Re parasites of terrestrial earthworms -
Earthworms are intermediate hosts for some nematodes (roundworms) the
final hosts of which are farm animals. They are also hosts for a protozoan
parasite called Monocystis which lives in the reproductive organs of the
worm. Earthworms are the only host so Monocystis is no threat to worm-fed
fish. [When I taught freshman biology labs, I used to demonstrate
parasitism by squashing worm gonads (actually seminal vessicles) under a
cover slip and letting students look for Monocystis under a low-power
microscope. We were rarely disapppointed. Infestation was nearly 100%! It
was cheap, fast, no-fail, and not as "destructive" as some other educational
demonstrations of parasitism]
Re earthworm survival in water -
Aeration makes the difference. A physiologist told me that earthworms
are not truly terrestrial since they require moist skin and a wet
environment in which to live, but because they have no special respiratory
organs, oxygen concentration must be high. In the gravel of a working
undergravel filter, there is sufficient oxygen for them to breathe. In
stagnant water of a bucket there is not. The reason earthworms surface
after rains is not because they are "drowning," but because they are
suffocating. Oxygen tensions are lower in water than in moist air pockets,
and biological oxygen demand of freshly wetted soil depletes it furthur.
They surface not because they cannot extract oxygen from water, but because
there is not enough oxygen in the water.
re types of aquatic earthworms -
Although this group is shunned by many freshwater invertebrate
biologists, the taxonomy of aquatic earthworms is very well established [I
used to ID them as part of my job. Identifications of segemented worms
(Oligochaetes) are based on number and shape of their bristles (setae) and
on dimensions of the male reproductive organ.]. The biggest worms really are
aquatic "earthworms," smaller versions of worms in the family Lumbricidae.
There are slightly smaller versions in another family called Lumbriculidae.
The smaller, skinny ones are Tubificidae, and some of the tiniest ones are
Naididae. This last group is the most interesting. They can form chains of
buds called zooids. Some species have a proboscis, some have eyes, and one
species is either an intimate commensal or an ectoparasite of snails [which
of course provides another convenient and easy-to-do classroom demonstration
of parasitism]
-----Original Message-----
From: Dave Neely
Sent: Sunday, April 30, 2000 6:23 PM
To: nanfa_at_aquaria.net
Subject: Re: NANFA-- Aquatic earthworms
>...That's what I thought was probably the case, but I wasn't sure.What >I
>don't understand is how earthworms could survive in your aquarium >for
>months but die in a pail of water and turn to mush after only >being in
>there for a day for me.The only plausible reason that I can >think of is
>aeration and filtration of the water in the aquarium vs. >no
>aeration/filtration in the bucket I had them in.
One other, perhaps more parsimonious possibility is that the greenish worms
in question aren't just your everyday garden variety Lumbricus. According to
Pennak (1978), there are 10 families of aquatic oligochaetes in North
America. These range in size from the Tubifex worms you're probably familiar
with, to some orders that routinely reach 30cm in length. One order in
particular, the Haplotaxidae, are primarily terrestrial but occasionally
occur in mud, debris of stream and lake margins, or sewage treatment plant
ponds. I wouldn't want to collect fish food from the latter.
cheers,
Dave
Pennak, R.W. 1978. Fresh water invertebrates of the United States, 2nd ed.
John Wiley and Sons, New York.
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