Download Latitudinal and bathymetric trends in egg size variation: a new look

Marine Ecology. ISSN 0173-9565
REVIEW ARTICLE
Latitudinal and bathymetric trends in egg size variation:
a new look at Thorson’s and Rass’s rules
Vladimir Laptikhovsky
Falkland Islands Fisheries Department, Stanley, Falkland Islands
Keywords
Egg size; fish; invertebrate; larval; Rass;
reproduction; reproductive strategy; Thorson.
Correspondence
Vladimir Laptikhovsky, Falkland Islands
Fisheries Department, Stanley, FIQQ 1ZZ,
Falkland Islands.
E-mail: [email protected]
Accepted: 5 December 2005
doi:10.1111/j.1439-0485.2006.00077.x
Abstract
The inverse relationship between egg size in marine animals and water temperature was simultaneously described by two outstanding marine scientists: G.
Thorson and T.S. Rass. This rule consists of two different phenomena. Thorson’s rule describes ecological processes related to changes in larval biology and
morphology that are caused by a selective pressure of natural selection on the
different types of larval development. It belongs to the realm of macro-evolution. Rass’s rule describes physiological processes within populations and species, and between closely related species. This is not related to changes in
reproductive strategy, and therefore belongs to the realm of micro-evolution
and to the early stages of macro-evolution. Populations begin to produce larger
eggs in colder environments because of phenotypic plasticity. Thorson’s rule
describes temperature-dependent changes in the relative abundance of smalland large-egged species, whereas Rass’s rule describes a temperature-dependent
relative position of both groups within an adaptive range of reproductive
strategies.
Problem
Egg size and fecundity are very important features of a
reproductive strategy and are inversely related. This is
because the energy that can be invested into reproduction
is restricted, leading to a trade-off between quality and
quantity of offsprings (Kasyanov 1999). For this reason,
differences in egg size between closely related populations,
species and higher taxa could be a ‘hallmark’ of habitatdependent evolutionary patterns of reproductive strategies. Egg size is generally species-specific, although
within the species it may vary within the population, e.g.
with female size, food availability or spawning season.
Between species, there is also an increasing trend in egg
size with a decrease of temperature.
This trend is called the ‘Thorson–Rass’s rule’ or ‘Thorson’s rule’ (Mileykovsky 1971) and was described simultaneously by these two scientists. Over the years it has
attained a paradigm status (Levin & Bridges 2001) and is
well known to all marine biologists. The expression
‘Thorson–Rass’s rule’ is more common in Russian literature, whereas ‘Thorson’s rule’ is widely recognised in the
rest of the world. The third name, ‘Rass–Thorson–
Marshall’s rule’ has also been proposed (Alekseev 1981)
in recognition of the work of Marshall (1953) who
brought data from Antarctica into the discussion.
Marshall (1953) compared the results of both authors
and indicated that ‘while both workers consider that
selective processes have been responsible for these parallel
trends, Thorson has taken into consideration biological as
well as physical factors, while Rass has been more
impressed with temperature condition’. Marshall (1953)
also concluded that ‘… in trying to appreciate the factors
which have conditioned the increase in egg size of polar
and deep-sea fishes, Thorson’s (1950) approach has been
followed rather than that of Rass (1941). The ecological
rule proposed by Rass… is an oversimplification’. This
point of view remains unchallenged to date.
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7
Egg size variation: Thorson’s and Rass’s rules
Laptikhovsky
Half a century has passed since the synthetic work of
Marshall (1953) was published. New data cast doubts as
to whether both authors really described the same phenomenon, as commonly thought, or if there are several
different ecological phenomena provoking the same ‘visible’ result? Further doubts arise from the temperaturedependent changes in egg size observed in genetic studies.
The aim of this paper is to have a closer look at both
rules, their implementation in natural habitats and their
relation to one another.
Results
Thorson’s rule (changing reproductive strategy)
Thorson (1936, 1946, 1950) described a trend toward
increased egg size and non-planktonic, non-feeding development with increasing latitude and water depth. Both
gradients involve a decrease of water temperature. Most
of his work was based on gastropod molluscs from Thailand, Denmark, and Greenland, i.e. on a taxon, whose
early stages of development range from pelagic larvae to
that of direct development.
Thorson provided a mostly ecological explanation for
the phenomenon, attributing these trends to restricted
food availability for planktotrophic larvae in colder
waters. Later, exceptions were found in some of the most
abundant species in polar seas that have feeding larvae
(Pearse & Lockhart 2004). Poulin & Feral (1996) suggested that Thorson’s explanation was specific to the iceage when ice shelves when sea ice covered much more of
the sea, limiting productivity, and now we probably
observe its evolutionary consequences.
Currently the original formulation of Thorson’s rule is
replaced by one in which latitudinal and bathymetric
shifts are observed in the proportions of larval planktotrophy and larval lecithotrophy (Vance 1973a,b; Levin &
Bridges 2001). It assumes a generalisation to taxa which
are not necessarily closely related. For example, in echinoids and bivalves, latitudinal gradients in development
exist because predominantly brooding taxa dominate in
the Southern Ocean (Levin & Bridges 2001). This phenomenon is almost certainly not an adaptation to low
temperatures and low larval food supply, as was supposed
by Thorson. Instead, the species-rich clades of brooders
probably reflect enhanced speciation under unique conditions in the Antarctic that did not happen in the Arctic,
where brooders are rare (Pearse & Lockhart 2004).
Because Thorson’s rule involves a generalisation of data
gathered from many different taxa and ecological groups,
there are many exceptions to this rule (reviews: Clarke
1992; Levin & Bridges 2001; Pearse & Lockhart 2004). For
example, it was found that the reproductive pattern
8
observed in marine gastropods along the Pacific coast of
South America fits the predictions of Thorson’s rule very
well, whereas benthic development predominates along
the Atlantic coast. The scarcity of pelagic development
among the Atlantic prosobranch gastropods is related to
the near-continuous soft-bottom habitat and consequent
prevalence of non-pelagic patterns of development that
evolved (Gallardo & Penchaszadeh 2001).
Reproductive strategies of most marine invertebrate
taxa exist in an adaptive range from relatively highly
fecund species with small eggs to relatively low-fecundity
species with large eggs. Within this adaptive range of
reproductive strategies, two more or less separate units of
small- and large-egg species exist. This phenomenon has
been shown by many authors both theoretically and
empirically. The mathematical model of Vance (1973a,b)
predicted that only the extremes of the range of egg size
and method of nutrition (i.e. planktotrophy, lecithotrophy) were evolutionary stable. Christiansen & Fenchel
(1979) presented a modified model with more realistic
parameters (sigmoid larval growth and size-dependent
mortality) and obtained results similar to those of Vance
(1973a,b). Further advance has been made since then. For
example, Roughgarden’s (1989) model included all
aspects of the life cycle. This model also demonstrated
that intermediate egg sizes have a disadvantage. One of
the interesting results of Roughgarden’s (1989) model was
that at some particular combination of growth and mortality rates, natural selection will favour the production of
both smaller and larger eggs simultaneously (Havenhand
2001), i.e. contributing to a separation of one species into
two with different-sized eggs. In this respect it is remarkable, particularly as most early reports of poecilogony
have proved to involve cryptic or sibling species (Levin
et al. 1991; Levin & Bridges 2001). The existence of sister
species differing in egg size was found, for example, in
octopods (Octopus bimaculatus – O. bimaculoides) (Pickford 1949) – a taxon which is very far from poecilogony.
However, this phenomenon could form an important
basis for the divergence in reproductive strategies and
thus species evolution.
Since then, much evidence of bimodal egg size distribution has been found in molluscs (Fig. 1), echinoderms
and fish (Kock 1992; Winemiller & Rose 1992; Nigmatullin & Laptikhovsky 1994; Sewell & Young 1997; Laptikhovsky 1998; Jaeckle 2001; North 2001) mirroring the
evolution of alternative reproductive strategies (see:
Brockmann 2001). Despite the fact that there are some
exceptions, it is likely that the mechanisms outlined by
Vance (1973a,b) and by Christiansen & Fenchel (1979)
are indeed valid (Havenhand 2001). These exceptions
occur because the evolution of egg size and larval type is
influenced by more factors than those considered by these
Marine Ecology 27 (2006) 7–14 ª 2006 The Author. Journal compilation ª 2006 Blackwell Publishing Ltd
Laptikhovsky
Egg size variation: Thorson’s and Rass’s rules
ronmental temperatures, but biotic factors probably play
the most important role in the strategy ‘choice’.
Frequency (%)
40
30
Rass’s rule (a strategy unchanged)
20
10
0
0
2
4
6
8
10
12
Egg length (mm)
14
16
28
35
Frequency (%)
30
25
20
15
10
5
0
0
4
8
12
16
20
24
28
Relative egg length (% mantle length)
Fig. 1. Distribution of egg size in 82 bottom octopod species representing about a half of all known species (data from Sweeney 1992;
Voss et al. 1998).
authors (Havenhand 2001). Environmental patterns may
diminish the occurrence of a particular reproductive style
until it disappears from the habitat.
For example, notothenoid fishes exhibit two reproductive strategies (small and large eggs) in the zone of seasonal pack ice presence, but are represented by only largeegged species in the proximity of Antarctic shores (Kock
1992). Both small- and large-egged groups have the same
type of development although their larval biology is different (North 2001). Octopods and some fish have a
direct development imitating the ecological larval stage
(Boletzky 1974; Poulin et al. 2001). In the tropics and in
temperate latitudes they are represented by species with
both small- and large-eggs, whereas in polar regions only
the large-egg species exist. These two examples also demonstrate the pressure of natural selection on the production of either small or large eggs, which can occur in
ecological groups with only one extant developmental
mode.
Therefore, Thorson’s rule reveals itself within the adaptive range of taxon’s reproductive strategies in the past as
the result of different pressure of natural selection on species with alternative developmental strategies favouring one
developmental mode and suppressing another one. This
leads to differences in species diversity between small- and
large-egg groups, and occasionally can result in the total
removal of a particular reproductive strategy from a habitat. To some extent, this phenomenon is related to envi-
A similar phenomenon was described by Rass (1935, 1941,
1986), who stated it as follows: ‘‘Egg size in aquatic animals
is in accordance with breeding temperatures. Among closely related species the egg size is inversely proportional to
the temperatures at the moment of spawning: the eggs
being larger, the nearer to the pole lies the species range’’
(Rass 1941). His results were based mostly on marine teleost fish, i.e. on a taxon with only one developmental type
(in contrast to most benthic invertebrate taxa), although
some data on amphibians and invertebrates were also used.
Initially, he applied his discovery to closely related species
(Rass 1941), but later indicated that this phenomenon
could be found within species when populations from different latitudes were compared. He never extrapolated this
assumption to large taxa.
This phenomenon has been reported not only in marine fish, but also in fresh-water crustaceans (review:
Khmelyova 1988), newts, frogs and toads (review: Rass
1986), although there are some exceptions (Lawrence &
Sastry 1985; Kokita 2003). An inverse relation between
egg size and temperature is also displayed by reptiles and
birds but in a different altitudinal direction: from lowland
plains to mountains (review: Badyaev & Ghalambor
2001). It is evident that there are no changes in developmental mode involved in this phenomenon, and as such
ecological mechanisms proposed by G. Thorson and
others are not applicable. Also, the trend cannot be
explained by a combination of Bergmann’s rule (an
increase of animal size from equator poleward) and a
positive relationship between egg size and female size.
Polar fish species, which produce the largest eggs, are
usually much smaller in body size than their temperate
relatives from the same family (Rass 1986).
The phenomenon described by T.S. Rass likely has different reasons from those of Thorson’s rule, and these
reasons are not precisely understood to date. Rass (1986)
hypothesised that it was caused by non-proportional
slowing down of the different stages of oogenesis at lower
temperature that causes species-specific egg size to be larger. A temperature decrease has a similar effect on the
timing of embryogenesis and provokes an increase in the
species-specific number of vertebrae regardless of adult
fish size. Among closely related fish species the number of
vertebrae unambiguously increases poleward with egg size,
not with adult size (Rass 1986). Changes in egg size, described by T.S. Rass happen in every fish family regardless
of whether those representatives produce small or large
eggs. Egg size increases with a decrease of temperature in
Marine Ecology 27 (2006) 7–14 ª 2006 The Author. Journal compilation ª 2006 Blackwell Publishing Ltd
9
Egg size variation: Thorson’s and Rass’s rules
Laptikhovsky
females from the same population produce larger eggs at
lower temperature. This phenomenon was briefly mentioned by Rass (1986) but not defined separately despite
the fact that it is not evidently subjected to natural selection. However, both phenomena are tighly intermingled.
For example, when females from three different latitudinal populations of fish, Pomacentrus coelestis, were
reared in captivity at different temperatures, egg size
decreased with increasing temperature in all populations
(Kokita 2003). However, at each temperature egg size was
always larger in the population with larger egg size in the
natural habitat, demonstrating that changes in egg size
were caused by interaction between both genotypic and
phenotypic variation. Thus latitudinal patterns of egg-size
variation may differ at the intra- and interspecific level
(Kokita 2003).
It is possible that this phenotypic plasticity could be
partially attributed to lower demands on metabolism at
lower temperature by poikilotherms, allowing them to
invest more energy into individual eggs with the same
genetically pre-determined fecundity. For instance, the
fecundity of genetically identical (Shaw et al. 2004)
autum–winter and spring–summer spawners of the squid
L. gahi at the same body size is similar, but the egg size is
always larger in autumn–winter spawners (Fig. 2), which
reproduce at lower temperatures (Laptikhovsky et al.
2002). However, it is not a general rule, because egg size
usually is inversely related to fecundity (Vance 1973a).
Phenotypic plasticity in egg size was revealed in insects,
particularly in Drosophila melanogaster, in which egg size
Oocyte diameter (mm)
A
B
Actual fecundity
the small-egg fish family Carangidae (egg size 0.6–
1.3 mm) as well as in a large-egg genus Cateroproctus, Liparidae (egg size 4.5–9.0 mm) (Rass 1986).
Latitudinal and bathymetric differences in temperatures
and egg size between ranges of related species probably
invoke different physiological adaptations, particularly
maturation patterns such as yolk composition. Eggs of
echinoderms and molluscs of the White Sea produced at
lower temperatures (warm season spawning versus cold
season spawning) contain more lipids in their yolk (Kaufman 1977). A relative increase of lipid content with an
increase of egg size was observed not only when small
eggs of planktotrophic species were compared to large
eggs of lecithotrophic species, but also when compared to
different-sized eggs of planktotrophic species (McEdward
1986; Jaeckle 2001). Specifically, planktotrophic eggs contained relatively more proteins than lecithotrophic eggs
(40.8–59.9, mean 49.5% versus 22.2–38.4, mean 29.2%),
and less lipids (19.6–23.6, mean 21.5% versus 34.5–50.3,
mean 41.7%) (Jaeckle 2001).
In contrast to Thorson’s rule, Rass’s rule is not related
to changes in larval biology and does not assume a switch
from one reproductive strategy to another, although some
elements of heterochrony are possible. Yolk accumulates
in greater amount at a lower temperature to such an
extent occasionally that it provides nutrition not only to
embryos/larvae development but also for juveniles (Lawrence & McClintock 1994). For example, eggs of the coldwater boreal loliginid squid Loligo forbesi are larger than
those of relatively warm-water L. vulgaris, but the duration of embryogenesis is not sufficient to utilise all of the
yolk. While L. vulgaris spends almost all of its yolk before
hatching, L. forbesi goes on to use it for growth long after
that (Boletzky 1987). The same was found in the sole
(Baynes & Howell 1996). Another cold-water loliginid
squid, Loligo gahi, whose embryogenesis is shorter than
the period expected from the relation between developmental time, egg size, and temperature (Arkhipkin &
Middleton 2003), also hatches with a big yolk sac,
demonstrating an excess of accumulated yolk.
Given this, Rass’s rule describes synchronous temperature-dependent changes in egg size in both small- and
large-egg species. In colder environments, these distinctive
groups shift within the adaptive range of reproductive
strategies to larger eggs and lower fecundity, while the
‘ruthless’ natural selection, following Thorson’s rule,
re-distributes the occurrence of these strategies within
different habitats.
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
100
120
140
160
180
200
180
200
6 000
5 000
4 000
3 000
2 000
1 000
0
100
120
140
160
Mantle length (mm)
Phenotypic plasticity
Rass’s rule also overlaps with a third phenomenon: phenotypic plasticity in egg size. This occurs when similar-sized
10
Fig. 2. Egg size (A) and fecundity (B) in the squid Loligo gahi.
Autumn–winter spawners are shown by black circles, spring–summer
spawners – by empty circles.
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Laptikhovsky
increases with a decrease of temperature, In this species
there was also no indication of genotype–environment
interaction for egg size, and evolution in egg size cannot
be accounted for by differences in adult body size
between populations (Azevedo et al. 1996). A similar phenomenon of plastic egg size increase at lower temperature
under experimental conditions (with respective fecundity
decrease) was found in a tropical butterfly Bicyclis anynana and some other arthropods (review: Steigenga et al.
2005). This trend could be disguised and even reversed by
biotic factors. For example, a copepod Tortanus discaudatus produced the largest eggs in the warmest time
(August), although it happens at the lowest rates of daily
egg production–four times lower than in January and 12
times lower than in May (Lawrence & Sastry 1985).
Therefore, this phenomenon could probably be explained
by some stress-induced changes in oogenesis provoked by
extremely high temperature.
An increase of the cell size with a decrease of temperature is not specific to oocytes only. It was found in epidermal cells of D. melanogaster (Azevedo et al. 2002), in
cells of a planarian (Romero & Baguna 1991), and in the
red blood cells of a fish (Van Voorhies 1996). It is
remarkable that the increase in cell size in the fruit fly
occurred with no significant change in cell number, and
flies reared at lower temperature were larger (Azevedo
et al. 2002). Subsequently, one may suppose that the
Thorson–Rass rule and the Bergmann rule are general
expression of the same natural phenomenon (Van Voorhies 1996). This is generally true, because an egg (at least
– a fertilised egg) and an adult animal are both literally
entire organisms at a different developmental stage.
Remarkably this phenotypic plasticity can be inherited,
bridging a phenomenon which is of no evolutionary
importance, and Rass’s rule which is subjected to a natural selection. Populations of the fruit fly, D. melanogaster
that were held at 16.5 C for 9 years produced larger eggs
at 25 C than populations that evolved at 25 and 29 C
(Azevedo et al. 1996). A supposedly phenotypic trait
became inherited! Recent investigations in insects revealed
that egg size responds in a plastic manner to oviposition
temperature, that egg size is heritable, and there seems to
be genetic variation in the phenotypic reaction to temperature. The supposed existence of genetic variation in
the temperature reaction norms challenges the widely
held notion that temperature-mediated plasticity (in egg
and body size) might be a purely physiological constraint
(review: Steigenga et al. 2005).
Discussion and conclusions
The problem of latitudinal changes in egg size and reproductive modes was tackled in the middle of the 20th cen-
Egg size variation: Thorson’s and Rass’s rules
tury, when the two authors began to investigate it from
different points of view. Thorson (1936, 1946, 1950)
emphasised an importance of evolution of the different
developmental styles, partially because he was working
mostly on gastropods – a taxon which possesses multiple
developmental modes. He attributed developmental
trends to different food availability in different latitudes.
Rass (1935, 1941, 1986) worked mostly on closely related
fish species that possess the same developmental mode.
He attributed egg size trends to temperature-dependent
changes in yolk accumulation patterns. Both of them
drew different conclusions because they worked on different materials and investigated different phenomena,
which had the same phenotype appearance: an increase in
egg size with a decrease of temperature. Another explanation was proposed by Alekseev (1981) who suggested
that this increase in egg size is caused by a decrease in
larval mortality with a gradual shift from complex food
webs with high predator diversity in tropical seas to
simpler food webs in seasonal seas and in deep waters.
An extermination of eggs and larvae in tropical seas by
numerous diverse predators with different hunting strategies does not allow most tropical species to adopt any
other reproductive strategy but to produce as many offspring as possible and to disperse them as widely as possible. In tropical species the large eggs could be produced
only in combination with extreme parental care. It was
hypothesised that with a decrease of the predator diversity
and simplification of the ecosystem structure in cold seas,
an individual’s fitness becomes more important for survival because the competition for food becomes more
intense. So, species begin to invest more energy into individual eggs (Alekseev 1981). This interesting hypothesis
was published in a short abstract and was never properly
investigated and described. It should also be emphasised
that in a particular female the egg size could depend on
food availability, body size and other internal and external factors that could change between seasons and generations (Ivankov 1985; Khmelyova 1988).
It is possible to conclude that there is no single rule
describing changes in egg size, but three phenomena with
the same expression are under environmental influence:
1. Thorson’s rule, which acts on the macro-evolutionary
level. Egg size increases with decrease of temperature,
leading to changes in developmental mode and larval biology. This rule has an ecological background and is
caused mostly by natural selection acting on taxa with
different types of larval development. It provokes a predominance of either small- or large-egged strategists in
different habitats depending on many environmental
features.
2. Rass’s rule, which pertains to the micro-evolutionary
level and early stages of macro-evolution. Egg size
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Egg size variation: Thorson’s and Rass’s rules
Laptikhovsky
increases with a decrease in temperature without important changes in larval biology and morphology. This rule
is likely caused by temperature-dependent changes in the
species physiology. It does not change the ratio between
the number of small- and large-egged species in a habitat,
could apply within the same species of marine organisms,
and occurs even in amphibian and terrestrial animals.
3. Phenotypic plasticity, which provokes invertebrates
(not only aquatic) to produce larger eggs at lower temperature, and as in the case of Rass’s rule, is also caused
by temperature-dependent changes in physiology. This
appears to be an ‘extension’ of Rass’s rule. This phenomenon is not subject to natural selection unless it is inherited and becomes a subject of Rass’s rule.
Both Thorson’s and Rass’s rules have different causative
agents, which are still insufficiently known, and display
themselves at different levels of evolution. Thorson’s rule
describes the relative abundance of both small- and largeegg species. Rass’s rule describes a relative position of
representatives of both these groups within the adaptive
range of reproductive strategies. Processes that reveal
themselves in Rass’s rule probably are a starting point,
where the micro-evolution of reproductive strategies
morphs into macro-evolution. Physiologically determined
changes in patterns of yolk accumulation (both qualitative and quantitative) at different temperatures could be a
basis first for minor changes in morphology and biology
of larvae, and then in the entire developmental mode.
It is difficult to say whether inheritance of acquired characters of developmental style is applicable to the temperature-dependent evolution of female gamete dimensions in
poikilothermous invertebrates and vertebrates. There are
quite a few examples of such inheritance of morphological
features using different genetic mechanisms (reviews: Waddington 1957, 1975; Jablonka & Lamb 1995, 2005; Grodnitsky 2001, 2002; Flegr 2002; Pigliucci & Murren 2003; Lolle
et al. 2005; Pigliucci 2005). They do not support a traditional understanding of Lamarckism, but they suggest that
some heritable, adaptive changes come not from natural
selection, but from the action of evolved internal systems
that generate non-random ‘guesses’ in response to environmental changes. This would not require the abandonment
of neo-darwinism, but rather its expansion beyond what
Ernst Mayr contemptuously labelled ‘bean-bag genetics’
(Pigliucci 2005).
Possibly, there are different mechanisms involved at the
different stages of increase in egg size with decrease of
temperature. First, it is an environmentally induced phenotypic plasticity that could be inherited using genomic rearrangements. Then, the temperature impact on yolk
composition and accumulation patterns changes the
amount of nutritional sources available for embryonic
development. Eventually, it is the intensive pressure of
12
natural selection that removes all unsuccessful variations
and leaves only genotypes with better fitness and reproductive performance. At low temperatures, an increasing
amount of yolk makes development of a lecithotrophic
strategy possible if there are no environmental constraints
such as restricted opportunities for larval dispersal and
settling (see: Giangrande et al. 1994; Bhaud & Duchêne
1996; Giangrande 1997; Bhaud 1998; Gallardo & Penchaszadeh 2001).
The aforementioned mechanism of temperaturedependent evolution of egg size in poikilothermous marine organisms is not a final conclusion. It is rather a
working hypothesis proposed for coordinated investigations of the evolution of reproductive strategies by the
different methods used.
Acknowledgements
I would like to thank sincerely Dr. S.v. Boletzky (Lab.
Arago, Banyuls sur Mer, France), Prof. Dr J.N. Havenhand (Tjärnö Mar. Biol. Lab., Göteborg, Sweden), Prof.
Dr Lisa Levin (University of California, San Diego, USA),
Dr K.N. Nesis (IO RAS, Moscow, Russia), Prof. Dr J. Pearse (University of California, USA), Prof. Dr S.F. Timofeev (MMBI, Murmansk, Russia), Dr P. Brickle, Dr L.
Triantafillos, and Dr A. Arkhipkin (FIFD, Stanley, Falkland Islands), Dr Ch.M. Nigmatullin and Dr A.G. Arkhipov (AtlantNIRO, Kaliningrad, Russia), Dr M.R. Bhaud
(Lab. Arago, Banyuls sur Mer, France), and Prof. Dr R.N.
Burukovsky (KSTU, Kaliningrad, Russia) and two anonymous referees for valuable comments and consultations.
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