For a group of large organisms with such obvious global importance, the available data on kelp biogeography is surprisingly limited and still raises many questions. The techniques exist now to answer some of these questions using molecular methods, and priorities will be suggested. Some regions of the world are still not comprehensively studied, a good example being the Sea of Okhotsk and Arctic Russia, where systematic studies remain morphologically based, and there has been little recent work (e.g. Petrov 1972, 1974; Klochkova 1998; see also comments in Graham et al. 2007a; Bartsch et al. 2008). Selivanova et al. (2007) present the first study using molecular techiques in this region and also outline the many little-known taxa present. Also the occurrence and importance of kelps in tropical regions, where the clear water allows light to reach cooler depths, is an important and relatively recent area of detailed study (Graham et al. 2007a; Santelices 2007). The current contribution is still very much an interim report on global kelp biogeography.
The origins and diversification of kelps
Only a single fossil kelp is known to exist (Parker and Dawson 1965). This is Julescrania grandicornis from Miocene deposits, morphologically considered to be intermediate between extant Pelagophycus and Nereocystis. These genera are closely related (Neushul 1971; Coyer and Zaugg-Haglund 1982; Coyer et al. 1992) with both now considered members of the Laminariaceae although Pelagophycus appears from molecular data to be more related to Macrocystis than to Nereocystis (Lane et al. 2006). There is, thus, no fossil evidence available which can shed light on the origins of either the order Laminariales or of the families which make up the order. What we can ascertain from our knowledge of the distribution and relationships of extant taxa is:
There are three separate clades currently included in the Laminariales (recognised as separate at the family level) which from molecular evidence are basal and thus represent likely ancestral lines of the order (Boo et al. 1999).
All three groups have one or more of the following morphological traits considered to represent ancestral traits: monoecious gametophytes, anisogamous gametes, meiospores with eyespots, chord-like sporophytes.
Six of the seven species in these three ancestral families grow in Japan, and only two of the species grow outside Japan (one in OKHO and one in TNEA).
As the majority of these ancestral Laminariales occur on the island of Hokkaido (Kawai 1986; Kawai and Sasaki 2000; Sasaki and Kawai 2007), this strongly suggests that the origins of the Laminariales were in cold-temperate regions of the Northwest Pacific. The island of Hokkaido is also the centre of diversity of kelp species and genera in Japan (Druehl 1968, 1981; Lüning 1990), which is part of the most species-diverse region in the current dataset although there are more kelp genera in the Northeast Pacific. The new molecular evidence removes the conundrums of the apparent occurrences of ancestral taxa outside this region. The Atlantic distribution of the Phyllariaceae (including Phyllaria and Sacchorhiza) and Halosiphon tomentosus (now Halosiphonaceae), then included in the Laminariales, caused difficulties for previous authors in discussing early origins (Estes and Steinberg 1988; Lüning and Tom Dieck 1990; Lüning 1990). The region including northern Japan is referred to by Lüning (1990) as cold temperate, using the 10°C winter isotherm as the border between warm and cold temperate despite the more typically tropical summer means of 25–28°C. This suggests that the early kelps required cool water, but may have been able to survive much higher temperatures, at least for short periods. Highest temperature tolerances of kelp gametophytes of up to 29°C have been recorded only for Japanese kelps (Undaria, Eisenia: Tom Dieck 1993).
All four ACLL families occur in both warm temperate North Pacific regions on either side of cooler waters of Alaska and the Aleutian Islands. These families are likely to have originated and diversified in a period when the temperate Northeast and Northwest Pacific were not separated by cooler waters. Estes and Steinberg (1988) present evidence that “limpets and herbivorous mammals associated with kelps or other stipitate brown algae appeared late in the Cenozoic”. This could be hypothesised to coincide with the advent of the ancestor of the ACLL families, and the start of their diversification (as most of the basal taxa, Chorda and Pseudochorda, do not have stipes which could support kelp limpets). It would be interesting to know whether there is a species of kelp-stipe limpet which occurs on Akkesiphycus.
Stam et al. (1988) presented the first molecular data for kelp phylogeography, using a DNA-DNA hybridization method, and proposed that radiation within the genus Laminaria occurred 20–15 Ma. These data need to be re-assessed using modern sequencing techniques, particularly as the genus Laminaria then included Saccharina, which is not now considered to be closely related to Laminaria within the Laminariaceae (Lane et al. 2006). There is much scope for work on molecular clocks using the data now available (see e.g. Hoarau et al. 2007 for a recent example with Fucus).
The invasion of the North Atlantic
The Laminariaceae and Alariaceae have a number of genera that occur only in temperate waters of either the Northwest or Northeast Pacific. Many of these are considered to be monospecific genera (e.g. Lessoniopsis, Pleurophycus, Pterygophora, Undariella, Pelagophycus, Postelsia, Streptophyllopsis). It is proposed that this genus-level evolution has occurred since the cooling of the Arctic Ocean and Bering Sea. It could thus be hypothesised that both these families have evolved warmer water forms, currently recognised at the genus level, separately on either side of the Arctic. It was put forward by both Estes and Steinberg (1988) and Lüning (1990) that this polar cooling occurred in the middle to late Miocene. Very few species in various families can survive both Arctic and temperate regimes (e.g. Chorda filum in the Chordaceae, Alaria esculenta in the Alariaceae, Costaria costata and Agarum clathratum in the Costariaceae, Saccharina latissima and Laminaria digitata in the Laminariaceae). The many species of Laminaria and Saccharina are generally regarded as occurring either in the Northwest or the Northeast Pacific, not both (possible exceptions being Laminaria yezoensis and Saccharina latissima). There are very few species occurring in the Arctic and both the North Pacific and North Atlantic temperate regions (possible examples being Agarum clathratum, Saccharina latissima), and they are geographically widespread, so, whether or not these populations are truly conspecific, detailed molecular population studies across their geographical range are necessary.
The current data clearly reveal a distinct kelp flora in the North Atlantic, linked closely with the Arctic flora. This concurs with the detailed discussion of previous authors (Lüning and Tom Dieck 1990; Lüning 1990) on the origins of the Atlantic kelps via the Arctic, hypothesised to have followed the opening of the Bering Strait. More recent evidence dates the first opening of the Bering Strait earlier than considered by these authors, at 5.4–5.5 Ma (Gladenkov et al. 2002). Adey et al. (2008) give a date of 3.5 Ma for this event, citing earlier references but not Gladenkov et al. 2002. There are very few genera of kelps in the North Atlantic (only Laminaria, Saccharina, Alaria, Agarum: plus the record of Ecklonia in North Africa and Northeast Atlantic islands, which will be discussed later). Thus, as noted by Lüning (1990), very few kelp genera and from those genera very few species have managed to survive this trip through the Bering Strait, despite the continuous coastline available. According to Lindstrom (2001) the only kelp species which have been recorded to occur in the North Atlantic and North Pacific are Alaria esculenta, Chorda filum, and Saccharina latissima. Lindstrom (2001) was of the opinion that all three of these “may represent vicariant pairs of cryptic species”, which has since been born out by the recent molecular study of Japanese Chorda (Sasaki and Kawai 2007). The latter authors demonstrate that indeed Chorda filum does not grow in Japan, and plants that previously were given this name in the Northwest Pacific were included in the new species Chorda asiatica. The order Laminariales is thus overwhelmingly a temperate rather than an Arctic group, in both likely origins and diversity. The existence of a warm-temperate kelp such as Laminaria ochroleuca in the southern Northeast Atlantic and the apparently closely related Laminaria pallida in the Southeast Atlantic (Tom Dieck 1992) is interesting with respect to the evolution of Atlantic Laminaria. Lüning and Tom Dieck (1990) commented that in the North Atlantic “warm-temperate species are probably ancestral to the cold-temperate species”. Recent evidence has shown that the prevailing sea temperatures in the Arctic after the opening of the Bering Strait were much warmer than today, with sea surface temperatures as high as 18°C in the mid-Piacenzian era of the Pliocene, 3.0–3.3 Ma (Robinson 2009). Did Laminaria ochroleuca evolve from a cold-temperate invader, or is it more related to a warm-temperate species currently occurring in the northeast or Northwest Pacific? Interestingly, the Arctic endemic Laminaria solidungula is the only true Arctic seaweed discussed in detail by Adey et al. (2008) which does not have a clear Pacific origin, having “18 sister species”, 11 of which have Pacific distributions. Molecular population studies combined with temperature tolerance studies in this genus have a great potential to shed light on the evolution of temperature tolerance in brown algae.
It is also notable that all three of the most species-rich genera of kelps (Alaria, Laminaria, Saccharina) are widespread in the Arctic and have reached the North Atlantic. Although the number of recognised species in these genera is gradually reducing (see especially the reduction to one species of Alaria in the temperate Northeast Pacific, Lane et al. 2006), from the literature, these genera are still recognised as the most species-rich. It is hypothesised that the level of morphological diversity in these genera, which has resulted in a relatively high number of species being described in each genus, is linked to their ability to colonise cold waters not available to most kelps. The relatively recent proposed time period for this colonization may offer an explanation for the presence of ‘species complexes’ in these genera but not in most other genera of kelps. The only exception to this pattern is the genus Agarum with three species, two of which are endemic to Japan and Kamchatka Island, and Agarum clathratum which occurs widely in the Arctic, Northeast and Northwest Pacific and Northwest Atlantic. The latter may be a more recent introduction into the Atlantic, based on the fact that it has reached Greenland (Pedersen 1976) but not the eastern North Atlantic.
Kelps in the tropics and the Southern Hemisphere
The most distant cluster from the origins in Japan in the analysis of world kelp floras represents the Southern Hemisphere. As kelps clearly originated in the Northern Hemisphere since the time when southern and northern temperate regions have been separated by permanent tropical waters—though fluctuating in extent—the ancestors of Southern Hemisphere kelps must have crossed the tropics. Kelps are warm- to cold-temperate and Arctic organisms, and they only survive in the geographical tropics under one of two conditions. Firstly, they can occur inshore in regions where large-scale upwelling brings about a temperate seawater temperature regime in shallow water, e.g. Laminaria pallida (the form previously known as L. schinzii, see Stegenga et al. 1997) in Northern Namibia (Molloy and Bolton 1996), or Eisenia and Lessonia along the coasts of Peru (Ramírez and Santelices 1991). Secondly, kelps can occur in cooler deeper water, where the clarity of tropical water enables the coincidence of enough light to grow with cool enough water for kelps to survive (Graham et al. 2007a). Examples are Eisenia galapagensis in the Galapagos Islands (Graham et al. 2007a) and, on the fringes of the tropics, Ecklonia radiata in Oman (Indian Ocean, Sheppard et al. 1992). In addition, where temperatures are extremely stable around 20°C throughout the year, it is possible for kelps to grow on coral reefs (e.g. Ecklonia radiata in the Houtman Abrolhos Islands off Western Australia; Hatcher et al. 1987), with both groups of organisms at the limit of their different temperature tolerances.
Which groups of kelps crossed the tropics, and where? With the new family arrangement within the Laminariales, the Lessoniaceae becomes particularly important. Three of the five genera in this family (all except Egregia and Eckloniopsis) occur in the Southern Hemisphere, although all except Lessonia occur in the Northern Hemisphere. It seems likely that the family Lessoniaceae arose in the North Pacific, as with the other ACLL families, but the genus Lessonia currently only occurs in the Southern Hemisphere. As Lessonia is not closely related to any other genus in the family Lessoniaceae, it is possible that, without fossil evidence, it may prove difficult to demonstrate whether the genus arose in the Southern Hemisphere, or evolved in the Northern Hemisphere where it has since become extinct. Available molecular data (Yoon et al. 2001; Lane et al. 2006) resolves most of the family into two clear clusters, with Lessonia species separating as a sister group from a clade containing Ecklonia and Eckloniopsis. These genera are closely related and may eventually be placed in a single genus in future, which would be Ecklonia. It is of interest that some of the genera have restricted distributions in the Southern Hemisphere: Lessonia has not reached southern Africa, and there is apparently no Ecklonia in South America. The outlier in the Lessoniaceae is the monospecific Northeast Pacific (Alaska to Baja California) genus Egregia, which has a more complex sporophyte morphology than other members of the family (Henkel and Murray 2007). There is much sporophyte morphological variation in the single species but little molecular variation, and the former does not coincide with the latter (Henkel et al. 2007). Egregia appears as an outlier, considered in its own family, the Egregiaceae, by Yoon et al. (2001). In many trees in Lane et al. (2006), it is also an outlier, but with a good deal of data manipulation these authors resolved Egregia into the Lessoniaceae. It is obviously an extremely interesting kelp from an evolutionary standpoint, but requires further study.
The distributional evidence suggests at least four crossings of the tropics by kelps, by Ecklonia, Macrocystis, Laminaria and Eisenia. Ecklonia has three species in Japan and three in Australasia. The diversity of the other genera in the Ecklonia clade in the North Pacific (presence of Eckloniopsis and Eisenia, also Egregia) suggests, however, that Ecklonia arose in that region, rather than in Australasia. Apart from Ecklonia brevipes in Australasia (which produces multiple uprights for a stoloniferous holdfast), Ecklonia species are morphologically rather similar in the Southern Hemisphere, with an individual solid stipe from each holdfast. The latter were considered part of an ‘Ecklonia radiata complex’ by Bolton and Anderson (1994). On the west coast of South Africa and southern Namibia, a larger species with a long hollow stipe, Ecklonia maxima, forms large kelp beds in the Benguela upwelling large marine ecosystem (Bolton and Anderson 1994, 1997). From hybridization experiments, this appears to be closely related to Ecklonia radiata, which grows on the adjacent south coast (Bolton and Anderson 1987), but no molecular data are yet available. In addition, Ecklonia has been reported in Atlantic North Africa and the Canaries and Cape Verde Islands (E. muratii, which was part of the Ecklonia radiata complex of Bolton and Anderson 1994) and in Oman in the northwest Indian Ocean (Ecklonia radiata, Sheppard et al. 1992). Thus, the most likely scenario for the dispersal of Ecklonia is that it spread from the Northwest Pacific to Australia and to South Africa. South African Ecklonia could have come either from Australia (via the West Wind Drift, see Waters 2008) or from Japan (perhaps via Oman), and the evolution of the long, hollow-stiped E. maxima is likely to be an adaptive response to the higher nutrient levels in the Benguela upwelling system (Santelices et al. 2009). A route of dispersal of many red seaweed species to South Africa from Japan across the northern Indo-Pacific, via cooler localised upwellings in epochs where tropical waters were cooler, was proposed by Hommersand (1986). Finally, it is hypothesised that Ecklonia colonised the eastern North Atlantic from South Africa which, if this were substantiated by a molecular phylogeographical study, would be the only example of kelps crossing from south to north.
Most species of Eisenia occur in the Northeast Pacific (2), western South America (3) and Japan (2). It seems likely that Eisenia arose in the North Pacific and crossed the Tropics to Peru, Chile and the Galapagos. The relationships between the species are in need of investigation, with one species endemic to the Galapagos (Eisenia galapagensis, Taylor 1945), and one species endemic to Guadelupe Island (Mexico; Eisenia desmarestioides, Setchell and Gardner 1930). Both Eisenia and Ecklonia are amongst the most warm tolerant of kelps, both from their distribution and from experimental evidence (Bolton and Levitt 1985; Bolton and Anderson 1987). The genus Ecklonia does not occur in true cold-temperate regions where minimum monthly mean temperatures are below 10°C (Bolton and Anderson 1994). They are thus particularly well suited to crossing the tropics and indeed occur currently, sometimes in abundance, in deeper, temperate, water conditions. Graham et al. (2007a) predict, from bathymetric data and observations, the existence of more than 23,500 km2 of tropical kelp habitats, on the western coasts of the Americas and Africa (see map in Santelices 2007). This refutes a remarkably widespread but erroneous view that, to quote Estes and Steinberg (1988), “extant kelps occur exclusively in cold-water habitats”. Kelps are better described as Arctic and temperate organisms, which hardly ever occur where prevailing minimum monthly mean seawater temperatures are above 20°C (for a rare exception, see Hatcher et al. 1987). Many kelps are limited in their distribution to warm temperate regions, where prevailing monthly mean temperatures are between 15 and 20°C. Graham et al. (2007a) did not have good bathymetric data from the east coast of Africa (M. H. Graham, personal communication), and certainly Ecklonia quite possibly occurs in these habitats between its currently recorded limits in Mozambique (Kerry Sink, personal communication) and Oman (Sheppard et al. 1992). Graham et al. (2007a) were of the opinion that rather than representing ‘stepping stones’ to dispersal between hemispheres, these populations may be more continuous across the tropics than previously thought. Tests of this hypothesis await further observations in deeper tropical habitats. Interestingly, future studies of the ecosystems containing coelacanths, which occur in canyons with temperate upwelled water in the East African tropics, (Ribbink and Roberts 2006; Roberts et al. 2006) could produce information on these hypothesised kelp populations.
Species of Laminaria occur on both sides of the South Atlantic. On the west coasts of South Africa and Namibia, L. pallida grows mostly below 10 m and has a shorter, solid stipe in the southern part of its distribution and is dominant inshore, with a long, hollow stipe in the north (previously known as L. schinzii; Molloy and Bolton 1996; Stegenga et al. 1997). Lüning and Tom Dieck (1990) and Tom Dieck and de Oliveira (1993) proposed the likely scenario that these populations are derived from a dispersal event involving an ancestor of the Southeast Atlantic L. ochroleuca. Little is known of the Brazilian tropical deep-water populations of Laminaria occurring in a small upwelling near Cabo Frio. Tom Dieck and de Oliveira (1993) concluded, using hybridization studies, that the two species described were conspecific and digitate, and closely related both to eastern North Atlantic L. digitata and also, but less closely, to warm-temperate L. ochroleuca and to South African Laminaria. Either there have been two crossings of the Atlantic tropics by Laminaria, or the Brazilian and southern African Laminaria arose by dispersal from a single crossing. The latter seems plausible, bearing in mind the presence also of L. pallida on the mid-South Atlantic island Tristan da Cunha (Baardseth 1941).
Graham et al. 2007a maintain that “the western tropical Pacific and Atlantic Ocean harbour three species of deep-water Laminaria, all of which have primitive traits for the Laminariales… suggesting an ancient origin (>30 Mya) and relative permanence of deep-water kelp refugia” and that “the deep-water tropical kelp, L. philippinensis, is the most morphologically and ultrastructurally primitive extant kelp taxon”. These authors are clearly only considering the Laminariales in the narrow sense (only the derived ACLL families), and it is not clear on what they base their contention that Laminaria is primitive among the ACLL families. The molecular evidence does not suggest that the Laminariaceae are more ancestral (basal) in this group than the Lessoniaceae, for example (Yoon et al. 2001; Lane et al. 2006). It is clear that single blades have been produced more than once in the Laminariaceae and that the genus with most species with entire single blades (Saccharina) is more derived than the genus with most species with split single blades (Laminaria). Splitting of the blade and splitting of the stipe have both evolved a number of times within and between different groups (Cho et al. 2006; Lane et al. 2006). The contention that L. philippinensis is a primitive member of the genus Laminaria is presumably based on the lack of mucilage ducts (Petrov et al. 1973), although there is no study on the evolution of mucilage ducts to back up this hypothesis. In their recent review of these taxa, Bartsch et al. (2008) make no mention of this and indeed express doubt as to the taxonomic position of the little-known L. philippinensis. Nevertheless, the study of tropical deep-water kelps is essential for our further understanding of kelp evolution, particularly in the Southern Hemisphere. It should also be mentioned that deep-water kelps occur in temperate regions with clear water, such as Laminaria rodriguezii in the Adriatic Sea (Lüning 1990), or L. ochroleuca in the Strait of Messina at 60 m (Drew 1972), and these should be included in any study of tropical deep-water kelps.
The genus Macrocystis occurs in the Northeast Pacific, western South America, much of the southern Ocean, and Australasia. It is absent from the Northwest Pacific and North Atlantic. It likely arose in the Northeast Pacific and has presumably spread across the tropics to South America and thence to Australasia and South Africa via the West Wind drift as floating ‘rafts’ which can carry many other organisms (Waters 2008). Molecular data (ITS sequences) places all Southern Hemisphere isolates (formerly considered to represent three separate species) in the same clade, most closely related to specimens from the most southern sites in the Northeast Pacific (Santa Catalina Island, California and Bahia Tortugas, Baja California, Mexico) (Coyer et al. 2001). This is evidence that the crossing of the tropics by Macrocystis has only been successful on a single occasion.
Towards a species concept for kelps
This contribution discusses distribution data for kelp genera and species, and it needs to be explained that there are problems with what constitutes a species and a genus. There is no commonly accepted species concept in seaweeds in general (Wattier and Maggs 2001), and kelps are no exception to this. There are many reports of successful hybridization between accepted closely related species of Laminariales, and even members of different genera have been suggested to be at least partially fertile (Coyer et al. 1992). If the biological species concept were to be followed in kelps, it would require a clear definition of what constitutes a successful cross (hybridization event). As Liptak and Druehl (2000) and Bartsch et al. (2008) point out, the hybridization to the F1 generation is not sufficient, and what is required is evidence of hybridization at least to the F2 generation, combined with molecular evidence of true hybridization. A discussion is also necessary on whether it is feasible or useful to apply a biological species concept to kelps. There is also now good evidence for the development of female gametophytes without fertilization, with full-size parthenosporophytes occurring in nature (Oppliger et al. 2007). Liptak and Druehl (2000), however, provided molecular evidence for their production of a true hybrid sporophyte blade in crosses between Alaria marginata and Lessoniopsis littoralis. Lessoniopsis has since been shown to be a member of the Alariaceae (Saunders and Druehl 1993: previously in the Lessoniaceae) and thus this is molecular evidence of at least one intergeneric hybrid (rather than inter-familial as described by the authors, provided Lessoniopsis is indeed distinct from Alaria).
Most kelp species have been described based on morphological and anatomical characteristics of the sporophytes, although morphological characters are few and many are highly plastic. The best studied group is Laminaria
sensu lato (comprising Laminaria and Saccharina), reviewed by Bartsch et al. (2008). They conclude, for example, that molecular evidence shows that L. saccharina (now Saccharina latissima) “is a huge Pacific-Atlantic species complex with a broad plasticity, as had already been assumed after the successful hybridization experiments in this species complex in the 1970s and 1980s”. This species complex includes five currently recognised species in the North Atlantic, and many in the North Pacific. It is possible that further studies including molecular techiques, will reduce this number of recognised species, as has been the case recently with global Macrocystis (Coyer et al. 2001; Demes et al. 2009) and Northeast Pacific Alaria (Lane et al. 2007).
A number of studies have been carried out on population-level variation along environmental gradients within a species or between closely related species (e.g. wave action, Roberson and Coyer 2004; Henkel et al. 2007, or temperature, Bolton and Anderson 1987). In recent studies incorporating molecular evidence, a pattern is emerging of molecular variation along environmental gradients which correlates with a level of morphological variation which in the past may have been used to describe different species. Authors describe this as “incipient speciation” (e.g. Roberson and Coyer 2004, with Eisenia), or the more erudite “species in statu nascendi” (e.g. Miller et al. 2000, with Pelagophycus). In the other major group of large brown algae, the Fucales, similar patterns exist, with hybridization between recognised species as well as hybrid geographical populations (e.g. Coyer et al. 2002b, 2006, 2007). There is an enormous amount of work still to be done at the interface between population differentiation and speciation in the Laminariales.
The level of genus in the kelp taxonomic hierarchy is also not without contention with Druehl and Saunders (1992), in what they describe as a “speculative scheme”, proposing a reduction to nine genera only. More generally, the genetic relatedness between taxa at the same level of the hierarchy is less in brown than in red algae, for example (Saunders et al. 1992). This proposal for reduction in the number of kelp genera has thus far not been followed by later authors (notably including Druehl and Saunders themselves), although Cymathaere, Kjellmanniella and Hedophyllum are now considered species of Saccharina by Lane et al. (2006) (see also Yotsukura et al. 1999). There has, rather, been a slight increase in recognised genera, with Pseudolessonia (Cho et al. 2006), Aureophycus (Kawai et al. 2008) and even Druehlia (Lane et al. 2007; reduced to synonomy with the earlier Eualaria, Wynne 2009) being recently described, as well as the re-recognition of Saccharina (Lane et al. 2006). In addition, the literature on the Far-Eastern Seas of Russia is now becoming more widely known, which includes genera such as Phyllariella, Costularia and Feditia (Selivanova et al. 2007).
The problem of taxonomic rank in kelps needs much more data, both molecular and morphological on a wider range of taxa, to be clarified. It is possible that there are too many species recognised in this contribution in the genera Laminaria, Saccharina and Alaria. Although this has been thought to be the case for at least two decades, we are not much closer to solving the problem, particularly because no consistent species concept is used by different authors, and because no comprehensive studies have been carried out over the whole range of distribution of these genera. A good example is the study of Lane et al. (2007), who on molecular evidence conclude that there is only one species of Alaria in the Northeast Pacific, while still accepting a number of species in the Northwest Pacific, which have not been studied in this way. Another example of these difficulties is in Saccharina latissima and S. longicruris, the former with a short, solid stipe, and the latter with a long, hollow stipe. Lindstrom (2001) states that these two taxa are generally accepted as being synonymous. Bartsch et al. 2008 also point out that there is much evidence that the taxa are conspecific, but as final taxonomic decisions have not been published, ecologists continue to publish studies on Saccharina (or Laminaria) longicruris (e.g. Lyons and Scheibling 2007; Saunders and Metaxas 2007). To my knowledge, there has been no definitive molecular taxonomic study on the possible conspecificity of these two taxa, although Baartsch et al. (2008) cite the published abstract of Cho et al. (2006) and are of the opinion that “there is much evidence … that S. longicruris is conspecific with S. latissima. There needs to be a consensus on how to deal with “incipient speciation” within genera: that is the existence of morphological forms with relatively slight differences within species, which have different ecologies, and can be separated molecularly (Yotsukura et al. 1999; Miller et al. 2000; Erting et al. 2004; Roberson and Coyer 2004; Lane et al. 2007; Uwai et al. 2007). As Lindstrom (2001) states: “more work is required to understand vicariance in seaweeds, especially in deciphering when a speciation event has occurred”. Combined studies of the morphology, molecular systematics, and ecology of the speciation process will be important for our future understanding of kelp evolution. This has been better studied to date in the other main family of large brown algae, the Fucales (see Coyer et al. 2006 for a recent study on the genus Fucus). It is interesting to note the maintenance of the taxonomic rank of species for entities which hybridise in these detailed molecular studies (e.g. Coyer et al. 2002a, b; Billard et al. 2005), as well of the erection of new species on molecular/morphological evidence (e.g. Bergstrom et al. 2005).
To conclude, a new and potentially very productive phase in the study of kelp evolutionary biogeography has been initiated, where molecular tools are being used to test hypotheses. Kelps clearly have sufficient dispersal barriers to enable us to hypothesise about early evolution from present-day distributions. This is borne out by the fact that all genera, even those including floating species such as Macrocystis pyrifera, Nereocystis luetkeana and Ecklonia maxima, are absent from many world regions where they could grow (note for example that only floating Nereocystis has been recorded in cooler waters of the Northwest Pacific, where it has not apparently established, Selivanova et al. 2007). In addition, accidental or deliberate human introductions of the important human food items Undaria pinnatifida (wakame) and Saccharina species (kombu) from Japan to many world regions have proved successful (Uwai et al. 2006; Suzuki et al. 2007; Li et al. 2008). Species numbers in the three most diverse genera are being reduced, and it is very likely that this reduction will continue, although some taxonomic system that can recognise the greater range of morphological variation in these genera may be useful. It is possible that a few further genera will be subsumed, but also a small number are being discovered or newly recognised. Detailed molecular studies need to be combined with morphological and ecological studies in regions and taxa where speciation is occurring, and in those which are particularly interesting from a phylogeographical standpoint. These include the mostly cool temperate genera Laminaria, Saccharina and Alaria, as well as the four genera, mostly warm temperate taxa, which have dispersed into the Southern Hemisphere (Laminaria, Macrocystis, Eisenia and Ecklonia). It is particularly important for collaborative research to occur between scientists in the northeast and Northwest Pacific, as work in the past on kelps from these two main sites of diversity have generally included mostly species (and ideas) from one or the other. These studies need to include the Far-Eastern Seas of Russia (called here ‘Sea of Okhotsk) which is a major centre of kelp diversity, with a literature only recently becoming generally available (see Selivanova et al. 2007). The relationship between the only exclusively Southern Hemisphere genus (Lessonia) and its relatives is in need of further study to see if any light can be cast on its origins. Further studies of tropical deep-water kelp populations, particularly molecular phylogeography, would be of great interest. Finally, the ‘molecular clock’ approach needs to be applied using modern data to test possible dates of evolution of taxonomic levels in the newly circumscribed Laminariales.