view of smaller specimen of X. alexisi n. sp., mesoplax in erect stage, not readily seen from dorsal view. Scale bars: 1 mm.

FIG. 3.— SEM image of the umboregion of a specimen of Xylophaga alexisi n. sp.: M, mesoplax; PD II, prodissoconch II. Inferior to the edge of the prodissoconch, note the shell records the growth series during which it developed the characteristic wood-boring shape. Also note the ventral elaboration and ventral reflection of the toothed ridges on the dorsal beak. Scale bar: 50 µm.

FIG. 4. — Excurrent siphonal opening of Xylophaga alexisi n. sp., SEM: A, lateral view; B, dorsal view. Scale bars: A, 10 µm; B, 100 µm.

FIG. 5. — Incurrent siphonal opening of Xylophaga alexisi n. sp., SEM. Note absence of cirri. Scale bar: 30 µm.

FIG. 6. — A, Xylophaga alexisi n. sp., lateral view to define “incomplete siphon” in which the excurrent siphon is considerably shorter than the incurrent siphon;B, Xylophaga pacifica Voight,2009,lateral view to define “complete siphon”,in which both siphonal openings are roughly equal in length. Scale bars: 1 mm.