Saturday, September 15, 2012

Pteropod Assemblages

In all ecosystems, whether terrestrial or marine, species combine to form assemblages. These assemblages are often specific to various environmental conditions at work in that ecosystem and so as you cross from one ecosystem to another the species assemblage changes. Although the ocean might seem like one big ecosystem made of water, this conception is wrong. The ocean is made of many different ecosystems, each characterized by different parameters such as seawater temperature, salinity, currents...even the exchanges with the atmosphere that occur at the surface can affect the species assemblages.

I described in a previous post how I am out here preserving pteropods for my dissertation work on the structure of pteropod shells. As a preliminary analysis, I decided to plot the number of shells of each species I have preserved so far to see if any assemblages are obvious along the geographical gradient of the first three survey lines (aka transects) we have sailed along. I quickly plotted the numbers of shells sampled at eight 'test stations' along our survey transect 0 (the first run from Newport to the study region) and transect 3 (the second transit out from Newport).
Map showing sea surface temperature (in color) and the location of our sampling stations. The regularly-spaced stations extending from 50N 150W to 35N 135W are our main study region. We also conducted a series of 'test' stations during our transit to the study region, which are the less regularly-spaced stations that together make a line from the survey start to Newport, Oregon, our port of departure.

At each of these stations we sampled with the Reeve net, and caught a few different pteropod species, including Limacina helicina helicina forma pacifica, Limacina helicina helicina forma acuta, Clio pyramidata and one more species labelled below 'AB'. To help you decode this, in Limacina helicina helicina forma pacifica, Limacina is the genus, helicina is the species, the second helicina indicates the sub-species, and pacifica denotes the forma. Formae describe sub-groups within a species where the individuals can be subdivided morphologically and geographically into several related groups that overlap and interbreed.

The morphological difference between pacifica and acuta are easily seen in the pictures below. Acuta is high-whorled, with distinct regular striations on each whorl; pacifica is low-whorled, without striations. These two formae have been found to cross-breed. In the graphs below I've grouped under the label 'AB' all the Limacina helicina helicina shells that do not fit in the forma pacifica or acuta. 'AB' individuals can be a combination of any of the features of acuta or pacifica: low-whorled with regular strations/with irregular striations, high-whorled without stration/with irregular strations.

Forma acuta
Forma pacifica

Clio pyramidata has a totally different shell morphology.

Clio pyramidata shell
So, here are the graphs...
Can you see a pattern? Here are a few hints:
-Compare test stations 05 and 02
-Compare test stations 06 and 03
-Compare test stations 07 and 04

If you look back at the map you can see the location of each station. Based on the changes in the composition (%) of the catch we can draw a range for each of the three species plotted here, along a geographical gradient going from southeast to northwest. Here is a new map to help you visualize where we crossed into the range of each species.

Map showing the relative abundance of the different pteropod species sampled.

We entered the range of Clio pyramidata near test stations 04 and 07. The pacifica forma of Limacina helicina helicina was present at test stations 02, 05, 03 and 06, with highest abundance at 06 and lowest at 03. This indicates that while we were deep inside its range at T06 we were only on the edge of it at T03. A very sharp change in the abundance could be explained by environmental parameters (seawater temperature, salinity etc).

Test station 08 was totally within the range of Limacina helicina helicina forma acuta but no other species or formae were seen there. With acuta being present at every test station we can presume its geographical range is much bigger than that of the other species or formae presented here. From the colors in the background of the map (representing the sea surface temperature) we can also see this forma prefers colder temperatures. 

Well, I hope this has given you a better idea of what we are seeing here. These analyses are very preliminary but already interesting. We hope the MOCNESS depth-stratified samples will help us further determine the species range with regards to geography, bathymetry, and seawater chemistry. Stay tuned!

That's all folks!

- Liza Roger

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