Fish are acutely aware of sea temperature; it’s one of the key
reasons particular species of fish live where they do. As the oceans
warm however, many tropical species are moving towards cooler climes. So
might the traditional cod and chips one day be replaced by Nemo and
chips?
It’s a big question, as the distribution of species across the Earth
is one of the most fundamental patterns in ecology. All plants and
animals are of course adapted to a limited set of climatic and
environmental conditions; if the climate changes, we expect
distributions to change. This matters not only because we like to eat
many of the species in question, but also because entire ecosystems
appear to depend on the number of interacting species present.
In general the tropics have more different species than the poles. This pattern, known as the latitudinal diversity gradient,
holds true for plants and animals across the world both on land and in
the sea. Compare a rainforest or a coral reef with icy tundra or the
Arctic ocean.
As ever in the natural sciences, it’s much easier to describe a
pattern than to explain its cause but we do know that temperature seems
to have a important role, as solar radiation increases levels of primary
production. Temperature impacts the food that species can eat and also
their metabolic rates and activity levels.
In aquatic systems, temperature also strongly affects the amount of
oxygen that can be dissolved in the water. Changes in temperature,
therefore, are very likely to lead to changes in the distributions of
marine species, and the current trend of warming temperatures is driving
fish away from the tropics and towards the poles.
Mapping fishy futures
We need to know what’s lurking round the ecological corner. Miranda
Jones and William Cheung of the University of British Columbia have
modelled the changes in marine species' ranges – and by summing up,
changes in overall biodiversity – expected under the IPCC’s different
climate change scenarios. The research, published in the Journal of Marine Science,
looks at the known distribution of around 800 marine fish and
invertebrate species, matches their distributions with environmental
conditions and then projects where these species are likely to be found
under future environmental scenarios. Abandoning the tropics: Predicted local extinctions 2000-2050. Red = extinction hotspots.Jones & Cheung.
The authors find that tropical seas, particularly the shallow highly diverse seas of South-East Asia are likely to suffer the most local extinctions, while polar – and particularly Arctic – seas are likely to see the greatest number of invasions. Consequently cold regions will generally see biodiversity increases, while tropical regions will suffer.
In total, marine fish and invertebrates are expected to shift 26km per decade towards the poles under the IPCC’s worst case scenario (3°C warming by 2100). Even under the best-case scenario, fish will move 16km per decade.
Similar predictions have been made before and the fishing industry has certainly seen this coming, with infrastructure already being put in place to exploit expected higher catches in Arctic regions. But what is new in this study is an approach combining several different models of species distributions. By looking at agreements between models, the authors identify likely regional hotspots of both extinction and invasion, marked with black diagonal lines in the maps above and below. Going polar Where new species will invade, 2000-2050. Red = more invasive species.Jones & Cheung
However, while the idea of tropical fish invading chillier waters might sound fun, we’re unlikely to eat Nemo and chips in London any time soon.
Temperature (or climate) is not the only limit to a species’ distribution; suitable habitat must also exist. Clown fish such as Nemo need to live in a coral reef and such reefs are complex ecological communities themselves with all sorts of environmental requirements.
At the moment ecological and climate models do not allow us to include interactions between species as additional factors that limit movements. This may significantly alter individual species' ranges. Competition from well-established resident fish may discourage new arrivals, for instance, or some fish may rely on a different species specifically to feed their young – if the food for the juveniles does not move in the same way as for the adults, then the species range will not change.
While it might be tough to predict exactly how fast things will change, or what it will lead to, the general message from this study is clear: change is coming to marine ecosystems. We can’t take the current distribution of marine life for granted.
Out to burst the bubble of Disney enthusiasts everywhere comes the
revelation that Flounder of the Little Mermaid might have been an
XX-male fish. It’s not just Nemo who is deceiving you! Then again, the fish named Flounder in the cartoon has no real resemblance to an actual flounder or any other flatfish.
Southern flounder (Paralichthys lethostigma), like many
flatfish, have a critical window during the juvenile stage when sex can
be reversed. When southern flounder are between 35–65 mm, phenotypic
sex is determined. This means: if flounder are genetically male (XY)
they stay male and develop as males. However, genetically female
flounder (XX) have plasticity and may develop phenotypically as females
or males. Under good conditions, the flounder develop as female. Under
poor, stressful conditions the flounder become sex reversed XX-males.
It is believed they do this because it is energetically less costly
to be a male than to be a female, who have higher growth rates and must
produce eggs. The XX-males grow just like normal males and produce
sperm (as seen in captivity), it’s just that they can only contribute
“X” chromosomes to the next generation. Unlike clownfish, once sex is
determined during this window it is locked in, flounder do not change
sex later in life.
So what’s the big deal? A few more males in the population, so what? More choice right?
In any population, throwing off the sex ratio can be detrimental for
population growth and sustainability. Generally females are needed more
as each female produces fewer eggs than a male counterpoint can produce
sperm.
In southern flounder populations, the additional twist of sexual
dimorphism makes a more male skewed population have greater negative
effects. Female southern flounder grow faster and larger than males.
Thus the fishery (both recreational and commercial where applicable)
consists almost solely of female fish. In many states, to take home a
legal sized fish is to take home a female fish, as males do not tend to
get longer than 12-13 inches, but regulations are set at larger minimum
sizes in most of the range of Southern flounder (NC-15 in, SC-14 in,
TX-14 in). These limits are not meant to target females, but rather let
them have one year of spawning before recruiting to the fishery.
30mm (hatchery) southern flounder.
The main driver and stressor causing sex reversal in southern
flounder is water temperature, which is on the rise. In general, higher
water temperatures cause sex reversals to male flounder. Studies of
flounder (southern flounder and a variety of flatfish species) sex
reversal in the lab have shown for years the connection between
non-optimum water temperatures and male skewed sex ratios. Another lab
study presented the effect of tank color on masculinization. More
importantly, tests revealed than an increase in the stress hormone
cortisol increase production of males.
For the first time we are testing the sex ratios of juvenile southern
flounder in wild fish. Our current research captures young-of-the-year
juvenile flounder after the period sex determination, but while they are
still in the habitats they settled into and grew during the sex
determination window. This is key because we can monitor the water
temperature and other environmental aspects of this habitat to be able
to connect to the juvenile southern flounder. At such small size, one
cannot visually determine the sex of the flounder gonads, but we can
test the gene expression levels of various hormones to determine the sex
of these individuals.
Our recent studies in North Carolina of wild fish have shown a trend
of masculinization of southern flounder in the mid-to-southern ranges of
flounder in the state. Water temperature records indicate these areas
are just 1-2°C higher than the northern areas. Thus, just small changes
cause big results. As water temperatures are predicted to rise 1-2°C
due to climate change in the future.
Interestingly, despite recordings of less than optimal temperatures
for Texas southern flounder, limited sampling of wild caught flounder
juveniles in the last two years have shown close to 50/50 sex ratios of
juveniles through gene expression analysis of hormones produced by
gonads. This sampling covered only a small part of the Galveston Bay
habitat, so we recommend more wide spread sampling before strong
conclusions can be drawn.
This early detection is key to possibly be able to predict if a large
or small year class of females will be moving through the ranks; which
could possibly help managers in adaptive with management policies and
inform population analysis. Sex ratio information could help in
classifying if particular places or habitats produce more females and
thus should be higher on the priority list of protection and
preservation. In other places, such as Texas, where a flounder stock
enhancement program exists, this testing can help guide areas as to
where the flounder are being stocked. We are also working with the
stock enhancement program in Texas evaluating the sex ratios of their
hatchery stocking fish to help develop best practices for potentially
stocking the most female fish into the population.
So what does this mean? States still report that females dominate
the adult population despite our studies showing skewing towards males
in the juvenile stage. So are males having differential (and less)
survival to adulthood? Or are they congregating in a completely
different area than other females? Either could be true. Though much
of the data about flounder population sex ratios is older or potential
biased by where fish are collected.
What happens if we get a decreasingly male population? Theoretically
as long as high water temperatures or other stressors cause sex
reversal and all XX population could continue with part of the
population sex-reversing during the juvenile stage to fulfill the role
of male producing sperm.
However, will this be enough to sustain the population over time? If
males do not survive as well as females (possibly due to slower growth
even at a young age) would enough live through this critical stage to
service the adult population? In the lab, we have found that XX-males
function the same as regular males, but does this hold true for fish in
the wild? Could the sperm they produce not really be up for the job?
All questions we hope to answer with future studies.
