The occurrence of Natsushima bifurcata ( Polychaeta : Nautiliniellidae ) in Acharax hosts from mud volcanoes in the Gulf of Cadiz ( south Iberian and north Moroccan Margins )

Miura and Laubier (1990) used the names Nautiliniellidae and Nautiliniella to replace the homonyms Nautilinidae and Nautilina (Miura and Laubier, 1989), because the genus name had already been occupied by a cephalopod mollusc. At present, this small family of uncommon polychaetes includes fifteen species ascribed to eleven genera found in the mantle cavity of deep-sea bivalve molluscs of the families Solemyidae, Mytilidae, Thyasiridae and Vesicomyidae (Miura and Hashimoto, 1996; Dreyer et al., 2004). Nautiliniellids are mainly restricted to Pacific hydrothermal vents and cold seeps (Blake, 1993; Miura and Hashimoto, 1993, 1996; Miura and Laubier, 1989, 1990; Miura and Ohta, 1991). Up to now, only two species, Petrecca thyasira Blake, 1990, found in the gill filaments of Thyasira insignis SCIENTIA MARINA 71(1) March 2007, 95-100, Barcelona (Spain) ISSN: 0214-8358

specimens from the Laurentian Fan, and Vesicomyicola trifurcatus Dreyer, Miura and Van Dover, 2004, found in the mantle cavity of the vesicomyid clams from the Blake Ridge cold seep off the coast of South Carolina, are known to occur in the Atlantic Ocean (Blake, 1990;Dreyer et al., 2004).Nautiliniellids are commonly quoted as commensals or parasites (Blake, 1990;Miura and Laubier, 1990) but the nature of the association between the polychaetes and their bivalve hosts is still unclear.
Communities of benthic animals associated with cold seeps are known from several locations on active and passive continental margins of the Pacific and have recently been discovered also in the Atlantic Ocean (Olu-Le Roy et al., 2004).The occurrence of mud volcanism, cold seepage, hydrocarbon venting and gas hydrates in the Gulf of Cadiz has been intensively investigated since 1996 (e.g.Pinheiro et al., 2003;Van Rensbergen et al., 2005).Within the framework of the UNESCO/IOC Training Through Research Programme, seven research cruises were conducted for this specific purpose: TTR9 (1999), TTR10 (2000), TTR11 (2001), TTR12 (2002), TTR14 (2004), TTR15 (2005) and TTR16 (2006).
The solemyid bivalve Acharax sp. is one of the most common species of the chemoauthosynthesisbased assemblage of the mud volcanoes from the Gulf of Cadiz (Rodrigues and Cunha, 2005).In three of these mud volcanoes some of the bivalve specimens were found to host in their mantle cavity nautiliniellid polychaetes identified as Natsushima bifurcata Miura and Laubier (1990), previously known only from Sagami Bay (Japan).These recently collected specimens verify the original concept of the species and provide the first record of N. bifurcata for the North Atlantic.

Study site
The Gulf of Cadiz is located at the crossroads of the European and African Atlantic margins and the Mediterranean.The compression between the Eurasian and African tectonic plates creates an interesting geophysical template shaped by volcanic activity and by the interaction between the topography and the circulation of the Atlantic and Mediterranean Ocean Waters.Geologically, the set-ting of the Gulf of Cadiz is extremely complex and still under debate but one of the most important structures is a large olistostrome complex emplaced in an accretionary wedge-type environment.(Sartori et al., 1994;Maldonado et al., 1999;Gutscher et al., 2002).Since the discovery of the first mud volcano in 1999, about 30 other sites at depths ranging from 200 to 4000 m, with varying degrees of hydrocarbon-rich gas seepage activity, have been located and sampled (Pinheiro et al., 2003;Van Rensbergen et al., 2005).

Collection of samples
Samples were collected during the TTR cruises (Training Through Research Programme, IOC-UNESCO) onboard the RV Prof. Logachev.A TVassisted grab was used to locate interesting sampling sites in the target mud volcanoes.Whenever Acharax specimens were collected, usually from sites within the crater of active mud volcanoes, they were opened and examined onboard for the presence of nautiliniellid polychaetes.The biological material was preserved in 70 or 96% ethanol (the latter will be used for future molecular analysis).
Diagnosis.Body vermiform, flattened ventrally and arched dorsally, with fairly uniform width throughout.Short prostomium, much wider than long, slightly incised anteriorly, with only a pair of lateral antennae and without eyes.Tentacular segment partially fused to prostomium, with dorsal and ventral cirri, neuroaciculae, and few neurosetae.Parapodia sub-biramous, with well-developed dorsal and ventral cirri.Notopodia short conical, similar throughout the body, with slender notoacicula and without setae.Neuropodia cylindrical, with neuroacicula and numerous setae of two kinds -up to 4 (usually 3) stout slightly curved hooks placed more dorsally and more than 100 smaller bifurcate setae placed ventrally.
Supplementary description.Largest specimens measure 17 mm in length and ~3 mm in width, for 72 setigers.Colour in life pink, and after preservation white.
Remarks.The original description of the holotype mentions that the specimens were found inside Solemya sp.bivalves (Miura and Laubier, 1989: 322) but the identification of the bivalve was corrected to Acharax johnsoni in a subsequent work by Ohta (1990, cited in Miura andHashimoto, 1996: 266).
The specimens from the Gulf of Cadiz are larger than the ones from the type locality (Table 1) and in some neuropodia (Fig. 2) there are 4 hooks (up to 3 in the original description).The diagnosis of the species was updated accordingly.
Ecology.The host bivalve, Acharax sp. has a wide distribution in the Gulf of Cadiz: it has been recorded from ten mud volcanoes in the Moroccan and Portuguese margins at depths varying from 358 to 3902 m.The nautiliniellid polychaetes were found in the mantle cavity, near the gill filaments, of Acharax specimens collected from the Jesús Baraza, Yuma and Ginsburg mud volcanoes, all located in the Western Moroccan field within a bathymetric range restricted to 920 and 1105 m (Fig. 3).At these three mud volcanoes, observed infestation rates varied from 12.5 to 75% and the number of nautiliniellid individuals found in each bivalve varied from 1 to 3 (Table 2).The infested Acharax specimens measured between 3.2 and 6.9 cm (total length).Miura and Laubier (1990).
iellids (up to 75% infestation rate).The majority (69.8%) of the 96 Acharax specimens were collected at the four shallowest mud volcanoes (358-701 m) but none of these specimens were infested.The populations from the three deepest mud volcanoes (1115-3902 m) were also not infested but the number of specimens collected was much lower (only 4.2% of the total).According to Neulinger et al. (2006), Acharax species differentiation based on shell morphology is likely to underestimate true species diversity within this taxon.A phylogenetic study based on some Acharax specimens from localities of the Pacific and Indian oceans (Aleutian Trench, off Oregon, Costa Rica, and Peru margins, and off Makran, and Java) resulted in two clusters that group populations located far apart (Makran, Oregon, and Peru in one cluster and Java, Aleutian Trench and Costa Rica in the other).These authors propose that the specimens are representatives of at least 2 different species and do not all belong to A. johnsoni, as assumed previously.
These results emphasize the need for further morphological and probably also genetic studies to solve taxonomic affinities within Acharax populations.Until then, we cannot assume that the Gulf of Cadiz and Sagami Bay specimens belong to the same species but nor can we discard this possibility.The same applies to the Natsushima specimens.We could not find morphological evidence to support the establishment of a new species.The comparison with the holotype only revealed differences in the proportions of the specimens (Fig. 2; Table 1).The largest specimens from the Gulf of Cadiz are 2 to 3 times larger than the holotype, the setigers are broader and the parapodia, including cirri, are thicker.The holotype seems to have proportionally longer and thinner dorsal cirri and antennae, and it is  also smaller.This different appearance is due to the more corpulent body of the larger specimens, resulting in proportionally broader and shorter cirri.In support of this, medium-sized specimens, such as the DBUA 00765 from the Gulf of Cadiz, and the USNM 172135 from Sagami Bay, show "intermediate" morphological features between the larger, more corpulent specimens and the holotype.It is possible that N. bifurcata from Sagami Bay and the specimens from Gulf of Cadiz are cryptic species but this question can only be answered with more knowledge on reproductive patterns and by the analysis of DNA sequences.As the preservation of the Japanese specimens does not allow DNA analysis, we must consider that both the Cadiz and the Sagami specimens belong to the same morphological species.
There are several cases of specimens from distant locations of other deep sea species that are ascribed to the same morphological species.An example is the commensal polychaete Branchipolynoe seepensis Pettibone, 1986 that was recorded in several bivalve host species both in the Gulf of Mexico and in the mid-Atlantic Ridge.In this case, subsequent studies carried out by Chevaldonné et al. (1998) determined the genetic divergence for the COI and 16S rDNA genes and showed that the Atlantic and Pacific populations should be considered as two isolated phylogenetic species.
Distribution.East Pacific, Sagami Bay: Hatsushima and Okino-yama cold seeps, 1114-1170 m, inside the mantle cavity of Acharax johnsoni Dall, 1891.North Atlantic, Gulf of Cadiz: Jesús Baraza, Yuma and Ginsburg mud volcanoes, 920-1105 m, inside the mantle cavity of an undetermined Acharax species.DISCUSSION The genus Natsushima comprises at present two described species, N. bifurcata Miura and Laubier, 1990 and N. graciliceps Miura and Hashimoto, 1996.The two species differ by the presence of respectively short conical and elongated notopodia in the middle segments, by the presence of a fine embedded notoacicula in the former and by the different size of dorsal and ventral cirri, which are smaller in the latter.Both Sagami Bay specimens of N. bifurcata occur in cold seeps associated with Solemyid bivalves at depths of around 1000 m, and the specimens recently collected in the Gulf of Cadiz corroborate this information.In fact, it is noteworthy that despite the wider bathymetric range of Acharax in the Gulf of Cadiz (358-3902 m), up to now only the populations from the mud volcanoes of the Western Moroccan field, located at depths of 920-1105 m, were found to be infested by nautilin-SCI.MAR., 71(1), March 2007, 95-100.ISSN: 0214-8358 98 • A. RAVARA et al.
FIG. 3. -Map of the study area: full triangles -sampled sites; empty triangles -other known mud volcanoes; full circles -sites with Acharax populations; empty circles -sites with infested Acharax populations.JB, Y and G -Jesús Baraza, Yuma and Ginsburg mud volcanoes.

TABLE 2 .
-Infestation rates; number of specimens of Natsushima bifurcata and Acharax sp.collected in each mud volcano (Number of N. bifurcata in each host Acharax in brackets).