Chapter 6. World-wide distribution patterns of squat ...

Chapter 6. World-wide distribution patterns of squat lobsters Kareen E. Schnabel1, Patricia Cabezas2, Anna McCallum3, Enrique Macpherson4, Shane T. Ahyong5 and Keiji Baba6 1

National Institute of Water and Atmospheric Research (NIWA), Private Bag 14 901, Kilbirnie, Wellington, New Zealand. [email protected] 2

Museo Nacional de Ciencias Naturales (CSIC), José Gutiérrez Abascal 2, 28006 Madrid, Spain. [email protected] 3

Museum Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia. [email protected]

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Centre d'Estudis Avancats de Blanes (CSIC), C. d'acces a la Cala S. Francesc 14, 17300 Blanes (Girona), Spain. [email protected] 5

Australian Museum, 6 College St., Sydney NSW 2010, Australia. [email protected]

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Faculty of Education, 2-40-1 Kurokami, Kumamoto 860-8555, Japan. [email protected]

Abstract Knowledge of squat lobster diversity has grown rapidly during the past two decades and has now reached a level where it is possible to attempt a biogeographic synthesis. Squat lobsters of the superfamilies Galatheoidea and Chirostyloidea are now represented by more than 1000 species worldwide (10% undescribed) across nearly all latitudes and depths; the potential for new species discoveries remains high. Patterns of global species richness indicate a distinct global centre of diversity in the tropical western Pacific from between New Caledonia, Indonesia and the Philippines, with progressively fewer species recorded with increasing distance from this centre. This pattern holds for taxa in both superfamilies and across all depths, although a single unified centre of diversity is less obvious for lower slope and abyssal species (>2000 m) compared with their shallow-water relatives. A search for broad-scale biogeographic patterns highlights the presence of at least ten distinct regions (north-eastern, south-eastern, western Atlantic Ocean, Indian Ocean, central western, south-western Pacific Ocean, Kyushu-Palau/Bonin, French Polynesia, north-eastern and south-eastern Pacific Ocean) with further indications of localised faunas and possible high levels of endemism. Boundaries between regional assemblages and differences observed for separate depth strata may be linked to different historical events and/or present-day oceanographic characteristics such as ocean currents or impact of oxygen minimum zones. Additionally, ecological processes are highlighted that may play important roles in driving distributions, such as biological associations between squat lobsters and other organisms, or life history characteristics that influence dispersal ability. Significant diversifications into deep water and high latitudes are probably derived and provide a useful comparative framework to examine the evolution of the different groups of squat lobsters in the future.

Schnabel KE, Cabezas P, McCallum A, Macpherson E, Ahyong ST and Baba K (2011) Chapter 6. World-wide distribution patterns of squat lobsters. In The biology of squat lobsters. Crustacean Issues Vol. 00. (Eds GCB Poore, ST Ahyong and J Taylor) pp. 000–000 (CSIRO Publishing: Melbourne and CRC Press: Boca Raton).

KE Schnabel, P Cabezas, A McCallum, E Macpherson, ST Ahyong and K Baba

Keywords Galatheoidea, Chirostyloidea, biogeography, distribution, diversity, endemism

Introduction Biodiversity and biogeographic studies on marine taxa have principally focused on shallow-water coral reef communities of macro-invertebrates and fish from the Indo-West Pacific. Benchmark studies include Carpenter and Springer (2005) for fishes, Wallace et al. (1991) for corals, Williams (2007) for gastropods, De Grave (2001) for commensal shrimps, Kensley (2007) for isopods, Reaka et al. (2008) for stomatopods and Castro (2005), Poupin (2008) and Malay and Paulay (2010) for selected decapods. These studies have contributed to widely accepted paradigms that posits the IndoAustralian Archipelago (IAA or ‘coral triangle’) as a major marine biodiversity hotspot, with the western Indian Ocean and the Caribbean Sea being at most secondary centres of diversity (Briggs 2003; Hughes et al. 2002; Roberts et al. 2002), views discussed by also by Malay and Paulay (2010). In contrast, few studies have incorporated deep-water (> 200 m) taxa in large-scale biogeographic studies (e.g., Bouchet and Kantor 2003; Cairns 2007; Gooday et al. 2004; Guinot and Richer de Forges 1995; Last and Yearsley 2002). Although most of these studies still remain geographically and taxonomically confined, they point to the existence of different centres of biodiversity in the Pacific. However, a similar pattern with a global peak around the Philippines was detected by Cairns (2007) based on a world-wide dataset of 706 azooxanthellate scleractinian deep-water corals. Regional biogeographic patterns will differ between taxa owing to complex interactions of biotic, abiotic and historical factors that affect local species richness and size of distributions (horizontally and vertically). However, only global and taxonomically inclusive analyses of widespread taxa across a wide depth and latitudinal range can determine whether these patterns hold across large geographic distances or across various taxonomic levels. This chapter presents a global dataset for the two squat lobster superfamilies Galatheoidea and Chirostyloidea (see Taxonomy and Phylogeny Chapters) and highlights the utility of these groups to address broad questions related to global diversity and diversification. Squat lobsters are abundant, widespread and biologically diverse, and, importantly, are well-resolved taxonomically, with recent documentation of all published records for all known species (Baba et al. 2008). Information on the distribution of squat lobsters, however, is still growing. Most recently, Macpherson et al. (2010) presented a biogeographic analysis for 402 squat lobster species of the galatheoid families Galatheidae, Munididae and Munidopsidae, between depths of 200 and 2000 m in the Pacific Ocean. Community composition and regional levels of endemism indicated a number of distinct assemblages that included the IAA, the south-western Pacific and French Polynesia. Some of their results were supported by a regional analysis of 504 species of both superfamilies in the southwestern Pacific region presented by Schnabel (2009a), highlighting possible commonalities and differences between the distributions of galatheoid and chirostyloid species. This chapter expands the geographic and taxonomic focus to examine global species richness and diversity patterns of the squat lobsters in the superfamilies Galatheoidea and Chirostyloidea across all oceans and all depths, in order to establish whether there is evidence for a global centre of species diversity and to identify distinct global assemblages of squat lobsters. It is important to note that this analysis covers a broad regional scale that includes orders of magnitude differences in sampling effort across the area. The aim is to identify large-scale general patterns of diversity that can provide a stimulus for more detailed and rigorous studies, rather than making strong quantitative inferences. 2

Chapter 6. World-wide distribution patterns

History of squat lobster collections The earliest records of squat lobsters were provided for the European munidid Munida rugosa by Fabricius (1775), with additional descriptions of primarily shallow-water galatheoids (< 200 m) reported by Leach (1814), Embleton (1834), Liljeborg (1851) and Lovén (1852) among others (see too Macpherson and Baba 2011, this volume). By the end of the 1850s, numerous galatheoids had already been reported from the far reaches of the Pacific such as Chile and Peru (Fabricius 1793), the Philippines and Fiji (Adams and White 1848; Stimpson 1858). Most early species descriptions were the result of deep-sea collections during the mid- to late 19th century scientific voyages by vessels such as the Blake around the Caribbean (Dana 1852; Milne-Edwards 1880; Milne-Edwards and Bouvier 1897) (presenting the world’s first records for Chirostylidae, five species of Uroptychus [as Diptychus] and one species of Gastroptychus [as Ptychogaster]), the Talisman and Travailleur in the Eastern Atlantic (Milne-Edwards 1881, 1882; Milne-Edwards and Bouvier 1900), the round-theworld voyage of the Challenger in 1873–1876 (Henderson 1885, 1888), expeditions in the Indian Ocean by the Investigator (e.g., Alcock 1901; Alcock and Anderson 1899; Alcock and McArdle 1901; MacGilchrist 1905) and Valdivia (Balss 1913; Doflein and Balss 1913), and the Albatross expeditions in the eastern and western Pacific Ocean (Baba 1988; Benedict 1902; Faxon 1893, 1895). Twentieth century expeditions continued to augment species numbers from new regions such as the Atlantide expedition to West Africa (Miyake and Baba 1970) and the Meteor expeditions around north-western Africa (Türkay 1975, 1976). Moreover, new species have been discovered by surveys exploring different depth strata, e.g., the North Atlantic deep-sea expedition of the Michael Sars in 1910 (Sivertsen and Holthuis 1956) that sampled to a depth of 4700 m, and the Danish Galathea expedition of 1950–1952 that collected two species of Munidopsis at 5243 m (M. petila and M. profunda) (Baba 2005). More recently, extensive geographic coverage has been provided through surveys by the French Campagnes MUSORSTOM in the south-western Pacific with a comprehensive description of the fauna in the region of Indonesia, New Caledonia, Vanuatu, Fiji, Tonga and French Polynesia (Baba and de Saint Laurent 1996; Macpherson 1994, 2004, 2006; de Saint Laurent and Macpherson 1990; de Saint Laurent and Poupin 1996). Further extensive sampling in the southwestern Pacific region around Australia and New Zealand has comprehensively characterised these regional faunas (Ahyong 2007; Ahyong and Poore 2004b; Ahyong and Poore 2004a; Schnabel 2009b, 2009a; Schnabel and Bruce 2006). Finally, additional explorations focusing on specific habitats such as seamount or chemosynthetic habitats have also increased our knowledge of squat lobster diversity and habitat associations (Hoyoux et al. 2009; Jones and Macpherson 2007; Macpherson and Segonzac 2005; Martin and Haney 2005; Rowden et al. 2010). In summary, squat lobsters have been recorded throughout all oceans and from depths ranging from shallow subtidal to 5330 m, Munidopsis parfaiti and M. thieli being the species recorded at greatest depths (Macpherson and Segonzac 2005), and habitats that range from subarctic to Antarctic (GarciaGuerrero et al. 2006; Khodkina 1975). Ongoing collections continue to explore new regions and habitats with the discovery of new species and genera continuing unabated.

How many species are there? In order to cover as much available information as possible, different sources were used to extract distributional information for squat lobsters. These included published and unpublished records obtained from:

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1. The world catalogue of squat lobster species listed of all published species records with notes on collection locations and depth distribution (Baba et al. 2008). Species are also listed on the World Register of Marine Species (WoRMS [2010] accessed through: World Register of Marine Species at http://www.marinespecies.org/). The majority of these records do not include geo-references, only location descriptors (country or feature names such as islands, seamounts, basins etc); 2. Records for the chirostylid genus Uroptychus include unpublished data contributed by KB. These include 70 undescribed species and geo-referenced location information; 3. South-western Pacific records for Galatheoidea and Chirostyloidea were derived from Schnabel (2009b). These include 38 undescribed species and geo-referenced location information; 4. Species records for 47 undescribed species including geo-referenced location information provided by works published after Baba et al. (2008) (Baba 2009; Baba and Fujita 2008; Baba and Lin 2008; Baba et al. 2009; Cabezas et al. 2009; Dong and Li 2010; Macpherson and Baba 2009, 2010; Schnabel 2009b; Schnabel et al. 2009; Taylor et al. 2010). FIG 1

In order to compare squat lobster assemblages between regions, species records were pooled within a global grid series of 20 × 20 degrees latitude and longitude (Fig. 1, Appendix 1). This grid roughly corresponds to regions identified for Pacific galatheoids (Macpherson et al. 2010) and allows for inclusion of primarily historic records with very coarsely constrained geographic information (e.g., Anderson’s (1896) record for Galacantha trachynotus in the ‘Arabian Sea’). Each species record was assigned to a grid number based on collection information, either based on geo-coordinates or manually assigned if latitude and/or longitude information were missing. The average depth was calculated from minimum and maximum depth where two depths were available, otherwise the single depth record was used. Depth classes were defined as follows: 0–199 m shelf; 200–899 m upper slope; 900–1999 m lower slope; 2000–3999 m continental rise; and >4000 m abyssal (following Macpherson et al. 2010). Each species record was assigned to a depth band by majority consensus. Taxonomic and geographic rules applied to the dataset are outlined in Appendix 2. The world list of squat lobster species (Baba et al. 2008) listed 869 species, 192 species of Chirostyloidea and 677 species of Galatheoidea. Additional species and locality information have been included for 47 species based on species described since publication of the catalogue resulting in a revised world list of 916 squat lobster species (710 Galatheoidea and 206 Chirostyloidea, as at September 2010). Further records for 126 undescribed species (101 chirostyloids and 25 galatheoids) of the south-western Pacific region are included that remain in the hands of the authors, bringing the total count of world squat lobster species to 1042 (307 Chirostylidae and 735 Galatheidae).

Global patterns of species richness Numerous hypotheses have been proposed for the origin and causes of the ‘bulls-eye’ pattern of diversity with a IAA hotspot with declining species numbers with progressive distance from it. These hypotheses have incorporated global and regional energy supply to the benthos (energy hypothesis), species-area relationships (area hypothesis) and the stochastic likelihood of range overlap in the area that roughly represents the geographic centre of the Indo-West Pacific (mid-domain effect) (e.g., Barber 2009; Bellwood et al. 2005; Bellwood and Meyer 2009). Irrespective of ultimate drivers, recent studies have also indicated the persistence of the IAA as the global centre of diversity for shallow coral-reef fauna over the last 50 million years, since the Eocene (Renema et al. 2009). However, the generality of this pattern cannot be assumed for all taxa, depths and area sampled (e.g., Abele 1982; Gray et al. 1997). 4


Squat lobsters of both Galatheoidea and Chirostyloidea are diverse and abundant and occur across a wide bathymetric range, which allows for a comparative review of distribution patterns across various environmental gradients. Our results on the global distribution of more than 1000 species of squat lobster across all depths generally conform to an IAA centre of diversity (Fig. 1). The western Pacific harbours by far the highest species richness compared to any other oceanic region with 738 species (249 chirostyloid and 489 galatheoid species, Table 1). Within this area, two peaks of maximum diversity are apparent, one in the region around New Caledonia and adjacent waters with a total of 316 species representing the area of globally highest numbers of species (cell 75), and a secondary peak around the central Indonesian/South Philippine archipelago, the central IAA, with 219 species (cell 58, see Appendix 1). Tails of diversity (cells containing > 100 species) reach as far as New Zealand, Tonga/Samoa, north-eastern Australia, and north from the Philippines to Japan. Outside the western Pacific region, combined squat lobster diversity per cell does not exceed 100 species and regional centres include the eastern Caribbean region with a total of 80 species, around Madagascar (77 species, cell 21), south-western Australia (47 species, cell 47) and the Arabian Basin (52 species, cell 41). Considering the distribution of species numbers at an oceanic scale, the Pacific Ocean harbours around 80% of the global species (831 species), the Indian Ocean harbours 23% of all species (254 species) and the Atlantic Ocean (including the Caribbean and the Mediterranean seas) approximately 15% (156 species, Table 1). The eastern Pacific region (including Hawaii) shows the smallest overall fauna (118 species or 11%), particularly with respect to chirostyloids with only 14 species recorded along the entire margin. This general pattern remains when galatheoids and chirostyloids are considered separately, although the chirostyloids are proportionally more dominant in the western Pacific, with 82% of all known species compared to 66% for galatheoids, and under-represented in all other oceanic regions where the proportion of galatheoids is significantly higher than chirostyloids (Table 1). This discrepancy is most distinct along the eastern Pacific margin where 5% of all chirostyloid species have so far been recorded, compared to 14% of galatheoids. Indications that this pattern is unlikely to be primarily a sampling effect are provided in the discussion below. FIG 2

On the other hand, by focussing on species distributions according to separate depth strata, it is apparent that the western Pacific diversity centre is common to all separate shelf, upper and lower slope faunas (0–1999 m). However, a more southern western Pacific centre is more prominent for the slope fauna compared to the shallow shelf species (Figs 2A–C). Peaks in diversity for the deep continental rise and abyssal depth fauna appear more scattered with comparably high numbers around Madagascar and the north-eastern Pacific for the continental rise (2000–3999 m), and the Atlantic for the abyssal fauna (>4000 m) (Figs 2D, E). Nevertheless, the region representing the cell around the Philippine basin that includes Taiwan, the southern part of the South China Sea and the northern Philippine archipelago (cell 59) is significant for squat lobster communities of nearly all depths, from the shallowest to the deepest (the area is only marginally less speciose for the upper slope fauna than the fifth-most speciose cell and was not included in Fig. 2B; it still contains 79 species of squat lobsters). However, it is important to note that global species records and abundances for the deepest squat lobsters (>2000 m) are extremely few compared to the shelf and slope faunas and may not allow for global generalisations across these depth ranges until more samples are available. Overall, a unified centre of diversity for both Galatheoidea and Chirostyloidea is apparent with peaks of diversity in the tropical and temperate western Pacific. This pattern appears to be consistent across most depth strata although peaks of diversity are more dispersed world-wide at depths exceeding 2000 5


m. However, the global distribution and varying degrees of species richness of squat lobsters also clearly highlight conspicuous gaps in regional coverage. Large portions of all the central ocean regions remain entirely blank or at most contain very few species (Fig. 1). Although most of these regions include some of the deepest oceanic habitats, and species numbers may be expected to be low, this trend may primarily be an artefact of limited sampling effort. On the other hand, low species numbers, particularly at shelf and upper slope depth in areas such as the northern Atlantic or eastern Pacific that have historically been sampled well, are likely to reflect true patterns. Nevertheless, for large parts of the globe, species inventories are far from complete and any large-scale generalisations have to be considered tentative.

Species distribution ranges The geographic range of species is influenced by biotic and abiotic factors operating from generational to evolutionary time scales. Distribution ranges are usually geographically confined by limitations imposed by biotic factors such as larval duration, specific niche requirements and behaviours, and abiotic factors such as ocean floor topography, ocean currents, environmental conditions across latitudinal and depth gradients, and historical events such as continental plate movements (Brown et al. 1996; Gaston 1998; McClain and Mincks Hardy 2010). Larger distributions, for example, are often a feature of organisms with high dispersal capabilities (Levin 2006; Malay and Paulay 2010; Schüller and Ebbe 2007), but also of progressively deeper-water taxa owing to the relative temporal and spatial homogeneity of the deep-sea habitats (see recent review by McClain and Mincks Hardy 2010; Rex and Etter 2010; Schüller and Ebbe 2007). Squat lobsters may provide excellent model taxa to examine distribution ranges, being abundant, speciose, with variable biological characteristics such as a variety of body sizes and motility and a world-wide distribution spanning nearly all depths and latitudes (Baba et al. 2008). The complete larval duration is known for only four species, Galathea intermedia (Christiansen and Anger 1990), Sadayoshia tenuirostris (Fujita and Shokita 2005), Munida gregaria (Pérez-Barros et al. 2007) and Chirostylus stellaris (Fujita and Clark 2010). Studies on the first zoeal stage for 11 other species clearly indicate differences across genera of both superfamilies (Baba et al. 2011, this volume). For instance, larvae of the species in the galatheoid genera Munida, Cervimunida, Pleuroncodes and Sadayoshia and the chirostyloid genus Eumunida hatch at an earlier developmental stage compared to the species of galatheoid genus Munidopsis and chirostyloid genera Chirostylus, Uroptychus and Gastroptychus (e.g., Clark and Ng 2008; Fujita 2007; Guerao et al. 2006; Pike and Wear 1969; Samuelsen 1972). Planktonic larval duration (PLD) is assumed to be directly related to dispersal ability. Hence, those taxa with abbreviated larval development would be expected to have proportionally smaller ranges and differences in duration of other larval stages, specifically the megalopa and first crab stage, would will have further implications for dispersability. Unfortunately, these details cannot yet be compared across a wide range of taxa. Geologically older taxa may also be expected to have wide distributions, although the impact of extinction and presence of palaeo-endemics has to be considered. The fossil record for galatheoids suggests a Tethyan origin with first fossils found in the European region being of Jurassic age, with a subsequent spread across the northern hemisphere along the Tethyan Seaway that corresponds with additional fossil records in North America and Japan from the Cretaceous (120–160 mya) onwards (Ahyong et al. 2011, this volume; Feldmann and Schweitzer 2006; Schweitzer and Feldmann 2000). These include the Recent genera Galathea and Munidopsis among other extinct genera such as Paleomunida and Paragalathea (see Schweitzer and Feldmann 2000). Range expansions resulted in a nearly cosmopolitan distribution since the beginning of the Tertiary, following the breakup of Gondwana and the formation of the Antarctic Circumpolar Current (ACC) that was accompanied by 6


dramatic climate shifts. The first fossils of Munida in Eocene deposits (50–60 mya) (Feldmann and Schweitzer 2006; Ghiglione et al. 2008). Chirostyloid fossil records are questionable and so far only exist for the extinct genus Pristinaspina from Cretaceous deposits in Alaska. However, the placement of Pristinaspina is not certain and it may well represent a munidopsid instead (Ahyong et al. 2011, this volume). A second proposed chirostyloid, Eumunida pentacantha (Müller and Collins, 1991), has been transferred to the munidid genus Sadayoshia (Schnabel and Ahyong 2010). Too little is known about the fossil history of chirostyloids to establish whether a Tethyan origin is likely, or whether this group has formed in the Palaeo-Pacific (Ahyong et al. 2011, this volume). Apart from Sadayoshia and Shinkaia, all Recent genera with a fossil record currently have cosmopolitan distributions. Contrasting these with range restrictions of other taxa may provide hints to more recent events shaping the regional faunas. Eighteen of the 42 known genera of squat lobsters only occur in the western Pacific. In contrast, other ocean regions do not harbour as many endemic genera as the western Pacific (none of which contains more than two species). Pleuroncodes and Janetogalathea are limited to the eastern Pacific, the Caribbean munidid genus Anomoemunida, with a single species, A. caribensis, is the only genus known solely from the greater Atlantic region and Hapaloptyx, Nanogalathea and Macrothea, all monotypic, are endemic to the Indian Ocean. At a species level only four species of the genera Munidopsis and Galacantha are so far considered ‘cosmopolitan’ (excepting the polar regions) (Baba 2005), of which at least one species, M. serricornis, originally described from the north Atlantic (Lovén 1852), represents a complex of undescribed species in the Indian Ocean and the South Pacific (EM, SA unpublished, see Appendix 2: species rules). The remaining three species, Galacantha rostrata, M. antonii and M. nitida, as currently recognised, have been collected across all oceans (Baba et al. 2008). Notably, most of these records are derived from collections of bathyal and abyssal depths indicating that global concepts of covariance of species ranges and depth shown for many other vertebrate and invertebrate taxa could apply for squat lobsters as well (McClain and Mincks Hardy 2010). In contrast, most remaining squat lobster species have limited distributions as shown in a frequency distribution of ranges.Allocation of records within a 20 × 20° grid shows a classic unimodal, highly right-skewed shape (Fig. 3). A total of 452 species have been recorded only from a single cell (43% of the global total of species) and two-thirds of all species (67%) occur in at most two cells. FIG 3

Chirostyloids show higher range restrictions with 52% (163 species) found in only a single cell, compared to 39% (288 species) of all galatheoids. This could support the hypothesis that chirostyloids, with a primarily abbreviated larval development, and hence shorter PLD, have a limited dispersal ability compared to most galatheoids and are less likely to maintain widespread panmictic populations (McClain and Mincks Hardy 2010; Meyer 2003). Other contributing factors that may generally restrict ranges for Chirostyloidea compared to Galatheoidea could be more specific habitat requirements by chirostyloids, which are predominantly associated with corals and other anthozoans (Baba 2005). These usually occur along steep continental margins, undersea ridges and seamounts, so the more discontinuous nature of this habitat could restrict long-range dispersal (Rowden et al. 2010). Not surprisingly, the three cosmopolitan species represent three of the six most widespread species, i.e. those that are present in the highest number of cells (between 16 cells for M. antonii and 19 for Galacantha rostrata). Furthermore, 11 of the 28 species with occurrences in more than ten cells are other Munidopsidae with deep-water distributions. On the other hand, some species are typical shelf forms ( 95%) of deepsea scleractinian corals are distributed within the upper 2000 m of the oceans world-wide (Cairns 2007) and proteinaceous corals such as gorgonian and black corals are not found deeper than about 3000 m world-wide (Roark et al. 2009). Only six records world-wide for Chirostyloidea are reported from below 2000 m depth, of which two are from hydrothermal vents (Martin and Haney 2005). This pattern remains intriguing and the associations between chirostyloid squat lobsters and corals remains primarily anecdotal (see Baeza 2011, this volume; Le Guilloux et al. 2010). The distinct differences in depth distribution between chirostyloid and galatheoid squat lobsters clearly warrants future examination. 9


Finally, general depth distribution across taxa (β-diversity) shown in figure 5 indicates a high degree of bathymetric overlap that appears to contrast with the high faunal turnover found along a bathymetric gradient in other megabenthic taxa (Howell et al. 2002; Rowe and Menzies 1969). Members of both superfamilies and most genera co-occur regionally. Multiple species and genera of squat lobsters are often collected together (authors, pers. obs.), which poses interesting questions related to niche selection in sympatry. Howell et al. (2002) found that Asteroidea across a 4800 m depth range in the north-eastern Atlantic occurred within a very narrow depth band where the species were abundant, while the total adult range was much wider, which was related to available habitat and resource partitioning. For squat lobsters, comparable abundance data are scarce and very little is currently known about species distribution at a small scale that may indicate variable habitat and resource use. Rowden et al. (2010) recently showed differences in squat lobster assemblages across seamount and surrounding non-seamount habitats of similar depths in the south-western Pacific, with chirostyloids being more characteristic of seamount habitat and galatheoids more prominent in nearby non-seamount communities. Such differences are probably related to characteristics such as larval development and body size in combination with environmental factors such as particulate organic input and host availability. However, this pattern did not appear to apply to all regions or habitats and further studies are still necessary.

Global biogeographic patterns Recent global biogeographic classifications have systematically evaluated large bodies of literature on species distributions in combination with reports on regional bathymetry, oceanography and nutrient availability (UNESCO 2009). The UNESCO (2009) GOODS classification subdivides the world’s oceans into 30 pelagic and 38 benthic provinces. Although fundamental biogeographic breaks along continents and across oceans are consistent, this differs slightly from other classifications. These are, for example, Spalding et al.’s (2007) Marine Ecoregions of the World (MEOW), a nested system of 12 realms, 62 provinces and 232 ecoregions for coastal marine systems, and Longhurst’s (2007) ‘Ecological Geography of the Sea’ that primarily covers pelagic ecosystems at a much larger oceanic scale, and proposes four principal biomes and 51 provinces. All three biogeographic classifications are used to examine emerging patterns of squat lobster distributions. FIGS 6-8

A global examination of regional patterns based on similarities between squat lobster assemblages in a grid of 20 × 20° cells (excluding cells with ≤ 10 species) revealed distinct gradients within and across ocean basins (Fig. 6 shows the results of NMDS ordination and Fig. 7 the cluster analysis). A fundamental biogeographic division between the Atlantic-East Pacific (AEP) and the Indo-West Pacific (IWP) was clearly evident with further significant regional differentiation into at least ten biogeographical assemblages, referred to as provinces, showing high species turnover between them. This is indicated by a high Global R value of 0.929, p