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\begin{document}

\title{Will shrinking body size and increasing species diversity of crustaceans
follow the Warming of the Arctic litoral ?}



\author[1]{Jan Węsławski}%
\author[1]{Joanna Legezynska}%
\author[2]{Maria Wlodarska- Kowalczuk}%
\affil[1]{Institute of Oceanology PAS}%
\affil[2]{Polish Academy of Sciences Institute of Oceanology}%


\vspace{-1em}



  
  \date{May 07, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Over thirty species of littoral marine Gammaridea occur along the coasts
of the North Atlantic. From one to several species can coexist in a
single region. There is an evident, inverse relationship between egg
incubation time and temperature (from 14 to \textgreater{} 120 days) and
consequent trends in the size of the animals on reaching maturity (from
5 mm in warmer waters to 30 mm in the coldest ones) and in lifespan
(from \textless{} 6 months to \textgreater{} 5 years). Littoral
gammarids are a good example of the shrinking size effect of increasing
temperatures and size-related species diversity. In large species the
annual cohorts of the population (3 to 5 annual size groups)
functionally replace the adults of smaller species. The ongoing warming
of the European Arctic seas may extend the distribution limits of boreal
species so that more Gammarus species may appear on northern coasts
hitherto occupied by just one or at most two species.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Temperature is widely regarded as a key factor both directly and
indirectly responsible for the diversity of marine fauna. The usual
patterns show species richness increasing from cold towards warmer
regions among the majority of taxa, at least in the Palearctic and
Nearctic (Gaston 1998, 2000). In poikilotherms, temperature directly
controls metabolism and growth rates, and hence, the size of an
organism. Species diversity is usually negatively correlated with the
size of an organism, that was well documented on terrestrial insects
(Siemann et al. 1996). That is why the temperature increase due to
global warming is likely to cause a change in average individual size,
or selection towards smaller species in communities (Atkinson \& Sible
1997). Such a pattern has already been demonstrated in pelagic Copepoda
(Beaugrand et al. 2000), but it is not yet universal in benthic
communities (Mazurkiewicz et al. 2020). The blurred size pattern among
soft-bottom benthos is due to the dominance of polychaete worms and
bivalves -- two groups in which linear growth is difficult to assess.
Peracaridan crustaceans, on the other hand, are likely to be the best
model taxon for studies of temperature-related size changes.
Peracaridans have no larval stage, and juveniles grow throughout life,
with the successive moults clearly demarcating the increments. Egg
incubation time, egg size and gammarid female size are inversely related
to temperature (see the review in Steele \& Steele 1975). In addition,
not only low temperatures but also oxygen levels are responsible for the
large size of cold-water Crustacea, a phenomenon known as ``polar
gigantism'' (Chapelle \& Peck 1999).

Atlantic sector of the Arctic is warming very fast (ACIA 2005), mainly
due to the increasing advection of Atlantic waters (Walczowski et al.
2012) that brings boreal species North of their previous limits of
distribution (Berge et al 2005, Beuchel et al. 2006, Fleisher et al
2007, Weslawski et al. 2018).

Here, we hypothesize that a larger body size (associated with perennial
longevity) in northern littoral \emph{Gammarus} populations reduces the
possibility of sibling species occurring sympatrically, as observed at
present in warmer waters. We explore this by comparing diversity and
size in \emph{Gammarus} populations inhabiting rocky North Atlantic
coasts across latitudes from 45 to 81\selectlanguage{ngerman}°N and water temperatures from -1.8
to 25°C. We expect that present-day patterns of distribution are likely
to change, as the temperature increase will tend to shorten gammarid
life cycles in the Subarctic. Such a phenomenon has already been
observed in pelagic Crustacea, where the same species -- \emph{Calanus
finmarchicus} -- may produce from one generation in cold water to three
in temperate seas per year, depending on the ambient temperature
(Irigoien et al. 2000).



\textbf{Material and methods}

The original material comes from the unpublished archive of the first
author, who sampled littoral \emph{Gammarus} species in the Gulf of
Gda\selectlanguage{polish}ń\selectlanguage{english}sk (Baltic Sea, 54\selectlanguage{ngerman}°N) and Hornsund fjord (Svalbard Archipelago,
77°N) in 1979-1982. They were collected with a hand-net on the shore, at
0-1 m depth, from under stones and algae. The animals were measured from
the tip of the head to the tip of telson, excluding spines.
Formalin-preserved specimens were wet-weighed after having been blotted
on filter paper. The temperatures for the different localities were
obtained from the cited references or meteorological data currently
available on the Internet. Some of the materials collected by the first
author were presented in the form of an unpublished manuscript -- an MSc
thesis at the University of Gda\selectlanguage{polish}ń\selectlanguage{english}sk (Wolska 1983).

Summer minimal temperature for the geographic region was taken as a
critical value for marine poikilotherms (Golikov et al. 1990). The
occurrence of the sympatric species was assessed for the region (sea)
from the literature cited, species names were checked after
Bellan-Santini and Costello (2001).

\textbf{Results and discussion}

At least thirty sibling species from \emph{Gammarus} and other species
from closely related genera (\emph{Marinogammarus, Pontogammarus,
Dikerogammarus} etc.) occur in the intertidal of both the eastern and
western North Atlantic (Table 1). Their size at maturity ranges from 4
to 52 mm and is related to lifespan and ambient temperature (Fig. 1).
Length frequencies in the summer samples of the Arctic population of two
sympatric sibling \emph{Gammarus} species indicate that there are three
annual cohorts, or fourteen size groups (2 mm intervals) (Fig. 2).
Compared to the Arctic, the temperate water population (Baltic) of the
same species in summer (after the death of the winter cohort and
juvenile release in spring) has a cohort of one age (length 6-15 mm)
that corresponds to five size groups (2 mm intervals). The summer length
frequency of the large (40 mm) species in the Arctic gives up to twenty
size groups (2 mm intervals).

The growth in two very different populations of the same species
(\emph{G. oceanicus} from the Baltic and Arctic, summer temperatures
plus 20 \selectlanguage{ngerman}°C and 4 °C respectively) is similar, although the cold water
population lives longer and grows to greater lengths (Fig. 3). A cold
water individual may not reach maturity in the first year of its life,
but will grow continuously for the next one or two years, ultimately
attaining a large size.

If the life cycle is to be completed within a year or less, the critical
phenomenon is the egg incubation time (Table 1). The relationship of
this time to ambient temperature has been studied many times in
poikilotherms like fish and crustaceans, as well as specifically in the
genus \emph{Gammarus} (Steele and Steele 1975, Whiteley et al. 2011).
Incubation in \emph{Gammarus setosus} or \emph{G. wilkitzkii} lasts 120
and more days in cold, Subarctic -- Arctic waters, at year round
temperatures \textless{} 2°C (Steele \& Steele 1974, Weslawski \&
Lege\selectlanguage{polish}ż\selectlanguage{english}y\selectlanguage{polish}ń\selectlanguage{english}ska 2002). In the Black Sea or southern Baltic, by contrast,
summer generations of \emph{Gammarus inaequicauda} and \emph{G.
salinus}incubate eggs at temperatures \textgreater{} 20\selectlanguage{ngerman}°C in less than
20 days (Ja\selectlanguage{polish}ż\selectlanguage{english}d\selectlanguage{polish}ż\selectlanguage{english}ewski 1970). The ability to shorten the egg incubation
period is probably governed by temperature only (as observed in
\emph{Calanus}copepods -- Irigoien et al. 2000 or pelagic hyperids --
Koszteyn et al. 1995). Other factors controlling the size of these
invertebrates are the availability and quality of food and/or oxygen
saturation (Chapelle \& Peck 1999). Adults of \emph{Gammarus} species
are omnivores (Tzvetkova 1975) and food availability is not a limiting
factor in the littoral (plant detritus, meiofauna and microorganisms are
plentiful; Weslawski et al. 1993). The oxygen concentration in coastal
waters is always high, or even supersaturated, as there the water
dynamics is the highest.\emph{Gammaru} s species have adequate food
resources, a high level of oxygen and an appropriate range of salinity.
The only limiting factor is suitable microhabitat, i.e. stones or
crevices under which they can hide from predators. Gammarids are a
preferred dietary constituent of coastal fish in the Baltic (MacNeil et
al. 1999), and of fish, seabirds and seals on Svalbard (Weslawski \&
Kulinski 1989, Lydersen et al. 1994). The interstices among loose
stones, providing adequate shelter, are quickly filled when some
hundreds of animals are trying to hide beneath one of them (Weslawski
1994). It is the occupation of this microhabitat by large, local species
that is the likely factor preventing boreal, eurytopic species from
successfully colonizing the North. Ca 300 large specimens (mean length
20 mm) or 2000 small ones (mean length 5 mm) can conceal themselves
under a stone 400 cm\textsuperscript{2} in area; this corresponds with
the average densities reported for Arctic localities
(300-500/m\textsuperscript{2}) (Weslawski 1994) and for temperate sites,
where the number of small gammarid species can exceed 10
000/m\textsuperscript{2} (Tzvetkova 1975).

Niche selection and competition was described as a critical factor for
the new species colonisation in littoral amphipods (Piscart et al. 2008,
Kotta et al. 2013).

The majority of marine littoral gammarids display a very wide tolerance
to salinity and temperature (Tzvetkova 1975). In the North Atlantic
intertidal, many different species occur in the temperature range
between 0\selectlanguage{ngerman}°C in winter to \textgreater{} 20°C in summer. Consequently,
most of the species listed in Table 1, have a potentially very wide
geographical distribution. If temperature were the only factor limiting
their occurrence, the North Atlantic coast would be divided into a
narrow zone with cold stenothermic \emph{Gammarus} species
(\emph{Gammarus wilkitzkii} and \emph{G. setosus} ), with the rest of
the area supporting the other, eurytopic species. In actual fact,
however, the littoral gammarids are spatially more limited, and the
number of species corresponds inversely to their size and life length
(Table 1, Fig. 1). In the Arctic, where the two large, cold-water
species (\emph{G. wilkitzkii} and \emph{G. setosus} ) co-occur, there is
almost no sympatric occurrence, as \emph{G. wilkitzkii} is an
ice-associated species and \emph{G. setosus} is a littoral species.
However, when the ice melts in coastal waters, \emph{G. wilkitzkii}
seeks the same shelter as its littoral congener (Weslawski 1994,
Polterman 1997). Another example of regions where two large species
occur together is Canada and Svalbard, where the local cold-water
\emph{G. setosus} is confronted with the boreal \emph{G. oceanicus} . On
Newfoundland, the coasts of which have an extensive tidal range from
three to twelve metres,\emph{G. setosus} was recorded higher up on the
shore and \emph{G. oceanicus} lower down (Steele \& Steele 1974). On
Spitsbergen, the two species co-exist recently, as the \emph{G.
oceanicus} is colonising the area after the glacial retreat (Grabowski
et al. 2019).

In the littoral, when a species is large, there are many size groups
that act as separate ecological units: size variations in gammarids lead
to differences in mobility, food and behaviour (see Weslawski et al.
2010). All the available space is occupied and the number of true
species is limited -- to two, according to published observations. In
areas where species are small, there are fewer size groups and more
species can co-exist (up to fourteen in a region like the Mediterranean
Sea, Table 1).

The sympatric occurrence discussed here is considering the regional
scale (gamma diversity). The co- occurrence on a small scale of one
sample, alpha diversity is difficult to assess, as there are very few
data. In the Baltic, where 9 local plus four alien (man introduced)
species occur, the actual occurrence of three to five species in one
spot was confirmed (Ja\selectlanguage{polish}ż\selectlanguage{english}d\selectlanguage{polish}ż\selectlanguage{english}ewski 1970, own observations).

As the size of gammarids is so closely related to ambient temperature,
we may speculate that with increasing coastal temperatures in the
Arctic, littoral gammarids will complete their life cycle at a smaller
size, which will create opportunities for the area's colonization by
southern species. This will be a direct effect not of temperature (those
eurytopic species are already capable of living there) but of the
favourable size structure of competitors.

In summary, two phenomena are well documented in littoral\emph{Gammarus}
species. One is the direct relationship of temperature to lifespan and
the size of an adult animal: at warmer temperatures, all known species
grow faster and reach maturity at a smaller size. The other is the low
number of sympatric species in areas where large species occur, and the
high number of such species where the animals are small. From these two
observations we can infer that climate change may shift the boreal
species northwards, where competition from large species will be reduced
as the temperature rises.

\textbf{Acknowledgements}

Preparation of this publication was supported by funds from the Svalbard
Miljovern SIP Project - RIS No. 3243, Polish Ministry for Higher
Education funds SPUB, the project ACCES - Belmont Fund project, the
ADAMANT-DAINA project and the Polish Norwegian Research Funding
Mechanism (project DWARF).

\textbf{Data availability statement}

All data presented in this paper are available through the project web
page ACCES\emph{https://www.iopan.pl/projects/Acces/}(operational since
June 2020).

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\begin{longtable}[]{@{}llllllllll@{}}
\toprule
Table 1 Gammaridae from Atlantic region & region & minimal & max. & min.
& max. & life & sympatric & eggs &\tabularnewline
\midrule
\endhead
& considered & female & length & summer & summer & span & species &
incubation &\tabularnewline
& & length & adult & water & water & & in region & &\tabularnewline
& & & & temp. & temp. & & & &\tabularnewline
species/population & & mm & mm & \selectlanguage{ngerman}°C & \selectlanguage{ngerman}°C & yrs & nr & days &
reference\tabularnewline
Chaetogammarus olivii (H. Milne Edwards, 1830) & Black, Medit. & 6 & 12
& 15 & 20 & 1 & 14 & & Greze 1985\tabularnewline
Chaetogammarus warpachowskyi Sars, 1897 & Baltic & 5 & 7 & 10 & 10 & 1 &
6 & & Zettler nad Zettler 2017\tabularnewline
Dikerogammarus haemobaphes (Eichwald, 1841) & Azov, Black & 8 & 20 & 15
& 20 & 1 & 9 & & Dobrzycka-Krahel et al. 2013, Zettler\&Zettler 2017,
http://www.iop.krakow.pl/gatunkiobce/\tabularnewline
Dikerogammarus haemobaphes (Eichwald, 1841) & Baltic & 10 & 22 & 10 & 20
& 1 & 6 & & Dobrzycka-Krahel et al. 2013, Zettler\&Zettler 2017,
http://www.iop.krakow.pl/gatunkiobce/\tabularnewline
Dikerogammarus villosus (Sowinsky, 1894) & Baltic & 8 & 30 & 10 & 15 & 2
& 6 & & Dobrzycka-Krahel et al. 2013, Zettler\&Zettler 2017,
http://www.iop.krakow.pl/gatunkiobce/\tabularnewline
Echinogammarus finmarchicus (Dahl, 1938) & White & 10 & 21 & 5 & 15 & 1
& 3 & 15 to 30 & Gurjavova 1951, Tzvetkova 1975, Lincoln 1979, Zettler
and Zettler 2017\tabularnewline
Echinogammarus foxi (Schellenberg, 1928) & Mediterr. & 6 & 8 & 15 & 25 &
1 & 14 & & Grintsov 2016\tabularnewline
Echinogammarus ischnus (Stebbing, 1899) & Baltic & 6 & 15 & 10 & 20 & 1
& 6 & &
https://nas.er.usgs.gov/queries/greatlakes/FactSheet.aspx?SpeciesID=23\tabularnewline
Echinogammarus ischnus (Stebbing, 1899) & Black & 5 & 10 & 15 & 20 & 1 &
9 & & Greze 1985\tabularnewline
Echinogammarus karadagiensis Grintsov, 2009 & Black & 4 & 6 & 15 & 25 &
0,5 & 9 & & Grintsov 2009, 2016\tabularnewline
Echinogammarus marinus (Leach, 1815) & North, W Atl. & 15 & 25 & 5 & 15
& 1 & 10 & 15 to 30 & Gurjavova 1951, Tzvetkova 1975, Lincoln 1979,
Zettler and Zettler 2017,\tabularnewline
Echinogammarus obtusatus (Dahl, 1938) & North, W Atl. & 9. & 20. & 5 &
15 & 1 & 10 & 15 to 116 & Gurjavova 1951, Tzvetkova 1975, Lincoln 1979,
Steele and Steele 1975\tabularnewline
Echinogammarus pirloti (Sexton \& Spooner, 1940) & North & 11 & 14 & 10
& 15 & 0,5 & 10 & 15 & Lincoln 1979\tabularnewline
Echinogammarus planicrurus (Reid, 1940) & North & 5 & 9 & 10 & 20 & 1 &
11 & & https://www.marlin.ac.uk/species/detail/1776\tabularnewline
Echinogammarus stoerensis (Reid, 1938) & North, W Atl. & 4 & 8 & 10 & 20
& 0,5 & 10 & 16 & Gurjavova 1951, Tzvetkova 1975, Lincoln 1979, Zettler
and Zettler 2017, Steele and Steele 1975\tabularnewline
Gammarus aequicauda (Martynov, 1931) & Black, Medit. & 10 & 20 & 15 & 25
& 1 & 14 & & Greze 1985\tabularnewline
Gammarus annulatus Smith, 1873 & W Atl. & 10 & 20 & 5 & 15 & 1 & 5 & &
Bousfield\tabularnewline
Gammarus chevreuxi Sexton, 1913 & North, E Atl. & 6 & 13 & 10 & 20 & 0,5
& 10 & 15 to 30 & Lincoln 1979\tabularnewline
Gammarus crinicornis Stock, 1966 & Mediterr. & 8 & 20 & 15 & 25 & 1 & 14
& & Lincoln 1979\tabularnewline
Gammarus crinicornis Stock, 1966 & Black & 10 & 20 & 15 & 25 & 1 & 9 & &
Greze 1985\tabularnewline
Gammarus daiberi Bousfield, 1969 & W Atl. & 8 & 12,5 & 5 & 20 & 1 & 5 &
& Bousfield 1969\tabularnewline
Gammarus duebeni Lilljeborg, 1852 & North, Baltic & 8 & 22 & 10 & 20 & 1
& 6 & 30 & Jazdzewski 1970, Lincoln 1979, Tzvetkova 1975, Steele and
Steele 1975\tabularnewline
Gammarus duebeni Lilljeborg, 1852 & White & 12 & 25 & 5 & 15 & 1 & 3 &
30 to 150 & Gurjanova 1951, Tzvetkova 1975\tabularnewline
Gammarus inaequicauda Stock, 1966 & North, Baltic & 8 & 10 & 10 & 20 & 1
& 6 & & Zettler and Zettler 2017\tabularnewline
Gammarus insensibilis Stock, 1966 & E Atl., Med.., Black & 5 & 21 & 15 &
25 & 1 & 14 & & Lincoln 1979, Greze 1985, Zettler and Zettler
2017\tabularnewline
Gammarus lacustris G.O. Sars, 1863 & E Atl., Baltic & 10 & 25 & 10 & 15
& 1 & 6 & & Zettler and Zettler 2017\tabularnewline
Gammarus lawrencianus Bousfield, 1956 & W Atl. & 5 & 10 & 5 & 15 & 0,5 &
5 & 17- 82 & Steele and Steele 1970, 1975\tabularnewline
Gammarus locusta (Linnaeus, 1758) & E. Atl. , North & 15 & 33 & 10 & 20
& 2 & 10 & & Gurjavova 1951, Lincoln 1979, Zettler and Zettler
2017\tabularnewline
Gammarus locusta (Linnaeus, 1758) & Baltic & 12 & 18 & 10 & 20 & 2 & 6 &
30- 60 & Ja\selectlanguage{polish}ż\selectlanguage{english}d\selectlanguage{polish}ż\selectlanguage{english}ewski 1970\tabularnewline
Gammarus mucronatus Say, 1818 & W Atl. & 1 & 4 & 5 & 30 & 0,5 & 5 & 12
to 15 & Bousfield 1969, Fredette and Diaz 1986\tabularnewline
Gammarus oceanicus Segerstr\selectlanguage{ngerman}åle, 1947 & Atlantic & 11 & 38 & 5 & 20 & 2 &
10 & 60 to 150 & Lincoln 1979, Steele and Steele 1972, Tzvetkova 1975,
Steele and Steele 1975\tabularnewline
Gammarus oceanicus Segerstr\selectlanguage{ngerman}åle, 1947 & Baltic & 10 & 30 & 10 & 20 & 2 &
6 & 25 to 94 & Ja\selectlanguage{polish}ż\selectlanguage{english}d\selectlanguage{polish}ż\selectlanguage{english}ewski 1970, Zettler and Zettler 2017\tabularnewline
Gammarus palustris Bousfield, 1969 & W Atl. & 4 & 14 & 5 & 25 & 1 & 5 &
& Bousfield 1969, Gable and Croker 1977\tabularnewline
Gammarus pulex (Linnaeus, 1758) & Baltic & 12 & 23 & 10 & 15 & 1 & 6 & &
Zettler and Zettler 2017\tabularnewline
Gammarus salinus Spooner, 1947 & E Atl. & 12 & 24 & 10 & 20 & 1 & 10 & &
Lincoln 1979, Tzvetkova 1975\tabularnewline
Gammarus salinus Spooner, 1947 & Baltic & 10 & 24 & 10 & 25 & 2 & 6 & 30
to 60 & Ja\selectlanguage{polish}ż\selectlanguage{english}d\selectlanguage{polish}ż\selectlanguage{english}ewski 1970, Zettler and Zettler 2017\tabularnewline
Gammarus setosus Dementieva, 1931 & Arctic & 13 & 44 & 0 & 15 & 3 & 1 &
35- 150 & Gurjanova 1951, Steele and Steele 1970, Tzvetkova 1975, Steele
and Steele 1975\tabularnewline
Gammarus subtypicus Stock, 1966 & Black & 6 & 20 & 15 & 25 & 1 & 9 & &
Greze, 1985\tabularnewline
Gammarus tigrinus Sexton, 1939 & North, Baltic & 4 & 14 & 10 & 25 & 1 &
6 & 30 & Bousfield 1969,Lincoln 1979, Zettler and Zettler
2017\tabularnewline
Gammarus wilkitzkii Birula, 1897 & Arcic & 20 & 52 & 0 & 5 & 5 & 0 & 37-
180 & Tzvetkova 1975, Poltermann 1997\tabularnewline
Gammarus zaddachi Sexton, 1912 & E Atl., & 10 & 20 & 10 & 25 & 1 & 6 &
30 to 60 & Jazdzewski 1970\tabularnewline
Gammarus zaddachi Sexton, 1912 & Baltic & 7 & 30 & 10 & 30 & 1 & 6 & &
Gurjanova 1951, Tzvetkova 1975, Zetler and Zettler 2017\tabularnewline
Obesogammarus crassus (G. O. Sars, 1894) & Baltic & 7 & 15 & 10 & 15 & 1
& 6 & & Grabowski et al. 2007, Dobrzycka-Krahel et al. 2013,
http://www.iop.krakow.pl/gatunkiobce/default2436.html?nazwa=opis\&id=35\&je=pl,\tabularnewline
Pontogammarus robustoides (G.O. Sars, 1894) & Baltic, North & 12 & 22 &
10 & 20 & 1 & 6 & & Zettler and Zettler 2017\tabularnewline
\bottomrule
\end{longtable}

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\end{document}

