Sunday, May 17, 2015



In my post of June 13, 2014 I noted some evidence that pogonophorans can easily live to be over 10,000 years old.  A brief discussion of factors leading to extreme longevity in the deep sea was presented earlier in a post on June 22, 2013.  The importance of the pogonophorans as an over-looked link connecting chordates to annelid ancestors makes it important to understand deep-sea conditions better in order to understand the reality of that relationship.


My first clue to understanding the problem preceded my understanding of the pogonophorans.  When teaching a marine biology class in my early years at Western Michigan University I was examining one of the fifty volumes of Challenger Reports of deep-sea research done in the 1870's.  A map or graphs included distribution of oxygen, salinity and temperature by depth and latitude.

An oxygen minimum zone, with little or no oxygen, was centered at about 500 meters depth in temperate, sub-tropical, and tropical latitudes.  In contrast to eutrophic fresh-water lakes where the oxygen rich zone is seldom more than ten meters thick, the oceanic counterpart may be over 100 meters thick.  Oxygen was most abundant at the surface, but, after a brief, rapid increase below the oxygen minimum zone, gradually increased with depth below 1000 meters until, at the bottom a few miles lower, oxygen concentration was almost comparable to surface concentrations.

Temperature decreases with depth, salinity increases with depth.  The change is most rapid near the surface and very gradual with increasing depth.


Density of ocean water increases with depth until about 2000 meters depth due to greater salinity and lower temperature.  So away from the polar regions bottom water is slowly rising with replacement from cold, salty, oxygen-rich water sinking along the bottom from polar regions at a relatively slow rate because of entrapment in the Arctic Ocean by shallower sea bottoms near most of its fringe.

The replacement of the sea-water beneath the oxygen-minimum zone takes over 10,000 years.  The rate can vary according to overall ocean levels and depth of sills where Arctic Ocean water spills over to sink and eventually reach topical latitudes.


Oxygen would be depleted and the deep ocean would be an anoxic wasteland if animals lived at the same rate they do in shallower waters.  Photosynthetic production of oxygen is limited to the first 100 meters or so of the ocean.  Enrichment from the atmosphere is the only other significant source of oxygen in the ocean and is limited to the surface and circulation by mostly wave induced surface currents.  The warmer surface and rain combine to lower density of water and make wave induced circulation ineffective below the thermocline (the zone of rapid drop in temperature).


The thickness and depth of the thermocline can vary within the 50 to 300 meter depths of ocean water.  Presumably a very intense hurricane induced wave episode could mix the ocean to a greater depth and compress the thermocline.  That could perhaps store excess heat for a few years and reduce surface water temperatures so they have less heat energy to produce another mammoth hurricane for a few years.  So the complications of predicting global warming rates is increased.


The ocean is generally more productive, with nutrients and organisms abundant, in shallow regions fringing continents, and in areas of up-welling currents which bring colder nutrient rich water to the surface.  Sedimentation rates are generally very slow beyond the continental shelves.  Most of the open ocean can be thought of as biological desert, but some very small organisms may be more abundant than others that are better known.  More recent research indicates the small organisms may be more important than generally is known.


As long as its descendants stay in abyssal waters they will remain to help remind us of the stage that survived extinction episodes and enabled those moving to shallow water to adapt again to shorter lives and greater variety that includes the chordates.  The previous post has a figure that shows one of the annelid-like features of the pogonophora and their similarity to near chordate relatives, the hemichordates.

Molecular clocks fail to place pogonophorans in the correct position in the "tree of life".  The problem is addressed in the February 2, 2015 post; a better guess at the correct position is in the post
and why the molecular clock estimates of phylogeny of phyla is wrong in the post

Inspiration for understanding evolutionary aspect of the embryological differences between protostomes and deuterostomes was presented in six posts from June 22, 2013 to June 30, 2013.


The long residence time of bottom water of the ocean would make it anoxic if animals lived at the same rate we see occurring in shallow water.  The alternative explanation would be extremely low populations and/or biomass in the abyss.  Life is less abundant, but not nearly enough to account for the difference.  The stability of conditions made it an ideal refugium to allow survival of forms that could later repopulate surface waters after an extinction event bad enough to cause the demise of over 90 percent of living species.

Joseph G. Engemann   Emeritus Professor of Biology, Western Michigan University     May 17, 2015

No comments:

Post a Comment