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The Decline of Seagrass Meadows

Zostera! Eelgrass, Zostera marina, is a flowering, marine vascular plant that remains submerged all the time. This is quite a feat for vascular flowering plants, and only a few dozen species world wide are capable of growing completely submerged in a marine environment. Eelgrass creates and extremely important habitat, its upright structures and complex root system create a 3-D living space for many different types of animals. It is (or was) the dominant habitat forming SAV (submerged aquatic vegetation) throughout much of the coastal waters in the northeastern United States. Unfortunately, for various reasons, eelgrass meadows have seen drastic declines, and in many locations eelgrass only exists in a mosaic of small patches. This is extremely bad news as many of the important, and formerly important, commercial and recreational fisheries of the northeast US are dependent on Zostera at some part of their life cycle as a nursery and foraging ground. Some of the species are finfish like tautog, bluefish, fluke, winter flounder, porgies, while others are shellfish such as blue mussels, hard clams, oysters, bay scallops, and blue crabs. Many of the aforementioned species support or once supported vibrant fisheries. Many of those fisheries have collapsed, also for various reasons. However, is it possible there is a link between the crash of the fisheries, the decline of Zostera and the failure for recovery on both ends?

Bay Scallop on Eelgrass

Argopecten on Zostera! Bay Scallops, Argopecten irradians , have developed a very close relationship with eelgrass, Zostera marina. As larvae, they are passively transported, and tend to settle in eelgrass meadows when the current is dampened by the 3D structure of the seagrass. This same 3D structure provides post-set juvenile scallops a spatial refuge from predation. Even as larger juveniles and adults, scallops are capable of, and have been shown to, actively select eelgrass habitats.

Other species also use eelgrass

grass shrimp A number of other species utilize eelgrass as a habitat. Included are grass shrimp, like the Palaemonetes pugio, other decapods such as blue crabs, bivalves such as hard clams, gastropods (snails), and numerous fish species, including winter flounder, tautog and cod.

“Snowball Earth” triggered animal evolution?

Now I am not going to try to pretend this entire Nature article. But I read about this as a small article on ScienceNow and decided it might be worth mentioning.  Noah Planavsky and his colleagues recently reported in Nature about the evolution of the marine phosphate reservoir, and surmised that phosphate enrichment after the earth was encapsulated by ice 635-750 mya led to the explosion of metazoan evolution around that time period.

What the snowball earth might have looked like

The basic idea is that photosynthetic organisms are limited by nutrients, most often nitrogen or phosphorous (although there are a myriad of other macro- and micro-nutrients that might also be limiting in certain conditions).  Plants utilize these nutrients and sunlight to undergo photosynthesis – creating sugar and oxygen from carbon dioxide and nutrients.  Oxygen is a requirement for higher organisms, as it is the primary and most efficient electron acceptor for metabolic reactions.  Without plants producing oxygen, there would be no animals.  This is the basic premise behind the authors leap from phosphorous enrichment to animal evolution.

Ok, back to the story at hand.  The researchers were able to investigate dissolved phosphorous concentrations in the ancient ocean over 3 billion years (!) by looking at iron-to-phosphorous ratios in sedimentary rocks.  (A chemist might be usef

ul to explain this linkage).  Over the geologic history of the earth, dissolved phosphorous remained relatively constant.  Well, with one exception.  A period starting ~750 million years ago and lasting ~100 million years experienced a dramatic increase in phosphorous, as indicated in the geologic record.  This time period was preceded by the snowball earth – the term describing the period of earth’s history when ice reached the low latitudes, essentially encapsulating the earth.  The thought is that this severe glaciation stirred up the terrestrial rocks and soil, and delivered this massive pulse of phosphorous to the oceans.

According to the authors, both snowball earth glaciations and Neoproterozoic oxidation have been suggested as triggers for the rise of metazoans that occurred immediately after this period in earth’s history.  The authors further conclude that these two events are linked via phosphorous.  Then, the hypothesis goes that this pulse led to an increase in primary productivity and pumping of oxygen into the atmosphere as well as burial of organic matter.  This dramatic rise in oxygen was necessary to support life.  So, the glaciations led to high phosphorous concentrations, which led to high productivity, which led to high oxygen in the oceans and atmosphere, which allowed for animal evolution to be triggered and thus the rise of the metazoans.  Sounds really good, right?

Obviously, there needs to be more solid evidence than just these correlations.  But it is a very interesting idea none-the-less.

Planavsky NJ, Rouxel OJ, Bekker A, Lalonde SV, Konhauser KO, Reinhard CT, & Lyons TW (2010). The evolution of the marine phosphate reservoir. Nature, 467 (7319), 1088-90 PMID: 20981096

3 comments to “Snowball Earth” triggered animal evolution?

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