INTRODUCTION
The family Mytilidae is one of the most widespread groups of bivalves which form the vast settlements in littoral and upper sublittoral zones of cold, boreal and subtropical waters of the World Ocean and often dominate in benthos communities (Seed, Reference Seed and B.L.1976; Scarlato, Reference Scarlato1981; Lukanin et al., Reference Lukanin, Naumov, Fedyakov and Lukanin1985; Zaika et al., Reference Zaika, Valovaya, Povchun and Revkov1990; Seed & Suchanek, Reference Seed, Suchanek and Gosling1992; Coan et al., Reference Coan, Scott and Bernard2000). The mytilids are also common in biofouling communities on anthropogenic substrata as well as being important objects in harvesting and cultivation (Mason, Reference Mason and B.L.1976; Lutz, Reference Lutz1980; Suprunovich, Reference Suprunovich1988; Hickman, Reference Hickman and Gosling1992).
On different data, from 13 to 16 mytilid species inhabit Peter the Great Bay in the Sea of Japan (Scarlato, Reference Scarlato1981; Kafanov, Reference Kafanov1991). Development of some mytilids is direct while in the life cycle of others there is a pelagic phase (Table 1). Although pelagic mytilid larvae have been intensively studied several species remained unidentified before our work began (Table 2). The purpose of the present study is to provide the most complete morphological descriptions of pelagic larvae of all Mytilidae species inhabiting the north-western part of the Sea of Japan in order to facilitate their identification in the plankton for population–biological studies including mariculture.
Developmental type in bivalve molluscs of the family Mytilidae from the north-western part of the Sea of Japan.
* As Mytilus edulis galloprovincialis.
List of studied pelagic bivalve larvae of the family Mytilidae inhabiting the north-western part of the Sea of Japan.
Larvae of 10 mytilid species from Peter the Great Bay in the Sea of Japan are described: Adula falcatoides Habe 1955; Crenella decussata (Montagu 1808); Crenomytilus grayanus (Dunker 1853); Modiolus kurilensis Bernard 1983; Musculista senhousia (Benson in Cantor 1842); Mytilus coruscus Gould 1861; Mytilus galloprovincialis Lamarck 1819; Mytilus trossulus Gould 1850; and Septifer keenae Nomura 1936 as well as an unidentified species Mytilidae indeterminate. The larval descriptions are based on both our own materials and literature data (Table 2).
MATERIALS AND METHODS
Bivalve larvae of the family Mytilidae from Vostok Bay and Possjet Bay of Peter the Great Bay in the Sea of Japan were used in this study. Larvae were sampled using a 96 µm plankton net every 5–10 d from water surface to the bottom in June–September, 2002–2005 in Vostok Bay and in June–September, 2002 in Possjet Bay.
Larvae were preserved in 96% ethanol. Shell morphology was studied by light microscopy and scanning electron microscopy (SEM). Specimens were cleaned with a 5–10% solution of sodium hypochlorite to remove the soft tissues (Rees, Reference Rees1950). The shells were thoroughly rinsed with distilled water and then embedded in glycerine jelly for light microscopy. For SEM the shells were dehydrated in 96% ethanol and sputter-coated with gold.
For the taxonomic identification of unknown mytilid larvae, planktonic specimens were kept in aquaria for some weeks up to identifiable postlarval or definitive stages. Also, the larval morphological features of the mytilid species from other parts of the World Ocean (Rees, Reference Rees1950; Loosanoff et al., Reference Loosanoff, Davis and Chanley1966; Chanley & Andrews, Reference Chanley and Andrews1971; Zakhvatkina, Reference Zakhvatkina and Vadyanitsky1972; Booth, Reference Booth1977; Le Pennec, Reference Le Pennec1980; Redfearn et al., Reference Redfearn, Chanley and Chanley1986; Fuller & Lutz, Reference Fuller and Lutz1989) were comparatively studied. Furthermore, our own knowledge of planktonic larval periods, gonad maturity of adult molluscs, water temperature in the areas investigated as well as the biogeographical characteristics of the species (Scarlato, Reference Scarlato1981) and data on life cycles of other mytilids (Table 1) were used for the specific identification.
Approximately 10–20 specimens of every species were used for studying the larval shell morphology. Since early developmental stages of bivalve larvae are hardly distinguished we here concentrate on pediveligers. Larval shell terminology is the same as in works of Rees (Reference Rees1950) and Chanley & Andrews (Reference Chanley and Andrews1971). Provinculum length represented the linear distance between the lateral extremes of the hinge. Shell length was the greatest dimension along any axis of the shell. Shell height was the greatest measurement perpendicular to the hinge line (Lutz & Hidu, Reference Lutz and Hidu1979; Fuller & Lutz, 1989).
RESULTS
In the family Mytilidae the larval shell is equivalve; its length is always more than its height. The shell shape varies from egg-shaped and triangular–oval to semicircular and D-shaped. Most of the examined species are non-equilateral, and the anterior shoulder is longer than the posterior one but in some species the shell is almost equilateral. In most species the anterior end is slightly pointed and the posterior end is more rounded. In some mytilids both ends are rounded and the anterior end is more obtuse. The ventral margin is mostly rounded but in some species it is almost straight. Umbones are usually well-marked and rounded, low, broad, massive or knob-shaped. In some species umbones are not pronounced or very low and hardly clear.
In most mytilids the larval shell surface is covered by more or less prominent co-marginal growth lines but in some species they are absent and the microsculpture is fine-grained. A dark eye-spot is either small and a dot, or large and in the form of a paint-brush stroke. The colour of preserved larva of different species varies from grey to yellow-brown and brown. Shell valves are thin and transparent or convex, massive and relatively non-transparent.
The larval hinge consists of large regular rectangular denticles of taxodont type located in both sides of provinculum and small denticles in its central part. The hinge line is straight or curved. In most species its length significantly exceeds an umbo length. The ligament is located in the central part of the provinculum and somewhat posteriorly displaced; the ligament pit is distinct.
Most mytilid larvae attain a size between 280 and 350 µm before settling, while in several species length of competent larvae does not exceed 190–200 µm.
Adula falcatoides (Figure 1A). The shell is triangular–oval, almost equilateral. The anterior end is raised in contrast to the posterior one; both ends are slightly pointed. The ventral margin is almost straight. The shell is low; a ratio of height to length is 0.75–0.77. Umbones are well-developed, broad and slightly inclined towards the anterior end. The shell is thick, convex and feebly transparent. Larvae are brown but umbones are more intensely coloured. The sculpture consists of distinct, almost regular co-marginal growth lines. The small eye-spot is well-marked.
The hinge is significantly curved and long. Its length amounting to 160–170 µm is considerably greater than umbo length. There are 11–13 large rectangular denticles in both anterior and posterior parts of the provinculum. In contrast to the other species the hinge lacks small denticles. The anterior margin of the umbo is located over the 6th–8th anterior provincular denticle. The ligament is placed in the central part of provinculum.
Larvae settle at a shell length of about 280 µm. Planktonic larvae occur from August to September in Vostok Bay and from July to September in Possjet Bay at water temperatures of 18–22°C and 17–21°C, respectively.
Musculista senhousia (Figure 1B). The shell is non-equilateral, triangular–egg-shaped. The anterior end is slightly pointed and somewhat raised; the posterior and ventral margins are rounded. The shell is high; a ratio of height to length is 0.89–0.92. Umbones are high, rounded and well-developed. The outer shell surface is covered by clearly marked co-marginal lines of growth. The valves are greatly convex and feebly transparent. Larvae are dark brown and umbones are more intensely coloured. The distinct eye-spot is small.
Each side of the provinculum bears 5–7 high rectangular denticles. In the central part of the hinge there are 10–12 denticles, which are smaller but well-marked, too. The hinge line about 100–110 µm long is arched and almost equal to umbo length. The anterior margin of the umbo is located over the first provincular denticle. The ligament is slightly shifted to the end of the central part of the hinge.
Larvae are relatively small; a shell length before settling is approximately 280 µm. In plankton of Vostok Bay and Possjet Bay larvae occur from July to September at water temperatures of 15–22°C and 17–21°C, respectively.
Crenella decussata (Figure 1C). The shell is equilateral, in the form of an almost regular semicircle. The anterior and posterior ends are rounded; the anterior end is more obtuse. The ventral margin is rounded. Umbones are absent. The shell is relatively low; a ratio of height to length is 0.78–0.79. The outer shell surface is fine-grained; the growth lines are absent. The inner broad marginal ridge is clearly visible from the outside of the shell. The larval valves are thin; larvae are dark-grey. The eye-spot is imperceptible.
The very narrow straight provinculum 120–125 µm long bears 8–10 anterior and posterior denticles; in the centre of the hinge line denticles are not distinct. The ligament is located in the central part of the provinculum nearer to its posterior end.
Larvae are small; they undergo metamorphosis at a shell length of 190–200 µm. In plankton of Vostok Bay larvae occur from August to September at a water temperature of 18–22°C.
Septifer keenae (Figure 1D). The shell is slightly non-equilateral, in the form of a semicircle. The anterior end is smoothly rounded; the posterior end is slightly pointed. The posterior margin is somewhat straightened when changing to the rounded ventral margin. The shell is very low; a ratio of height to length is 0.7–0.71. Umbones are absent. The outer shell surface is fine-grained; the growth lines are absent. There is a broad marginal ridge inside the valves. The shell is relatively thick; larvae are grey and slightly violet. The eye-spot is a dot.
The hinge line is straight; its length is about 125–130 µm. The hinge is represented by 4–5 large denticles on the provinculum ends and 18–22 small central denticles. Unlike other species of the family, in S. keenae the large denticles do not considerably differ from small ones by their height and width. The ligament is located in the central hinge part but displaced to the posterior provincular end.
Larvae are small; a shell length before metamorphosis is 180–190 µm. In plankton of Vostok Bay larvae occur from August to September at a water temperature of 18–22°C.
Mytilus coruscus (Figure 1E). The shell is non-equilateral, round–egg-shaped. The posterior shoulder is shorter than the anterior one; the obtuse and rounded posterior end is slightly oblique ventrally. The anterior end is somewhat pointed and slightly raised. The ventral margin is rounded. The shell is high; height/length ratio is 0.89–0.92. Umbones are rounded and small. The shell is relatively thin; larvae are greyish. The outer shell surface is covered by well-marked co-marginal lines of growth. The small eye-spot is clearly distinct.
The provinculum consists of 5–6 large denticles located on each side of the hinge and 15–21 small central denticles. The latter are broad in their bases and easily observed. The curved hinge line reaching 115–125 µm is somewhat longer than umbo length. The anterior margin of the umbo is located over the second or third provincular denticle. The ligament is placed at the end of the central provincular part.
Larvae are relatively large; larval shell length is 330–350 µm before settling. In Vostok Bay planktonic larvae occur from August to September at a water temperature of 18–22°C; in Possjet Bay they are common from July to September at a water temperature of 17–21°C.
Mytilus galloprovincialis (Figure 1F). The shell is non-equilateral, round–egg-shaped. The anterior shoulder is longer than the posterior one. The anterior end is slightly pointed and somewhat raised; the posterior end is broader but both ends are rounded. The ventral margin is also rounded. The shell is relatively high; a ratio of height to length is 0.86–0.88. Umbones are rounded and low. The shell is thick and larvae are yellowy-brownish. The shell surface is covered by regular co-marginal growth lines. The eye-spot is large and clear.
The hinge consists of 7–8 large denticles on each side of the provinculum and 16–18 small denticles in the centre. The hinge line 135–145 µm long is slightly curved and exceeds umbo length. The anterior umbo margin is placed over the 5th–6th provincular denticle. The ligament is located at the end of the central hinge part nearer to the posterior provincular denticles.
Larvae are relatively large; they settle at a shell length of approximately 330–350 µm. In plankton of Vostok Bay larvae occur from July to September and in Possjet Bay from June to August at water temperatures of 15–22°C.
Mytilus trossulus (Figure 1G). The shell is non-equilateral, triangular–egg-shaped. The anterior shoulder is slightly longer than the posterior one. The anterior end is pointed and slightly raised. The posterior and ventral margins are rounded. The shell is relatively high; height/length ratio is 0.84–0.87. Umbones are knob-shaped, rounded and narrow. The shell sculpture is represented by thin co-marginal lines of growth. The valves are relatively thin; larvae are yellowish. The distinct eye-spot is small.
The hinge consists of 8–10 large denticles on both sides of the provinculum and 17–20 small denticles in its central part. The hinge line is slightly curved; its length exceeding umbo length reaches 130–140 µm. The anterior margin of the umbo is located over the 6th–7th provincular denticle. The ligament is placed at the end of the central part of the provinculum.
Metamorphosis begins at a shell length of 300–330 µm. In plankton of Vostok Bay larvae occur from June to August at a water temperature of 15–22°C.
Crenomytilus grayanus (Figure 1H). The shell is non-equilateral, oval–elongated. The anterior shoulder is longer than the posterior one; the anterior end is slightly pointed and somewhat raised. The rounded posterior end is slightly drawn ventrally and smoothly changes to rounded ventral margin. The shell is not high; height/length ratio is approximately 0.83. Umbones are broad, rounded and low. The shell is transparent and thin. Larvae are yellowy-greyish, while umbones are more intensely coloured. The sculpture consists of fine co-marginal growth lines. The clearly distinct eye-spot is small.
The hinge line is slightly curved; its length exceeds the umbo length and reaches 130–135 µm. The hinge consists of 8–9 large denticles on both sides of the hinge and 21–25 small central denticles. The anterior margin of the umbo is located over the 4th or 5th provincular denticle. The ligament is placed behind the centre of provinculum at the end of the row of the small denticles.
Metamorphosis begins at a shell length of about 280–300 µm. In plankton of Vostok Bay and Possjet Bay larvae are common in July–September at a water temperature of 15–22°C and 17–21°C, respectively.
Modiolus kurilensis (Figure 1I). The shell is non-equilateral, oblong–egg-shaped. The anterior shoulder is longer than the posterior one; the pointed anterior end is slightly raised. The postero-ventral margin is clearly drawn ventrally; the ventral margin is rounded. The shell is relatively high; a ratio of height to length is 0.85–0.89. Umbones are large, broad and low. The shell is thick and convex resulting in very sharp shell outlines. The outer shell surface is covered by clear co-marginal lines of growth. Larvae are yellow-brownish. The large eye-spot has a distinguishing shape of paint-brush stroke.
The hinge consists of 8–9 large denticles on each side of the hinge row and 18–21 small denticles in its centre. The hinge line is slightly curved; its length reaching 150–155 µm is somewhat greater than the length of the umbo. The anterior margin of the umbo is located over the 5th–6th provincular denticle. The ligament is displaced to the end of the central part of the provinculum.
Larvae are relatively large; they settle at a shell length of 300–330 µm. In Vostok Bay planktonic larvae occur from August to September at a water temperature of 18–22°C; in Possjet Bay larvae are common in July–September at a water temperature of 17–21°C.
Mytilidae indeterminate (Figure 1J). The shell is non-equilateral, triangular–egg-shaped. The posterior shoulder is shorter than the anterior one; the broad rounded posterior end is greatly drawn ventrally. The anterior end is pointed and considerably raised. The ventral margin is somewhat bulging. The shell is not high; a ratio of height to length is 0.79–0.82. Umbones are very low, flat and broad. The shell is thin. Larvae are greyish; their postero-ventral parts are yellowish. The sculpture is represented by fine co-marginal growth lines. The small eye-spot is well-marked.
The hinge consists of 5–7 large denticles on each side of the provinculum and 22–26 very small central denticles. The hinge line is little curved; its length reaching 145–155 µm is greater than umbo length. The anterior margin of the umbo is placed over the 4th–5th provincular denticle. The ligament is located in the central part of the hinge but somewhat displaced towards its posterior end.
A shell length is approximately 280–300 µm before settling. In plankton of Vostok Bay larvae occur from July to September at a water temperature of 17–22°C.
DISCUSSION
Pelagic larvae of the family Mytilidae can be distinguished by their shell outlines; dimensional parameters including length of the anterior and posterior shoulders; shell height to length ratio; outlines and size of umbones; sculpture; colour, and shape and size of an eye-spot. The hinge of the taxodont type consists of large regular rectangular denticles located on each end of the hinge line and, in most cases, small denticles in the central part of the provinculum that is invariably a common internal feature of larvae of all mytilid species. Also in all mytilid larvae studied the ligament is placed in the central part of the provinculum but its closeness to the posterior provincular part varies in different species.
The larval shells of A. falcatoides and M. senhousia (Figure 1A & B) are characterized by well-developed large umbones. In both species shells are thick, convex, feebly transparent and dark with more intensely coloured umbones. The co-marginal growth lines are distinct. At the same time in A. falcatoides unlike other mytilid larvae the ventral margin is almost straight and a shell height is considerably less than its length. Larvae of M. senhousia are relatively small. In both species the hinge line is more curved than in other species of the family, and in M. senhousia it is almost equal to the umbo length but in A. falcatoides hinge length considerably exceeds umbo length. Adula falcatoides also develops more large provincular denticles than other mytilids and lacks central denticles. The anterior umbo margin is located over the 6th–8th anterior denticles but in M. senhousia it is placed almost over the beginning of the provinculum.
Morphology of larvae of the family Mytilidae: (A) Adula falcatoides; (B) Musculista senhousia; (C) Crenella decussata; (D) Septifer keenae; (E) Mytilus coruscus; (F) Mytilus galloprovincialis; (G) Mytilus trossulus; (H) Crenomytilus grayanus; (I) Modiolus kurilensis; and (J) Mytilidae indeterminate: 1, general view, scale bar: 50 µm; 2, hinge, scanning electron microscopy, scale bar: 20 µm (l, left valve; r, right valve).
Larvae of C. decussata and S. keenae (Figure 1C & D) distinctly differ from other mytilid larvae by D-shaped shell, absence of umbo, the straight hinge margin, and homogeneous fine-grained sculpture without co-marginal lines of growth. These shell characters indicate lecithotrophic development (Ockelmann, Reference Ockelmann, Cox and J.F.1965; O'Foighil, Reference O'Foighil1986; Jablonsky & Lutz, Reference Jablonski, Lutz, Rhoads and Lutz1980; Lutz & Kennish, Reference Lutz, Kennish and Gosling1992; Table 1). Larvae of S. keenae are smaller and their shells are lower, thicker and more antero-posteriorly extended than those of C. decussata. The hinge of C. decussata consists of a larger number of marginal provincular denticles and the central denticles are not so clear. In S. keenae the posterior, anterior and central provincular denticles do not greatly differ in shape and size.
In M. coruscus (Figure 1E) a ratio of shell height to its length is larger than in other species of the family. The anterior shell end is less pointed than in M. trossulus. The shell is thinner; the larva is lighter coloured. The shell sculpture is represented by more distinct co-marginal lines of growth. Quantity of the anterior and posterior provincular denticles is considerably less than in M. trossulus and M. galloprovincialis and its hinge line is shorter. The anterior margin of the umbo is located closer to the beginning of the provinculum.
Larvae of M. galloprovincialis and M. trossulus are superfacially similar (Figure 1F & G). But in M. galloprovincialis the larval shell is higher and its umbones are broader. The shell is thicker; the larva is darker. There are fewer large denticles than in M. trossulus and the anterior margin of the umbo is closer to the beginning of the hinge line. The ligament is placed closer to the posterior part of provinculum.
Unlike larvae of M. trossulus, larvae of M. kurilensis (Figure 1I) are larger and their umbones are also larger and broader. The anterior shell end is more pointed and the postero-ventral margin is distinctly drawn out ventrally. Because the larval shell of M. kurilensis is more convex and thicker the shell outline is better defined and the larva is less transparent. The co-marginal lines are clearer and more distinct than in M. trossulus. The larva of M. kurilensis is darker and its eye-spot is larger being in the form of a paint-brush stroke. In M. trossulus the anterior umbo margin is located somewhat farther from the beginning of the provinculum as compared to the provinculum of M. kurilensis.
Larvae of C. grayanus (Figure 1H) differ from larvae of M. trossulus and M. kurilensis by a lower shell, broader umbones and a more rounded anterior margin. The postero-ventral shell margin is considerably less drawn out ventrally than in M. kurilensis. The shell is thinner; the larva is light-brown. The co-marginal lines are less distinct. The eye-spot is smaller than in M. trossulus. The provinculum of C. grayanus bears more central denticles. The anterior umbo margin is located over the 4th–5th anterior provincular denticle, while in M. trossulus and M. kurilensis the anterior margin of the umbo is placed over the 6th–7th and the 5th–6th denticle, respectively.
An unidentified species Mytilidae indeterminate (Figure 1J) was found to belong to the family Mytilidae due to the comparative study of the larval morphological features in the mytilid species inhabiting the north-western part of the Sea of Japan (Table 2) and other areas of the World Ocean (Rees, Reference Rees1950; Loosanoff et al., Reference Loosanoff, Davis and Chanley1966; Chanley & Andrews, Reference Chanley and Andrews1971; Zakhvatkina, Reference Zakhvatkina and Vadyanitsky1972; Booth, Reference Booth1977; Le Pennec, Reference Le Pennec1980; Redfearn et al., Reference Redfearn, Chanley and Chanley1986; Fuller & Lutz, Reference Fuller and Lutz1989). Larvae of Mytilidae indeterminate are characterized by the morphological features peculiar to the mytilid larvae, namely shell outlines, its dimensional ratios, hinge of the taxodont type, location of ligament, sculpture, and presence of an eye-spot. Larvae of species examined mainly differ from other mytilid larvae, particularly M. kurilensis, by a more oblong and almost triangular shell, greatly drawn out postero-ventral margin, more raised anterior end, almost straight ventral margin, and very low flat umbones. The shell of Mytilidae indeterminate is thinner; a larva is greyish. The hinge line is relatively long and slightly curved. The provinculum bears a larger number of very small central denticles than in M. kurilensis, while the quantity of large denticles on each side of the hinge is considerably less.
Since the pelagic larvae of all mytilid species inhabiting the north-western part of the Sea of Japan are identified, it might be supposed that larvae of Mytilidae indeterminate are brought into Peter the Great Bay from warmer areas of the Sea of Japan or over subtropical waters from the Yellow and East China Seas through the Korean Strait (Yurasov & Yarichin, Reference Yurasov and Yarichin1991). Our observations carried out earlier would support this hypothesis. So, larvae of Mytilidae indeterminate settling on hard substrata do not survive during the cold seasons. The maximum length of juvenile specimens of this species, which were collected in different biotopes, does not exceed 400–450 µm. Nevertheless, no descriptions of larvae with similar morphological features, which inhabit other areas of the Asian part of the Pacific Ocean, were found in the literature (Miyazaki, Reference Miyazaki1962; Hayashi & Terai, Reference Hayashi and Terai1964; Tanaka, Reference Tanaka1979b; Sakai & Sekiguchi, Reference Sakai and Sekiguchi1992; Ozawa & Sekiguchi, Reference Ozawa and Sekiguchi2002).
The hinge morphology is one of the most important criteria in taxonomy both of adult bivalves and their larvae (Rees, Reference Rees1950; Cox, Reference Cox and Moore1969; Turner & Boyle, Reference Turner and Boyle1975; Lutz & Hidu, Reference Lutz and Hidu1979; Le Pennec, Reference Le Pennec1980; Lutz et al., Reference Lutz, Goodsell, Castagna, Chapman and Newell1982). The larval hinge as the most invariable and determinant character permits not only identification of the larvae but also defining the taxonomic status of the species. So, Coan & Scott (Reference Coan and Scott1997) and Coan et al. (2000) recently ascertained that M. kurilensis is a synonym for M. modiolus, which is widely distributed in the Atlantic Ocean and the Pacific waters off the American coast. However, the comparative study of the larval morphological features in M. kurilensis from the Sea of Japan and M. modiolus from the Atlantic waters (Lutz & Hidu, Reference Lutz and Hidu1979; Fuller & Lutz, Reference Fuller and Lutz1989) showed significant differences exceeding the specific level. In M. modiolus there are no more than 5–6 large denticles on each side of provinculum of late veliconch larvae, whereas the hinge of M. kurilensis bears 8–9 anterior and posterior provincular denticles. The anterior margin of the umbo is located over the 5th–6th denticle in M. kurilensis and over the first or second denticle in M. modiolus. In addition, in the latter the hinge line is appreciably longer than in M. kurilensis. Externally, the shell of M. kurilensis is less dorso-ventrally extended. Thus, based on the comparative study of the larval shell morphological features, particularly the provinculum morphology, it appears that Coan's revision of the specific status of M. kurilensis from the Sea of Japan is erroneous.
ACKNOWLEDGEMENTS
We are very grateful to Dr Valery A. Brykov (A.V. Zhirmunsky Institute of Marine Biology, Vladivostok) who provided us with plankton samples from Possjet Bay in the Sea of Japan. We greatly appreciate Dr Valentina A. Kulikova (A.V. Zhirmunsky Institute of Marine Biology, Vladivostok), Dr Nikolaus Malchus (Universitat Autònoma de Barcelona, Barcelona) and anonymous referees for their critical reading of the manuscript and very helpful comments. We are also very grateful to Dr Nikolaus Malchus for his thorough correction of the English.
This study was partially funded by grant 06-III-A-06-161 from the Presidium of the Far Eastern Branch of the Russian Academy of Science.
