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Biodiversity and phylogeography of Northeast Atlantic and Mediterranean
sponges
Bogalho Teixeira Xavier, J.R.
Publication date
2009
Link to publication
Citation for published version (APA):
Bogalho Teixeira Xavier, J. R. (2009). Biodiversity and phylogeography of Northeast Atlantic
and Mediterranean sponges.
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CHAPTER 3
D
EMOSPONGEFAUNAOFO
RMONDEANDG
ETTYSBURGS
EAMOUNTS(G
ORRINGEB
ANK, N
ORTHEASTA
TLANTIC):
DIVERSITYANDZOOGEOGRAPHICALAFFINITIES
Joana R. Xavier & Rob W.M. van Soest
Journal of the Marine Biological Association of the United Kingdom (2007), Vol.
87, 1643–1653
ABSTRACT
Seamounts, although abundant features of the world’s oceans, constitute one of the least studied marine ecosystems. In the present work we assessed the diversity and zoogeographical affinities of the demosponge assemblages of Gettysburg and Ormonde Seamounts (Gorringe Bank, NE Atlantic). Twenty three demosponge species were identified adding up to the thirteen previously reported for Gorringe shallow-water. Gorringe’s demosponge assemblage was found to be mainly composed of species with a wide Atlanto-Mediterranean distribution (61%) and a group of species (28%) that are endemic to this Bank or have a restricted geographical distribution. This high level of endemism suggests the Gorringe Bank as a hotspot for demosponge fauna on the NE Atlantic. Gorringe demosponge fauna was found to present an overall moderate similarity to the Mediterranean, Iberian coasts and Macaronesian archipelagos. We compare our findings with patterns reported for fish and molluscan faunas for this Bank, and discuss some evolutionary aspects on the role of these ecosystems for the Northeast Atlantic marine biota.
INTRODUCTION
Seamounts are undersea mountains that steeply rise from the seabed with an elevation of more than 1000 m but not emerging above the surface usually associated with volcanic activity (Epp & Smoot, 1989; Rogers, 1994). Although abundant features in the world’s oceans - there are an estimated 14.000 seamounts worldwide (Kitchingman & Lai, 2004) - these constitute one of the least studied marine ecosystems.
Seamount ecosystems harbour rich communities of benthic filter-feeding organisms, such as corals, sponges, anemones, ascidians, as well as aggregations of commercially important fish species in relatively high abundances, which has lead to descriptions of seamounts as “underwater oases” (Rogers, 1994; Stocks, 2004). However, due to the intrinsic biological factors characteristic of seamount species (e.g. slow growth rates, late maturation), these communities are extremely vulnerable to human activities (e.g. trawling) and require urgent conservation measures (Rogers, 1994; Probert, 1999; Morato et al., 2006; Clark et al., 2006).
As large topographic features, seamounts are known to interact with the main current systems causing special meso- to large-scale effects, such as current deflections, local upwelling, eddies and Taylor caps generation, which have major biological implications (see White & Mohn, 2004 for a review). Some of these effects are: (1) the enhancement of primary production over seamounts and therefore an increased energy availability for higher benthic and pelagic trophic levels; (2) the retention of planktonic stages (such as larvae) in the immediate vicinity of the seamounts (Mullineaux & Mills, 1997; Parker & Tunnicliffe, 1994) inducing high levels of endemism (e.g. Lévi, 1969; Wilson et al., 1985; Borets, 1986; de Forges et al., 2000). However, because seamounts can occur in clusters or chains, they might also act as bridges for long-distance oceanic dispersal and colonization following a stepping–stone model (Hubbs, 1959; Wilson & Kauffman, 1987). Combined, these effects have major implications both on the diversity and biogeographical affinities of seamounts and oceanic islands’ marine biota (Boehlert & Genin, 1987).
The North Atlantic comprises more than 800 large seamounts (Epp & Smoot, 1989), most of which are located in the proximity of the Mid Atlantic
Ridge. Due to its geographical, geological, geomorphologic and oceanographic
context, the Gorringe Bank constitutes an excellent system to study evolutionary aspects of the North-east Atlantic marine biota such as historical biogeography
CHAPTER 3
and speciation because: (1) it is part of the Horseshoe seamount chain that stretches between the SW coast of Portugal and Madeira archipelago; (2) it is a shallow seamount with summits at approx. 28-46 m depth (de Alteriis et al., 2003) enabling direct sampling, and episodically constituted an island during the glacio -eustatic sea-level fluctuations of the Late Pleistocene (Pastouret et al., 1980); (3) it is influenced by different current systems such as the Azores current, Portugal current and the Mediterranean outflow; (4) it has a fossil record dating back to the Mesozoic (145-155 My) suggesting the Gorringe Bank was a seamount at the early opening of the Atlantic ocean (Conti et al., 2004).
Although it is considered an important seamount that urgently requires investigation (WWF, 2001) few studies have so far focused on the fauna of the Gorringe Bank (e.g. Ramil et al., 1998; Ávila & Malaquias, 2003; Dijkstra & Gofas, 2004; Gonçalves et al., 2004).
Sponges constitute a dominant invertebrate group in hard-bottom benthic communities (Sarà & Vacelet, 1973), and this is no exception for seamount ecosystems (Stocks, 2004; Brenke, 2002). Previous to the present study, 23 demosponge species had been reported from the Gorringe Bank as result of broader scale expeditions. Topsent (1928) reported and described 15 demosponge species mostly from Gorringe’s deep-water, and Lévi & Vacelet (1958) reported eight species from above 100 m depth for the same area.
The aim of this work was to study the demosponge assemblages of the Gorringe Bank and evaluate the role of this seamount in the long-distance dispersal and colonization of oceanic islands of the Macaronesian archipelagos. To address these topics we assessed Gorringe’s demosponge diversity and inferred its zoogeographical affinities.
MATERIAL AND METHODS
Study area
Approximately 270 km off Cabo S. Vicente (SW coast of Portugal) the Gorringe Bank is a large seamount - 200 km long and 60 km wide - located at the eastern tip of the Azores-Gibraltar plate boundary separating Eurasia and Africa (Girardeau et al., 1998). It rises from the seabed at nearly 5,000 m depth to peaks of approximately 20-28 m depth (Gettysburg peak), and 33-46 m depth (Ormonde peak), separated by an 800 m depth saddle. It is enclosed to the North and South by the Tagus and Horseshoe Abyssal plains, respectively (Figure 1). According to
WWF’s report “This site is important as a large seamount in the NE Atlantic and part of
the Horseshoe Seamounts range. The biology of this site urgently requires investigation using modern survey techniques” (WWF, 2001).
Figure 1. Location of the Gorringe Bank in the north-east Atlantic. Insert: detail of the
bathymetric contour of Gettysburg and Ormonde Peaks. Bathymetric data from GEBCO Digital Atlas (2003). AP, Abyssal Plain; G, Gettysburg Seamount; O, Ormonde Seamount.
Sampling
The Gorringe seamount fauna and flora has been surveyed during LusoExpedição 2006, a scientific expedition organized by the Universidade Lusófona (Portugal), on board the N.T.M. Creoula. A total of eight scuba dives were performed, of which six at Gettysburg (Latitude 36º 31’ 10” N; Longitude 11º 34’ 10” W) and two at Ormonde (Latitude 36º 42’ 70” N; Longitude 11º 09’ 70” W) peaks. Given the oceanic character of the Gorringe Bank, its depth and potentially strong currents, two support cables (attached to surface buoys) were anchored at each of the peaks. The sampling was performed within close (visual) distance of the cables, in order to ensure diver safety. Collected material was sorted on board
CHAPTER 3
and preserved in 96% ethanol for taxonomic and molecular studies. All collected material is deposited at the Porifera collection of the Zoological Museum of Amsterdam.
Zoogeographical analyses
In order to study the zoogeographical affinities of the Gorringe demosponge fauna we compared it with that of other areas across the North-east Atlantic and Mediterranean (including the Macaronesian archipelagos).
Given that shallow and deep-water sponge assemblages are different in species composition, we only took into account for this comparison species inhabiting the sublittoral down to a maximum of 120 m depth. Therefore we extracted from the literature presence/absence data of the species for adjacent regions such as: British Isles (BRI) – Picton et al. (1997); Bay of Biscay (BIS) – Solorzano (1991), Preciado (2002); Coast of Portugal (PTC) - Lopes & Boury-Esnault (1981), Lopes (1989), Naveiro (2002), J. Xavier (unpublished data); Azores (AZO) – Topsent (1892, 1904), Boury-Esnault & Lopes (1985), de Weerdt & van Soest (1986); Madeira (MAD) – Topsent (1928), Lopes (1995), R. Pestana (unpublished data); Canaries (CAN) – Cruz (1980, 1984, 2002), Cruz & Bacallado (1983, 1985); Cape Verde (CAP) – van Soest (1993); Gibraltar (GIB) – Carballo (1994), Boury-Esnault et al. (1994), Carballo et al. (1996). Some studies with a larger geographical coverage were also used for this purpose (Topsent, 1928; van Soest et al., 2000). For the Mediterranean areas we followed the review made by Pansini & Longo (2003) and divided the Mediterranean locations into western (wMED - Alboran Sea, South eastern coasts of Spain, France and the Ligurian Sea and Algerian coast), central (cMED - Tyrrhenian, Adriatic, Ionian Seas and Tunisia, Malta, SW Sicily) and eastern Mediterranean (eMED - Aegean Sea, Levant Basin, Cyprus and Egyptian coast). This work was complemented with some later studies (e.g. Voultsiadou, 2005).
Many comparative methods (such as PAE, biogeographical indexes, etc) are available to infer affinities between areas based on taxon distributions (Crisci et al., 2003). However, given the scarcity of data available for some locations (such as Madeira) and the prospective character of this study (the Gorringe fauna studies are in their infancy), we chose a method that, although simple, allows an overview of the main affinities of the species so far reported for this Bank. Thus, in order to assess the zoogeographical affinities of Gorringe’s demosponges we made a comparison of % of similarity between locations by calculating the ratio of
shared species between any location and the Gorringe and the total number of species recorded for the Gorringe Bank.
Previous to the present study 23 demosponge species had been reported by Lévi & Vacelet (1958) and Topsent (1928) for Gorringe, but of these, nine correspond to deep-water species and are therefore not included in the biogeographical analysis.
RESULTS
Systematics and distribution
A total of 86 specimens were collected during LusoExpedição 2006. These were identified and assigned to 23 species that constitute new records for the area, adding up to the 13 species previously reported for shallow-water at the Gorringe Bank. Several specimens were identified only to the genus level. These may constitute rare or even undescribed species and will therefore require further investigation. Of the identified species, 17 were found at the Gettysburg and 12 at the Ormonde (Table 1). Only 6 taxa (Erylus sp., Aaptos aaptos, Tethya aurantium,
Thymosia guernei, Batzella sp., and Myxilla rosacea) were found to be shared between the two seamounts. Only one of the species previously reported for the Gorringe was collected in this expedition - Stelletta lactea.
We present here an annotated checklist of all (shallow and deep-water) demosponge fauna reported to date for the Gorringe Bank.
ANNOTATED CHECKLIST OF THE DEMOSPONGE FAUNA OF THE
GORRINGE BANK
Species marked with an * were found in the present study and d correspond to
deep-water records.
Order ASTROPHORIDA Sollas, 1888 Family ANCORINIDAE Schmidt, 1870
Genus Stelletta Schmidt, 1862
Stelletta lactea Carter, 1871*
Topsent (1928: p. 124) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen.
CHAPTER 3
Genus Ancorina Schmidt, 1862
Ancorina sp.*
Present study - Gettysburg Peak, 31-42 m depth, three specimens; Ormonde Peak, 38-40 m depth, one specimen. Massive globose to cup shaped sponges, bright yellow in colour, with a conspicuous cortex. Only two Ancorina species are reported for the North-east Atlantic and Mediterranean areas – A. cerebrum Schmidt, 1862 and A. radix Marenzeller, 1889– and our specimens clearly differ from these in external morphology (colour, habit, consistency) and skeletal composition (orthotriaenes, oxeas, sanidasters, and oxyasters).
Genus Stryphnus Sollas, 1886
Stryphnus mucronatus (Schmidt, 1868)*
Present study - Gettysburg Peak, 36-42 m depth. One small globose specimen, black outside and grey inside. Skeleton composed of oxeas, dichotriaenes, oxyasters and amphiasters. This constitutes the first record of the species for the Atlantic.
Family GEODIIDAE Gray, 1867 Genus Geodia Lamarck, 1815
Geodia geodina (Schmidt, 1868)
Lévi & Vacelet (1958: p. 226, fig. 3) – station 149, East Gettysburg, 95 m. Genus Erylus Gray, 1867
Erylus euastrum (Schmidt, 1868)
Lévi & Vacelet (1958: p. 228, fig. 4 as Erylus stellifer) – station 149, East Gettysburg, 95 m.
Erylus sp.*
Present study - Gettysburg Peak, 31-42 m depth, several massive specimens; Ormonde Peak, 38-40 m depth, two specimens. Massive globose, big sized specimens (up to 10 cm thick x 30 cm long). Dark brown outside and beige inside with a conspicuous cortex. It has all the characteristics of an Erylus but presents deformed triaenes and no aspidasters were observed. This may constitute a new (and endemic) species but further investigation is required.
Family PACHASTRELLIDAE Carter, 1875 Genus Dercitus Gray, 1867
Dercitus bucklandi (Bowerbank, 1858)*
Present study - Gettysburg Peak, 31-38 m depth, one thickly encrusting specimen on a crevice. Black on the outside and dark grey inside with a firm consistency. It has a confused skeleton composed of calthropses, microxeas and toxas, and many pigmented cells. It conforms in all respects to Dercitus bucklandi and this constitutes the southernmost record of this Atlantic species.
Genus Pachastrella Schmidt, 1868
Pachastrella monilifera Schmidt, 1868 Lévi & Vacelet (1958: p. 231) – station 149, East Gettysburg, 95 m.
Genus Stoeba Sollas, 1888
Stoeba plicata (Schmidt, 1868)
Lévi & Vacelet (1958: p. 231, figs. 10, 11 as Dercitus plicatus) – station 149, East Gettysburg, 95 m.
Order HADROMERIDA Topsent, 1894 Family HEMIASTERELLIDAE Lendenfeld, 1889
Genus Stelligera Gray, 1867
Stelligera mutilus (Topsent, 1928)
Topsent (1928: p. 188; Pl. VI fig. 20 as Vibulinus mutilus) – 1904 campaign, station 1664 Gorringe Bank, 116 m. This species has been originally described from Gorringe specimens.
Family SUBERITIDAE Schmidt, 1870 Genus Aaptos Gray, 1867
Aaptos aaptos (Schmidt, 1864)*
Present study - Gettysburg Peak, 36-42 m depth, one massive specimen; Ormonde Peak, 38-40 m depth, one specimen.
Genus Terpios Duchassaing & Michelotti, 1864
Terpios gelatinosa (Bowerbank, 1866)*
Present study - Gettysburg Peak, 31-38 m depth, one encrusting specimen on a boulder.
CHAPTER 3
Family TETHYIDAE Gray, 1848 Genus Tethya Lamarck, 1814
Tethya aurantium (Pallas, 1766)*
Present study - Gettysburg Peak, 31-42 m depth, two specimens; Ormonde Peak, 38-40 m depth, one specimen.
Order CHONDROSIDA Boury-Esnault & Lopes, 1985 Family CHONDRILLIDAE Gray, 1872
Genus Thymosia Topsent, 1895
Thymosia guernei Topsent, 1895*
Present study - Gettysburg Peak, 31-42 m depth, four specimens; Ormonde Peak, 38-40 m depth, one encrusting specimen. This is a white dense sponge, with a firm consistency. The skeleton is composed of nodular spongin fibres. This constitutes the southernmost record of this Atlantic species.
Order POECILOSCLERIDA Topsent, 1928 Family ACARNIDAE Dendy, 1922
Genus Cornulum Carter, 1876
Cornulum cheliradians (Topsent, 1927)
Topsent (1928: p. 227; Pl. VII, figs. 21, 22 as Cornulotrocha cheliradians) – 1904 campaign, station 1664 Gorringe Bank, 116 m, One specimen encrusting a stone.
Genus Damiria Keller, 1891
Damiria testis Topsent, 1928d
Topsent (1928: p. 325; Pl. X fig. 28) – 1904 campaign, station 1898 Gorringe Bank, 270 m. One specimen encrusting Hamacantha johnsoni. This species has been originally described from Gorringe material, and has not been reported reliably elsewhere (but see Van Soest et al., 1994) thus constituting a deep-water endemic.
Family MICROCIONIDAE Carter, 1875 Genus Antho Gray, 1867
Antho (Acarnia) signata (Topsent, 1904) d
Topsent (1928: p. 306 as Plocamiopsis signata) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen encrusting a stone.
Antho (Antho) involvens (Schmidt, 1864)*
Present study - Ormonde Peak, 38-40 m depth. One thinly encrusting specimen on a boulder.
Family RASPAILIIDAE Hentschel, 1923 Genus Eurypon Gray, 1867
Eurypon hispidulum (Topsent, 1904) d
Topsent (1928: p. 291; Pl. X figs. 12, 13 as Acantheurypon hispidulum) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen encrusting a stone.
Eurypon pilosella (Topsent, 1904) d
Topsent (1928: p. 291 as Acantheurypon pilosella) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen encrusting a stone. This species was originally described from Azorean deep-water specimens (Topsent, 1904 as
Hymeraphia pilosella).
Eurypon lacazei (Topsent, 1891)
Topsent (1928: p. 290; Pl. II, figs. 17d, Pl. VIII fig. 13) – 1904 campaign, station 1664 Gorringe Bank, 116 m. One specimen encrusting a stone.
Family CRELIIDAE Dendy, 1922 Genus Crella Gray, 1867
Crella (Yvesia) ridleyi Topsent, 1892 d
Topsent (1928: p. 229; Pl. VIII, fig. 3 as Grayella ridleyi) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen encrusting a stone.
Family HYMEDESMIIDAE Topsent, 1928 Genus Hymedesmia Bowerbank, 1864
Hymedesmia (Hymedesmia) flaccida Topsent, 1928 d
Topsent (1928: p. 267; Pl. IX, fig. 8 as Hymedesmia (Holorodesmia) flaccida) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen encrusting a stone. This species was originally described from Gorringe material and has been reported from Rockall Bank by van Soest & Lavaleye (2005).
Hymedesmia (Hymedesmia) occulta Bowerbank in Norman, 1869 d
Topsent (1928: p. 250 as Hymedesmia occulta) – 1912 campaign, station 3182 Gorringe Bank, 780 m. One specimen.
CHAPTER 3
Hymedesmia (Hymedesmia) pansa Bowerbank, 1882*
Present study - Gettysburg Peak, 36-42 m depth. One brownish yellow thinly encrusting specimen.
Hymedesmia (Hymedesmia) peachi Bowerbank, 1882*
Present study - Gettysburg Peak, 31-38 m depth. One orange specimen encrusting hydrozoans.
Hymedesmia (Stylopus) coriacea (Fristedt, 1885)*
Present study - Ormonde Peak, 38-40 m depth. Two greyish-white thinly encrusting specimens.
Family CHONDROPSIDAE Carter, 1886 Genus Batzella Topsent, 1893
Batzella sp.*
Present study - Gettysburg Peak, 31-38 m depth, one encrusting specimen; Ormonde Peak, 38-40 m depth, one encrusting specimen. Orange thinly encrusting specimens with a soft texture. Skeleton composed of anisostrongyles of one size category. Our specimens are similar to another collected in the Cape Verde islands, during the CANCAP expeditions (van Soest, 1993). The assignment of these specimens to a particular species will require further investigation.
Family MYXILLIDAE Dendy, 1922 Genus Myxilla Schmidt, 1862
Myxilla (Myxilla) rosacea (Lieberkϋhn, 1859)*
Present study - Gettysburg Peak, 31-38 m depth, four specimens; Ormonde Peak, 38-40 m depth, three specimens.
Family DESMACELLIDAE Ridley & Dendy, 1886 Genus Desmacella Schmidt, 1870
Desmacella informis (Stephens, 1916)
Lévi & Vacelet (1958: p. 235, fig. 15 as Tylodesma informis) – station 149, East Gettysburg, 95 m.
Family HAMACANTHIDAE Gray, 1872 Genus Hamacantha Gray, 1867
Hamacantha (Hamacantha) johnsoni (Bowerbank, 1864) d
Topsent (1928: p. 198) – 1904 campaign, station 1898 Gorringe Bank, 270 m. One specimen.
Family PODOSPONGIIDAE De Laubenfels, 1936 Genus Podospongia du Bocage, 1869
Podospongia loveni du Bocage, 1869
Topsent (1928: p. 219; Pl. II fig. 17b; Pl. VII fig. 17) – 1904 campaign, station 1664 Gorringe Bank, 116 m. Three specimens.
Order HALICHONDRIDA Gray, 1867 Family AXINELLIDAE Carter, 1875
Genus Axinella Schmidt, 1862
Axinella polypoides Schmidt, 1862
Lévi & Vacelet (1958: p. 233) – station 149, East Gettysburg, 95 m.
Axinella vasonuda Topsent, 1904
Topsent (1928: p. 174; Pl. II fig. 17a) – 1904 campaign, station 1664 Gorringe Bank, 116 m. One specimen encrusting a rock.
Axinella damicornis (Esper, 1794)*
Present study - Ormonde Peak, 38-40 m depth. Three erect specimens. Family DICTYONELLIDAE van Soest, Diaz & Pomponi, 1990
Genus Dictyonella Schmidt, 1868
Dictyonella obtusa (Schmidt, 1862)
Lévi & Vacelet (1958: p. 238, fig. 19 as Dictyonella plicata) – station 149, East Gettysburg, 95 m.
Dictyonella incisa (Schmidt, 1880)*
Present study - Ormonde Peak, 38-40 m depth, one thickly encrusting specimen. Family HALICHONDRIIDAE Gray, 1867
Genus Ciocalypta Bowerbank, 1862
Ciocalypta aderma (Lévi & Vacelet, 1958)
Lévi & Vacelet (1958: p. 236, fig. 18 as Coelocalypta aderma) – station 149, East Gettysburg, 95 m. Originally described from Gorringe material, this species has not been reported elsewhere and thus constitutes a Gorringe endemic.
CHAPTER 3
Genus Ciocalapata De Laubenfels, 1936
Ciocalapata almae Carballo, Uriz & Garcia-Gomez, 1996*
Present study - Ormonde Peak, 38-40 m depth. One thinly encrusting yellow round patch on a boulder, with an easily detachable surface. This specimen presents a tangential skeleton composed of styles and oxeas (both slightly curved). Our specimen perfectly fits the description of Carballo et al. (1996) for
Ciocalapata almae, from the Strait of Gibraltar (Algeciras Bay), type (and only) locality for which the species has been recorded. This constitutes the second record of the species and the first record for the Atlantic.
Genus Spongosorites Topsent, 1896
Spongosorites placenta Topsent, 1896 d
Topsent (1928: p. 157; Pl. III fig. 10) – 1904 campaign, station 1898 Gorringe Bank, 270 m. One small specimen on Hamacantha johnsoni.
Order HAPLOSCLERIDA Topsent, 1928 Family CHALINIDAE Gray, 1867
Genus Haliclona Grant, 1836
Haliclona (Haliclona) simulans (Johnston, 1842)*
Present study - Gettysburg Peak, 31-42 m depth. Three repent specimens.
Haliclona (Soestella) mucosa (Griessinger, 1971)*
Present study - Gettysburg Peak, 36-42 m depth. One thickly encrusting specimen, cream yellow and with mucous in life (brownish in spirit). Its skeleton is composed of paucispicular lines of oxeas. Our specimen perfectly fits the description of Griessinger (1971) of Haliclona mucosa, constituting the second record of this species in the Atlantic (previously reported from the Canaries).
Order DICTYOCERATIDA Minchin, 1900 Family DYSIDEIDAE Gray, 1867
Genus Dysidea Johnston, 1842
Dysidea sp.*
Present study - Ormonde Peak, 38-40 m depth. One globose encrusting specimen, colour a mottled purplish blue. A similar specimen collected on the southern coast of Portugal by Gonçalo Calado has been examined, and it matches our
material. The external morphology resembles the photo in Cruz (2002, p. 211) identified as D. fragilis. However the colour and conules (0.5 mm high and less then 0.5 mm spaced) differ from this species. The only other characteristically bluish Dysidea are D. tupha (reported for the Mediterranean) and D. etheria (Caribbean Sea, and Cape Verde) but our specimen does not match those descriptions. Another Mediterranean bluish-grey species is D. perfistulata (Pulitzer-Finali & Pronzato, 1980) but this species clearly differs in habit from our specimen.
Order VERONGIDA Bergquist, 1978 Family IANTHELLIDAE Hyatt, 1875
Genus Hexadella Topsent, 1896
Hexadella racovitzai Topsent, 1896*
Present study - Gettysburg Peak, 31-42 m depth, three encrusting specimens.
Hexadella pruvoti Topsent, 1896*
Present study - Gettysburg Peak, 36-42 m depth, four encrusting specimens. Bright yellow in life turning dark purple in spirit. No spicules or spongin fibres.
Hexadella pruvoti has been described from shallow-water in the western Mediterranean (Cap d’Abeille) and this constitutes the first record of the species in the Atlantic.
Zoogeographical affinities
The Gorringe demosponge fauna showed an overall moderate similarity (mostly around 50% of shared species) to the adjacent locations included in this study, with highest affinity to Mediterranean and Iberian regions. Its fauna was found to be most similar to the western and central Mediterranean (64% and 58% shared species) followed by a group composed by the Straits of Gibraltar and the Iberian coasts with 50% of shared species. The locations with which Gorringe revealed the lowest similarity were Madeira and Azores archipelagos with 28% and 42% of species overlap (Table 1).
CHAPTER 3 S P ECI ES GO RRI NG E NE ATL A N T IC AN D ME DI TER R A N EAN Aff in ity Get Or m BRI BI S PT C AZO MAD CAN CAP GI B wME D cMED eMED St elle tt a lac te a Carter, 1871* AM + + + + + + Ancorin a sp .* pE + St ryph nus m u cr on at us ( S chmidt, 1868) * AM + + + + Geod ia geo d in a (S chmid t, 1868) AM ? ? + Er yl us e uas tr u m (S chmi d t, 1868) AM ? ? + + + + + + Er yl us sp .* pE + + Der ci tu s bu ck la n d i ( B ow er bank, 1858) * A + + + + P a chas tr ell a mo ni life ra S chmi d t, 1868 AM ? ? + + + + + + + + + St oe ba p lic at a (S chmidt, 1868 ) AM ? ? + + + + + + + + St ellige ra mut ilus (T ops ent, 1928) R ? ? + Aapt o s a apt os (S chmi d t, 1864) * AM + + + + + + + + + + + T erp io s g el a tin o sa ( B ow er bank, 1866 )* AM + + + + + + + + + + T eth ya a u ran ti um (Pallas , 1766) * AM + + + + + + + + + + + + Th ym os ia g u er ne i T o ps ent, 1895 * A + + + + + + + C o rnul um c h elir a d ians (T ops ent, 1927) E ? ? Ant ho (A nt ho ) in vo lve n s (S chmi d t, 186 4) * AM + + + + + + + + + + + + Eur y po n lac a ze i (T ops ent, 1891) AM ? ? + + + + + + + Hyme de sm ia (H yme d es m ia ) pans a B o w erbank, 1882 * AM + + + + + + + + + + Hyme de sm ia (H yme d es m ia ) pe ac h i B o w erbank, 1882 * AM + + + + + + + + + + + Hyme de sm ia (St y lopus ) c o ri ac ea ( F ris ted t, 1885) * AM + + + + + + + + + + + B a tz ella sp . * pE + + M y xi lla (M yx ill a) r o sa ce a (L ieber kϋhn , 1859) * AM + + + + + + + + + + + + + Des mac el la i n fo rmis (S teph ens , 1916) A ? ? + P odos pon gi a love ni d u B o cage, 1869 R ? ? + + Axine ll a p o ly p o ide s S chmidt, 1862 AM ? ? + + + + + + + + + + Axine ll a v a so nu da T o ps ent, 1904 A ? ? + Axine ll a d a m ic o rn is (E sp er, 1794) * AM + + + + + + + + + + + Dic tyo ne ll a obt u sa (S chmidt, 1862 ) AM ? ? + + + + + Table 1 . Zo o g eo gra p hical a ffi nities o f t h e Go rrin g e B a nk De mo sp o n ge fa una .
Abbr evi a ti o ns : Get, Gettys bu rg a nd Or m, Or mo nde s ea m o u nts (Go rr inge Ba nk); BR I, Br it is h Is le s; BI S, B a y o f Bi sc a y ; P T C , P o rt u g u es e co a st; AZO , Azo res ; M AD, M adei ra ; C A N, C ana ri es ; C A P , C a p e V er d e; G IB, Gi br a lt a r; w,c ,eM ED , w es ter n, c entr al a nd e a ster n M edi ter ra nea n. A ff in it ie s: AM , At la nto -M edi ter ra ne a n ; A, Atl a n tic; M , M edi ter ra n ean ; R , r es tri cted di st ri b u ti o n ; E , end emi c to Go rr inge; p E , po tenti al ly endem ic (th e s ta tus a nd d is tri bu ti o n o f these s p eci es a re s til l u n kno wn). T he s p eci es i n d ic a te d i n the t a b le w ith ? a re r epo rted fr o m l iter a tur e fo r Gor ri nge bu t wi th o u t cl ea r r ef er ence to whi ch o f the s eamo u nts. *, fo u nd in the pr es ent s tudy. Dic tyo ne ll a inc is a (S chmi d t, 1880) * AM + + + + + + + + + C ioc a lypt a ade rm a (L
evi & Vacelet,
1958) E ? ? C ioc a lap at a a lm a e Carballo , Uriz & Ga rcia -G omez, 1996 * R + + Hal ic lo na ( H a lic lo n a ) s imu lans ( Johns to n, 1842) * AM + + + + + + + + + + + + Ha li cl o n a ( S o estel la ) m u co sa (Gries si n g er, 1971) * AM + + + + + + Dys ide a sp .* R + + Hexad el la r a covit z ai T o ps ent, 1896 * AM + + + + + + + + Hexad el la pr uv ot i T o ps ent, 1896 * R + + SI M IL A RI T Y (s ha re d s p e cie s) To ta l Abs o lute number 36 30 25 17 18 18 15 10 17 17 18 23 21 16 Percentage (%) 47. 2 50. 0 50. 0 41. 7 27. 8 47. 2 47. 2 50. 0 63. 9 58. 3 44. 4 Table 1 (cont .) . Zo o g eo gra p hical a ffi nities o f t h e Go rrin g e B a nk Demo spo nge fa una.
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More than half (61%) of the species found at the Gorringe Bank are widely distributed in the A t l a n t i c a n d Mediterranean areas (Figure 2). Of these, t h r e e s p e ci e s – Stryphnus mucronatus,
Dictyonella obtusa and
Haliclona mucosa – are
widely distributed in the Mediterranean and have a discontinuous Atlantic distribution, usually at one of the Macaronesian archipelagos. Twenty eight percent of Gorringe’s fauna is composed of species that are either endemic or exhibit a very restricted distribution at nearby locations: Geodia geodina,
Podospongia loveni, Ciocalapata almae and Hexadella pruvoti have been reported from the immediately surrounding areas (either Portuguese coast, western Mediterranean or Gibraltar). Ciocalypta aderma and Cornulum cheliradians, originally described from Gorringe material have not been reported elsewhere and constitute to our current knowledge species that are endemic to this seamount. Ancorina sp. and Erylus sp., given their still unknown status may also constitute Gorringe shallow-water endemics. The remaining 11% account for five species - Dercitus bucklandi, Thymosia guernei, Stelligera mutilus, Axinella vasonuda and Desmacella informis – that are limited to the Atlantic coasts and which presence in the Gorringe constitutes in most cases their southernmost records.
DISCUSSION
This constitutes to date the only study exclusively dedicated to the diversity and zoogeographical affinities of the demosponge assemblages on the Gorringe Bank, and one of the few studies reporting sponges from seamount ecosystems on the NE Atlantic. Our results demonstrate that the Gorringe Bank exhibits a diverse sublittoral demosponge fauna with a total 36 recorded species, some of which occurring in high abundances. This result is especially noteworthy given the fact that due to topographic (depth) constraints we only surveyed horizontal rocky
AM A R/E/pE
28% 61%
11%
Figure 2. Biogeographical affinities of Gorringe
demosponge fauna. Abbreviations: AM, Atlanto-Mediterranean; A, Atlantic; R, restricted distribution; E, endemic to Gorringe; pE, potentially endemic.
substrates, which cover a rather small part of the total summit area. The diversity found on the Gorringe Bank may be correlated to high productivity levels resulting from local upwelling of deep nutrient-rich waters. This phenomenon is typical for seamount systems, and it was also demonstrated for the Gorringe Bank through both model simulations as well as field observations (Coelho & Santos, 2003).
Interestingly we found, in our samples, only 6 out of the 23 species to be shared between the Gettysburg and Ormonde seamounts, corroborating patterns that were observed in other studies. For instance, Gonçalves et al. (2004) found only 6 out of 14 fish species shared between Gettysburg and Ormonde, and Gillet & Dauvin (2003) found only 9 out of 53 polychaetes shared among 5 other North Atlantic seamounts (also see de Forges et al., 2000). These observations suggest that individual seamounts can represent distinctive systems, with a strong shift in species composition.
Seamounts, such as the Gorringe, on the intervening areas between continental shelves and oceanic islands (Macaronesian archipelagos) may shorten the dispersal distances between areas, providing suitable habitat for population establishment, and therefore play a role on marine species range expansions following a stepping-stone model, as originally suggested by Hubbs (1959). The Gorringe Bank is located on the pathway of several main surface streams, including the southward flowing Portuguese current and the south-eastward flowing Azores current. These currents can provide means of dispersal for marine organisms to the Gorringe Bank from the Iberian coasts. These organisms can subsequently disperse to southernmost locations such as Madeira, Canaries and Cape Verde islands leading to the strong biogeographical affinities between these areas that have been reported in previous studies. For instance, the molluscan fauna of the Gorringe Bank, shows the greatest affinity with the Mediterranean (93.5%) followed by the Portuguese Coast, Canaries (80%) and Madeira island (77%) (Ávila & Malaquias, 2003).
The zoogeographical affinities found in our study have to be taken with some caution for two reasons: (i) survey times were limited and it is expected that the demosponge fauna of the Gorringe Bank will prove to be more diverse if sampling effort is intensified; (ii) it is possible that our results are biased due to the fact that the fauna of some areas (such as Madeira) are markedly understudied compared to the Mediterranean. Notwithstanding this, the similarity found between the demosponge fauna of the Gorringe Bank, the
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Mediterranean (approx. 60%), Iberian coasts (50%) and Macaronesian archipelagos (mainly above 40%) is consistent with the patterns observed in other organisms.
Similarly to what has been reported for fish (Gonçalves et al., 2004) and molluscan (Ávila & Malaquias, 2003) faunas, our data show that the Gorringe demosponge assemblages are largely composed of species with a wide Atlanto-Mediterranean distribution (61%). However, in contrast with molluscs and fish, the demosponge fauna of the Gorringe Bank shows a relatively high level of endemism. Our study showed that 28% of the sponge species found on the Gorringe Bank are endemic, potentially endemic to these seamounts or have a restricted distribution at close-by areas.
If the species that could only be identified to the genus level in our study prove to constitute new species this will add up to the two species found to be endemic to the Gorringe Bank (Cornulum cheliradians and Ciocalypta aderma) and increase its endemic element to 13.9%. Although modest when compared to the ranges found for invertebrate faunas of other seamounts (Wilson et al., 1985; de Forges et al., 2000), or to the 53% of endemic demosponges found for Vema, a South Atlantic seamount at a similar depth range (Lévi, 1969), the level of endemism found in our study suggests the Gorringe as a hotspot for demosponge fauna in the NE Atlantic.
Endemism associated with oceanic islands is considered to be the outcome of speciation events given isolation through time, in which evolutionary mechanisms such as natural selection and genetic drift acting on the founder populations play a major role (Grant, 1998; Whittaker & Fernández-Palacios, 2007). Thus, in analogy to islands, seamount endemisms could be interpreted as a result of local enhanced speciation, and seamounts perceived as source areas of diversity.
The role of seamounts as areas of endemism has recently been questioned based on a phylogeographic study, and the apparent high level of endemism attributed to sampling bias (Samadi et al., 2006). From well supported data they suggested that seamounts do not represent areas of endemism, but highly productive zones with many species in high densities (the oases model). In another study O’Hara (2007) showed that seamounts did not exhibit elevated level of species richness or narrow-range endemism in ophiuroids when compared with non-seamount areas. These contrasting patterns reflect the complexity of seamount ecosystems and suggest that seamounts can play
different evolutionary roles as its communities’ assembly constitute a dynamic balance between intrinsic life-history traits (e.g. dispersal potential, bathymetric range, habitat preference) and extrinsic environmental factors (e.g. distance to the source area, other seamounts or islands; depth, topography, prevailing oceanographic conditions, geological age and origin).
Sponges, as sessile organisms, find in the larval stage the main means to disperse and colonize distant areas (Maldonado & Bergquist 2002; Maldonado, 2006; but see also Maldonado & Uriz, 1999). However, sponges produce lecitotrophic larvae with a relatively short planktonic life that can last from a few days up to two weeks (Maldonado, 2006), which makes dispersal across large areas of deep sea unlikely. From this view point, seamounts with relatively shallow summits are more likely to constitute endemism centres for shallow-water sponges, as supported by our and Lévi’s (1969) studies.
In order to further test some evolutionary hypotheses on the uniqueness of these ecosystems and their role as stepping-stones and/or speciation centres a thorough survey of other relatively shallow NE Atlantic seamounts such as Ampère (60m), Joséphine (170m) and the application of molecular approaches on a multi-taxa framework would prove useful. Molecular studies both at supra- and intraspecific levels could provide insights on the phylogenetic position of taxa and populations of these seamounts, and enable inferences of divergence time, dispersal direction and populations’ connectivity and structure. It is our aim to perform such studies within the near future, with some of the material collected at the Gorringe Bank.
In conclusion, given the level of diversity found and the uniqueness of its faunal assemblages the Gorringe Bank should be subject of further investigation and regarded as a priority area for conservation.
ACKNOWLEDGEMENTS
The authors wish to thank Universidade Lusófona (Portugal) for the opportunity to participate in the LusoExpedição 2006, in particular to Gonçalo Calado, Frederico Almada and Prof. Pinto de Abreu. We also warmly thank all the crew of N.T.M Creoula and the technical diving team (leaded by José Tourais) in particular to the diving colleagues Nuno Álvaro, José Pedro Borges, Paulo Cardoso, Isabel Alpiarça and Frederico Cardigos. We also thank Paola Rachello-Dolmen for her help in the construction of figure 1, Rosa Pestana for sharing her
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unpublished data on Madeira’s sponge fauna and Paco Cárdenas for useful discussions on the astrophorid specimens. Saskia Marijnissen and two anonymous reviewers are acknowledged for their constructive comments. This study is part of J. Xavier PhD project concerning the Phylogeography and Taxonomy of marine sponges from the Azores and North Atlantic Seamounts, funded by Fundação para a Ciência e Tecnologia (FCT – Portugal) under the grant no. SFRH/BD/16024/2004.
