From: fiddler@concertina.Eng.Sun.COM (steve hix)
Newsgroups: rec.arts.sf.science
Subject: Re: Merpeople
Date: 13 Aug 1992 23:47:08 GMT
Organization: Sun Microsystems, Mt. View, Ca.
NNTP-Posting-Host: concertina

In article <59662@mimsy.umd.edu> kuijt@umiacs.umd.edu (David Kuijt) writes:
>
>Kathryn Whitworth asks for thoughts about gills;
>
>Gills have one big advantage over lungs: lungs are a sack.  We fill
>them with air, allow the alveoli (sp?) to do their thing (passive
>CO2 for O2 exchange), then we expell the air and start over.  Gills,
>on the other hand, work continuously.  The fish/shark/whatever
>keeps a constant flow of water through a fancy tube (or set of them)
>in which are set frills containing the gill-equivalents of alveoli
>which do their thing (passive CO2 for O2 exchange) constantly.
>This allows continuous effort, rather than the gulp/expel technique
>us earthbound dudes have.  

Our current breathing method still permits continuous effort expenditure.
Or are you talking about just the effort expended in breathing?

Don't forget that most muscles need *some* resting time between
doing work...even though the heart does well without lots of rest.
(What is its duty cycle, anyway, 50%?)

In some cases, it also requires continuous effort to continue
breathing water.  Some fishes have to keep moving all the time to keep
moving water over their gills.  Other fish don't, and do what
*looks* like typical breathing (watch a moray eel), as they pump water
over their gills while waiting for lunch to bumble by.

One reason for the continuous-flow method may be hydrodynamic:  if they
exhaled back the way the water came in, they'd move backward on
exhale.  Not too useful.

>(Speaking of which--is there any reason
>why lungs couldn't be built on a continuous flow model, evolutionarily?
>Enquiring minds want to speculate!).

One reason is that such a system would require more holes...a more
complicated system, with new avenues for parasitic invasion, potential
injury, increased water loss in dry environments, increased access for
dust/sand/other foreign particles.

>As a thought, it might be a lot easier for a gill-based being to
>modify the surface area of its gills in times of great need than
>it is for us lunger-types to take "deep breaths".  Some water
>breathers have external gills (molluscs, I think?).  

Lots of marine worms, arthropods, some juvenile amphibians...most
of these are either sessile or slow-moving and putting the gills out
here simplify exygen exchange, some of them are filter feeders who
may also get some of their food while they're breathing (though most
have some other mechanism for feeding).  A disadvantage to external
gills is that they're a lot of drag if you're moving, and they're
pretty fragile.  Some tidepool beasties look for, say, tubeworms
that are breathing and try to nip the gills as an easy snack.  (Tube
worms resent this sort of thing and tend to be very quick at reacting
to movement, whisking their tender gills out of sight at the slightest
move.)

>Designing external gill extensions (extruded only under stress) might be
>an interesting feature in a Merperson.


--
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Of making many books there is no end...  |  What's wrong with chasing 
                       -- Eccl. 12:12    |  nebulae till 3 am?  |-(
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