Icy Inverts Cruise 2013 - Shipboard Blog - Jan 18th to Jan 21st

21 Jan 2013: In the Amundsen Sea  (71° 03.1’S, 108° 15.5’W)

Dichotomous Branches

As our ship, the R/V Nathanial B. Palmer, cuts a meandering path through the sea ice along Antarctica's shelf, we stop in clear sites to trawl and collect a vast diversity of animals from the sea floor.  If you are reading this blog, then I am sure you have already been introduced to the beautiful and sometimes strange forms of brittle stars, sea cucumbers, and arthropods that frequent our species sorting tables (and also our dreams).  Through most of my career as an evolutionary biologist, I have had an affinity for colonial invertebrate animals that build modular bodies composed of smaller, repeated functional units (or zooids), some of which are used for feeding and others for reproduction and defense.  Coloniality as a trait has evolved several times, as it is present in several evolutionarily distant taxa such as cnidarians, bryozoans, kamptozoans, pterobranchs, and tunicates.  Although the anatomies of the individual zooids among these animal groups vary greatly, there are often distinct similarities in the overall growth forms of the colonies.  Many of these colonies can be flat sheets covering a rock that grow through asexual budding or rows of zooids all budded from a common tubular structure (stolon).  In these benthic (bottom) habitats space for growth can be limited by lack of appropriate hard substrate or competition with other animals.  Common selective pressures such as these often result in colonial animals that can grow vertically producing erect branching forms consisting of single or multiple zooid rows (as the case with the endemic Antarctic bryozoan Camptolites).  In many cases these branching forms are shaped similarly to some types of macroalgae (e.g. the bryozoan, Nematoflustra, above picture), mosses, and ferns, which is why, bryozoans were first mistaken to be plants rather than animals (hence the name 'moss' animals). 

Cnidarian octocorals (above picture) are another of the more interesting colonial animals commonly found on the shelf habitats of Antarctica and are so named for the eight pinnate tentacles on each of their feeding zooids.  Like the other animal groups I have mentioned, octocoral colonies come in a variety of shapes and growth forms, but I tend to like the simpler, dichotomously branching colonies such as those of Fannyella.  Perhaps one reason for my bias comes from spending so much time investigating the evolutionary relationships among animals producing dichotomously branching phylogenetic tree diagrams (or cladograms) that depict a related group of species diverging over time from a common ancestor (below picture).  Even most taxonomic keys used for roughly identifying specimens to species are based largely on a series of trait-based choices, usually two of more per category.  And so more broadly, dichotomous keys and phylogenetic trees can be compared to a series of choices down a path from a single point, and it is these choices that define us.  This context is one of the reasons I have the following quote (albeit modified here) from Charles Dickens’ book, Great Expectations, posted on my office door at Long Island University.  “Pause you who read this, and think for a moment of the long chain of iron or gold, of thorns or flowers, that would never have bound you, but for the formation of the first link (zooid) on one memorable day.”

Contributed by: Dr. Scott Santagata, Assistant Professor of Biology, Long Island University-Post

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20 Jan 2013: In the Amundsen Sea (70° 45.7`S, 104° 59.7`W)

It has been a busy few days.   We have sampled at several locations in the northeastern Amundsen Sea.  Due to nearly year round ice cover, this sea is the least explored, with the first biological expedition occurring in 2008.  I was fortunate enough to be a part of that expedition, and it is great being back five years later.  During that 2008 expedition on the 

RSS James Clarke Ross I was impressed with the number and diversity of the brittle stars that came up in every sample.  As sample size can be so important when exploring various scientific ideas, I decided then that the brittle stars would be an interesting group to study.  There are 217 brittle star species recorded in the Southern Ocean, 127 of them are found nowhere else.  On the continental shelf - the shallower water around Antarctica - there are 79 species. 

One interesting aspect of sampling the sea floor is the patchy distribution of animals (above picture).  For example, at one site our catch was dominated by the brittlestar species Ophioperla koelheri and Ophiura caranifera.  Our next site, only a few miles away, had no Ophioperla or Ophiura, but was dominated by Ophionotus victoriae.  Our last sample as we were leaving the Amundsen Sea was almost entirely brittle stars (below picture), but this time the giant red spikey Ophiocamax gigas. What ecological factors dictate this patchiness is not fully understood as it is so difficult to appreciate the various characteristics of the seafloor at any one location.  Whether the sediment is soft or coarse is certainly one factor. 

The variety of animals found in a specific location on the Antarctic continental shelf, called an assemblage, is largely dictated by the frequency and time since the last iceberg scour.  The giant icebergs around us - some several tens of miles across - can be as deep as 500 meters and scrape across the seafloor demolishing everything in its path.  Over the last 10,000 years every bit of the shallow seafloor has been scraped many times.  The effect of this over a large area is a mosaic of patches where communities of animals have been established over different time periods - since the last iceberg scour.  In the very few places that are protected from icebergs, large long-living animals, such as giant sponges, can live, providing a complex and stable three-dimensional habitat, typically rich in diversity.

We are now crashing through the ice on our way out of the Amundsen Sea, perhaps to sample on the shelf break, where the "shallow" continental shelf (500 - 1000m deep) drop suddenly into the very deep oceanic or abyssal plains.

Contributed by: Dr. Chester Sands, Molecular Ecologist, British Antarctic Survey

Last updated: 11/12/2013