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2002 ICES Annual Science Conference, 1 – 5 October, Copenhagen CM / M: and Ecology of – Indications of Unique ICES CM 2002/M:24

Not to be cited without reference to the author

How important are seamounts for the dispersal of meiofauna? 1

Gunnar Gad and Horst Kurt Schminke

Gunnar Gad, Horst Kurt Schminke, AG Zoosystematik und Morphologie, Fachbereich 7: Biologie, Geo- und Umweltwissenschaften, Carl von Ossietzky Universität Oldenburg, Carl-von-Ossietzky- Straße 9-11, D-26111 Oldenburg, Deutschland (tel: +049 (0)441 798 3373 fax: +049 (0)441 798 3162 e-mail: [email protected], [email protected])

Keywords: seamounts, meiofauna, stepping stones, dispersal strategies, endemism, biogeography, Great Meteor .

Abstract Virtually nothing is known about meiofauna associated with seamounts. Since the sixties lots of biological samples have been taken on seamounts mainly for the study of . The invertebrate fauna of seamounts is only superficially known, the meiofauna practically not al all. The zoogeography of marine meiofauna presents a problem known as the ”meiofauna paradoxon” because there obviously are the same genera of meiofauna along the coasts of widely separated continents and even of remote islands. This is surprising because many meiofauna taxa with a wide distribution, in particular interstitial meiofauna, lack adaptations, which favour dispersal. For example they lack free swimming larvae and are bound to life in sediments. Many possible dispersal mechanisms have been discussed to explain the occurrence of meiofauna on young volcanic islands, e.g. the Galapagos. The potential role of seamounts for long distance submarine dispersal has so far not been taken into account. Seamounts are very common geological formations rising up from the ; many of them are arranged in long chains thus bridging gaps in transoceanic dispersal. The Great Meteor Seamount is one of the best explored seamounts in the world, and since an expedition in 1998 some detailed information on the meiofauna inhabiting its plateau is available for the first time. Seamounts such as the Great Meteor Seamount resemble isolated “islands” in respect to the colonisation by meiofauna, but the question is whether they can nevertheless function as “stepping stones” for long-distance dispersal.

Introduction In meiofauna zoogeography there is a phenomenon called the “meiofauna paradoxon” (Giere 1993). Despite the fact that most groups of the meiofauna in particular when they inhabit the interstices of coarse marine sands lead a life basically bound to the substrate and lack free swimming larvae for dispersal, there is a high percentage of cosmopolitan genera and even species (Gerlach 1977, Westheide 1991, Giere 1993). Study of the interstitial fauna of marine beaches of different continents and even remote islands usually reveal species, which belong to already known genera.

1 This publication is a result of part 3 “Seamount Ecology” (SEAMEC) of expedition No. 42 of R/V “Meteor” in 1998. 2

This is surprising because unfavourable non-sandy, soft sediments in the deep sea preclude dispersal of the interstitial fauna along the bottom of the (Westheide 1991). So, how else could the cosmopolitan occurrence of some genera be explained? Gerlach (1977) listed several possibilities for long-distance dispersal of meiofauna, which could be transported adhering to birds or floating material including the thick fibrous cover of coconuts washed ashore on oceanic islands. Another possibility is the release of ballast water or sand form commercial vessels. In contrast to this rafting hypothesis others have argued that plate tectonics and continental drift may be responsible for the cosmopolitan occurrence of genera of interstitial meiofauna (Rao 1972, Sterrer 1973, Westheide 1977). However, this hypothesis fails to explain the presence of a rich meiofauna on many geologically young volcanic islands like Galapagos, and Hawaii among others (Westheide 1991). Common to all these hypotheses about dispersal of interstitial fauna is that they seek an explanation for dispersal near the surface of the sea. The focus has been on oceanic islands reaching beyond the surface of the sea. But what is with islands not reaching the surface of the sea and being concealed under water? Such islands are called seamounts.

Seamounts According to a definition by Beckmann (1999) seamounts are submarine elevations mostly of volcanic origin, which rise more than 1 000 m up from the deep-sea floor. No two seamounts are identical. A typical seamount is conical in shape with a circular or elliptic base and diameter between 20 and 100 km. A classification of seamounts has to consider that they represent very different geological formations. Their variety has to do with their origin and age, their size and shape, and different physical and chemical condition characteristic for them. A particular class are the (tablemounts) with flat summits. What makes seamounts so attractive for marine science is their wide distribution throughout the oceans. They can be isolated peaks or stand together in clusters or be lined up in chains. It has been estimated that there may be 810 seamounts of a height of over 1 000 m in the Atlantic, while in the Pacific there may be 30 000 – 50 000 of them (Smith and Sandwell 1997, Smith and Jordan 1988). Relative to the open ocean or the deep sea seamounts with shallow peaks are often found to be areas of high biological (Rodgers 1994). As a consequence of this high productivity the summits of many seamounts harbour rich communities of planktic and benthic organisms. The special conditions on seamounts result from complex interactions of nutrient concentrations, and circular water currents as well as local biological processes (Boehlert 1987). Hubbs (1959) was one of the first to formulate some fundamental questions about seamount biota, which are by no means outdated. Can seamounts act as stepping stones for the dispersal of a wide range of marine taxa? What kind of dispersal strategy would allow the establishment of populations on seamounts? Is there a degree of isolation on seamounts favouring radiation processes and resulting in endemic species? Wilson and Kaufmann (1987) add the question of the effect of the geological history of seamounts on the composition of their biota.

Invertebrate fauna of seamounts Seamounts as a special marine for invertebrates has remained very poorly explored even to days. An inventory of the organisms found on seamounts2 and a review of their biogeography at the end of the 80ies reported 1045 identified species collected from at least 100 seamounts worldwide (Wilson and Kaufmann 1987). Over 596 invertebrate species representing 16 phyla have been reported form at least 59 seamounts. Only five seamounts have been investigated more intensively (Table 1) and account for 430 (73%) of all invertebrates known from seamounts. The invertebrates on seamounts are dominated by suspension feeders, such as corals, mollusks or sponges (Wilson

2 Seamounts as used in the review of Wilson et al. (1987) include guyots, large plateaus, some banks and submaine mountains isolated or in ridges. 3 and Kaufmann 1987, Richer de Forges et al. 2000). Endemism on seamounts has been estimated to be 15,4% for invertebrates and 11,6% for fishes (Wilson and Kaufmann 1987).

Tabel 1: Seamounts from, which most invertebrate species were collected (Wilson and Kaufmann 1987) Oceanic Region Seamount Depth (m) Km to shelf Species reported Pacific Cross 2745 >2000 47 Cobb 59 398 52 Hess Plateau 3800 >2000 128 Atlantic Great Meteor 210 1340 108 Vema 63 670 197

A more recent study in the Coral Sea and Tasman Sea reported 29-30% of the species found to be new to science and potential endemics (Richer de Forges et al. 2000). From 24 seamounts a total of 850 macrofauna species was collected. The study aimed at describing the whole benthic community with all abundant taxa. A low faunal overlap between the seamounts in this region of the Pacific indicates that the investigated seamounts grouped in clusters or along ridges function as isolated systems, leading to highly local species distributions. Despite these studies it can be maintained that the true diversity of most invertebrate taxa on seamounts is unknown. This is true in particular for the marine meiofauna, which was not part of the studies mentioned.

Meiofauna of seamounts There are enough seamounts in the oceans to play an important role in the dispersal of meiofauna. But the mere existence of seamounts is not enough; they must have the right sediment suitable for interstitial meiofauna colonisation. Sedimentary of seamounts are unique compared with other marine environments (Levin and Nittrouer 1987). Most sediments have a high percentage of carbonate derived from pelagic fallout or are coarse biogenic sands. The Great Meteor Seamount is a good example because its plateau is covered by coarse biogenic sand composed of fragments of corals and of mollusks’shells. This sand is the result of erosion of old coral reefs, which fringed the top of the seamount when the water level was lower (Nellen 1998). Such rippled and scoured sands on the top of seamounts are usually rather coarse because the finer fractions are removed by the action of strong currents (Levin and Nittrouer 1987). It is to be expected that there are many seamounts with substrates suitable for interstitial life. The Great Meteor Seamount is one of the best investigated in the world. It is a gigantic submarine of volcanic origin with an oval plateau about 1,465 km2 large. It reaches up form a depth of almost 4 800 m to about 270 m under the surface of the sea. Since the late sixties it was the subject of an intensive study of its , geology, and biology (Hempel 1968, Hinz 1969, Thiel 1970, Horn et al. 1971, Ulrich 1971, Hempel and Nellen 1972, Bartsch 1973a, 1973b, Nellen 1973, Andres 1977, Ehrich 1977, Hartmann-Schröder 1979, Dietrich et al. 1994, Grevemeyer 1994, Rogers 1994). Further studies were made during the expedition no. 42/3 of R/V “Meteor” in 1998 (Nellen 1998, Pfannkuche et al. 2000). Theses studies centered around the question as to whether the shallow plateau in conjunction with specific hydrographic conditions led to the origin of an isolated and largely self-contained species association. For this it was necessary to identify the species of a number of taxonomic groups and to ascertain their abundance and range of distribution on the plateau as well in the surrounding deep sea. The investigations showed that the Great Meteor Seamount like many others represents a “littoral habitat”, which despite its isolation by a permanent circular current (Taylor column) and despite the fact that it is surrounded by soft bottoms in the deep sea, harbours an astonishingly diverse interstitial meiofauna. In the samples 25 taxa of meiofauna were recovered (George and Schminke 4

2002). First results are available on patterns of distribution, species richness and detailed taxonomy of Harpacticoida (Copepoda), Epsilonematidae as well as Draconematidae (Nematoda), and Loricifera. Most of the species found on the plateau of the Great Meteor Seamount turned out to be new to science. The plateau represents an isolated area with its own and highly endemic fauna (George and Schminke 2002). A colonisation of the plateau by shallow water species seems to be more common in the case of Epsilonematidae, but the small amount of Harpacticoida and Loricifera from coastal indicates that their arrival may be rather accidental. In Draconematidae there also are more shallow water or interstitial species, but about 11% of the species originate from the deep sea. Even in the Harpacticoida 7% of the species also occur in the deep sea (George and Schminke 2002). Two mechanisms could explain the scarcity of bathymetric exchange. The Great Meteor Seamount is an undersea area of large size. According to the theory of island biogeography (McArthur and Wilson 1967) there should be many small potential niches for meiofauna not completely occupied by meiofauna of costal origin. The possibility remains that there is a constant step by step addition of deep-sea species to the fauna of the plateau, a mechanism described by Emschermann (1971). The other possibility is that the plateau fauna originated from ancestors lifted up with the seamount and having adapted over time (George and Schminke 2002). What is more probable is difficult to decide because the upwelling processes around seamounts favouring bathymetric exchange are not studied well enough (Beckmann 1999). An indication of the isolation of seamounts may be found in different morphotypes of invertebrates based on genetically isolated populations. Grasshoff (1972) e.g. found obvious variations between populations of a horny coral on Josephine and Great Meteor Seamount. An explanation could be that microhabitats on the seamount are sufficiently different and that the populations on the seamount undergo strong natural selection. Or it could just be a result of the “bottle neck” effect when the initial population colonising the seamount consisted of few individuals only. This phenomenon could explain the high number of new species of Epsilonematidae. Decraemer et al. (2001) pointed out that there are only minor differences between many species of coastal Epsilonematidae worldwide, which could be an indicator for cosmopolitanism. At the present state of investigation it remains unclear whether the morphological differences found in the Epsilonematidae of the Great Meteor Seamount are an indication for species status in all cases or for extreme morphotypes based on isolation.

General Conclusions In conclusion it can be said that transport through the seems to be the most plausible explanation for dispersal of meiofauna and genetic exchange (Giere 1993). The regularly drifting meiofauna in the water column, which is due either to passive erosion of sediments or to active swimming plays a role in small-scale dispersal but may have no effect over long distances (Palmer 1988). Studies of sinking velocity by Hagerman and Rieger (1981) suggested that long-distance transport across oceans may be impossible. It should be mentioned, however that meiofauna could well adhere more often to floating transported by demersal drift or to marine snow than to sediment particles, which need stronger currents to stay in the water column (Giere 1993, Dahms 1997). The question now is whether there are submarine places endowed with the necessary environmental conditions for interstitial life, i.e. coarse sand, which could bridge the gap by shortening the distance for trans-oceanic dispersal. So far seamounts have not been taken into account as submarine stepping stones for the dispersal of marine meiofauna. The investigation of the Great Meteor Seamount may at first sight speak against seamounts as possible stepping stones for the dispersal of meiofauna in general, because single seamounts, especially large ones, represent isolated areas as pointed out already by Wilson and Kaufmann (1987). This isolation is not simply due to the distance form the next or the next seamount, but a number of flow phenomena add to the faunal isolation of seamounts in particular when they are shallow (Boehlert 1987). Perhaps most important from a biological perspective are 5 trapped waves and a Taylor column. Such a Taylor column was found above the Great Meteor Seamount and isolates its plateau in time and space (Beckmann 1999). This circulation system traps planktic organisms, planktic larvae and may be a hindrance to meiofauna dispersal. Yet a Taylor column will be present or remain stable only under certain conditions. To offer these conditions a seamount must be single, large, relatively high, and have a more or less ideal conical shape. Around seamounts of unequal shape or along seamount chains (Roden 1987), which bridge long distances across the ocean floor there are different current regimes, which are no hindrance to dispersal so that such seamounts could well qualify as stepping stones for meiofauna disperal. The best seamounts, for this purpose are those which are: (1) shallow guyots (tablemountains) with a more or less flat summit, (2) which are covered by coarse biogenic sand, (3) which do not stand in isolation but together in groups, and (4) which are arranged in long chains.

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