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Biological jigsaw

Vanda Carmo has just begun a research project that will hopefully provide some pieces of the large jigsaw puzzle MAR-ECO researchers are trying to assemble in order to have a more complete picture of the trophic relationships among the organisms living along the mid-Atlantic Ridge.

Vanda Alexandra Santos do Carmo is a young PhD student studying at the Department of Oceanography and Fisheries (DOP) at the University of the Azores. Her research project aims to determine some specific trophic interactions among several key fish groups collected during the research cruise aboard the RV G.O.Sars summer 2004.

 Not only is the MAR-ECO pelagic nekton collection at Bergen Museum extensive in terms of numbers of specimens, but, in addition, the researchers used depth stratified gear during the G.O.Sars cruise so that the samples collected could be better situated in the water column, which would, in turn, provide more information about the structure of life in the water column. They also used several different kinds of sampling gear, which reduced the gear selectivity of any one type of equipment.

Road to becoming a marine biologist

Vanda explains that she has known since she was a little girl that she wanted to do something relating to the sea. This fascination became more quantified in junior high school when she first was exposed to biology. “It has been like an obsession!” she says!

After graduating from university in Lisbon, however, Vanda was unable to find work relating to marine biology. While waiting for a suitable opportunity, she began to work as a volunteer fire rescue worker, beginning as a trainee and working her way up the ranks. She combined this activity with her love of the sea by pursuing diving rescue training and this led her to the Azores and eventually into the research programme led by Dr. Ricardo Serrão Santos at DOP.

Becoming a MAR-ECO student has given Vanda her first experiences of travelling abroad. In her project work she will travel to work with supervisors among the MAR-ECO researchers in Norway and the US. Her first trip was to Scotland for the annual MAR-ECO conference summer 2006.

What is eating what?

The 2004 MAR-ECO cruise collected nearly 60 000 fish specimens. Vanda is not going to study all of these! Her work will focus on the more characteristic mesopelagic families; Stenoptychidae, Gonostomatidae, Phosichthyidae, Stomiidae and some species of Myctophidae. While there are many different species in each group, each consuming quite different diets, Vanda points out that more or less all except the Stomiidae could be described as zooplankton eaters.

Unlike a terrestrial food web, there is no light, and thus no photosynthesis or primary production in the interior of the sea. At the higher tropic levels deep sea food webs begin with zooplankton. Many of the fishes that Vanda will be studying engage in diel (daily) vertical migrations up to the more productive surface layers during the night and down again to the dark zones during the day.

Vanda explains that the mesopelagic zone of the ocean is dimly lit, but receives enough sunlight that the fauna in this zone can distinguish a diel cycle. Unlike the ecosystems of the continental margins, the mid-ocean ridge systems do not receive terrigenous (land-derived) nutrient inputs.

Despite this, the limited surface production of organic matter can be transferred vertically to the pelagic (dark) zone by the sinking of aggregates of organic particles (marine snow) or the carcasses of larger animals, as well as by the vertical migration of living animals (both prey and predators) in the water mass.

This vertical process is critical to the cycling of organic matter in the deep sea. It is the only means by which photosynthetic-based nutrition from the surface of the ocean can enter the deep sea food web.

Equipped with a selection of around 6000 specimens from the MAR-ECO G.O.Sars collection from Bergen Museum, Vanda is going to begin three months of intensive work at the research station in Flødevigen in southern Norway. There she will dissect each specimen!

First she will measure and weigh each specimen. Then she will carefully cut each specimen open and extract the entire digestive tract. The stomach contents will be assessed both qualitatively and quantitatively as completely as possible. Since the contents of the intestine will be more digested than that of the stomach, the material here will only be assessed qualitatively.

Most fishes swallow their prey whole. Any relatively undigested food items will be used to identify prey items to the lowest taxonomic level possible. Vanda will be working to assemble a zooplankton reference collection that will enable her to do this work as efficiently as possible. Tracey Sutton is working to help build and document a “bone” library that will be an invaluable resource for this kind of work.

Thousands of bone fragments have been analysed from the excellent osteological preparations by the Bergen Museum staff. Some of these fragments are characteristic and can be used for species identification and to calculate the original size and weight of the fish prey.

What will she find?

Gut contents studies, such as this one that Vanda is undertaking, provide “shapshots” of a short time period in a fish’s life (its most recent meals). Whether or not a fish has eaten recently depends on the time of day for those that migrate vertically, or whether a larger predator has been lucky enough to find prey recently! Vanda explains that those eating zooplankton tend to feed more frequently and so will probably have something in their stomachs. For each recognisable food item, Vanda will prepare and label a sealed glass slide where the item can be preserved for later study and identification.

The project is a huge undertaking, but the results will give marine researchers valuable new insights into the trophic relationships among marine organisms and provide them with a greater understanding of the trophic pathways among the pelagic nekton of the northern mid-Atlantic Ridge system It focuses on organisms representing one of the dominant nekton components, the meso- and bathypelagic fishes, and their food resources, including zooplankton and other nekton.

DOP

Harbour Branch Oceanographic Institution

Flødevigen Marine Research Station

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