At 10:04 AM 12/2/98 EST, you wrote:
>Dear all:
>   Just a random question, but do any members of Gastropoda (or any other
>mollusks for that matter) display either Mullerian or Batesian mimicry? I
>can't seem to think of any examples and it is driving me up the wall. Also are
>there any members of all classes of Mollusca that display Convergent
>evolution, Adaptive radiation, Co-evolution, or Character displacement?  I
>can't seem to think of mollusks but I sure can think of insects
>Thanks
>Sarah Watson
>
 
I'm going to start with definitions, just in case. Sorry for the length, but
exact definitions are essential. Evolutionary ecology is kind of my
specialty, so forgive the rambling.
 
Mullerian mimicry: Two seperate species that display a similar color pattern
because both are toxic. A classic example is the Milkweed bug and the
Monarch butterfly. Both species are toxic, and both are red and black. Let's
say the Monarch evolved first. Birds eat the monarch, get sick, and never
touch another red and black bug again. A Milkweed bug with similar patterns
could benefit from the Monarch's sacrifice.
Remember, it's useless (for your genetic success) to be toxic if a predator
has to eat you to figure out that you're toxic.
 
Batesian mimicry: Two species that display a similar color pattern, however
only one is toxic and the other perfectly edible. The classic example is the
Monarch butterfly (toxic) and the nearly identical Viceroy butterfly
(edible). As above, the Monarch's red and black coloration is a warning of
toxicity. The perfectly edible Viceroy looks so much like a Monarch that
predators will avoid it.
 
Convergent evolution: Two species that display similar traits not because of
relatedness but because of similar ecological demands (analogous traits).
The pectoral fins of fish and the flippers of whales are similar in shape
because that is an optimal shape for aquatic locomotion, not because of a
specific relatedness (whales are closer to bats, hedgehogs, cows and humans
than to any fish). The opposite of an analogous trait is a homologous or
derived (from a common ancestor) trait. I believe that no other area of
evolution has caused more confusion than this important distinction
 
Adaptive radiation: When a single species exploits a diverse habitat and
subsequently speciates into several new species. The most commonly cited
examples are the finches of the Galapagos Islands. Presumably a single South
American finch made to the Galapagos. Being the only land bird, quickly
speciated into the divergent forms seen today. While island examples are the
most obvious, there is not a species alive that is not the product of
adaptive radiation.
 
Co-evolution: When two species each become each other's primary ecological
adaptive influence. All species in a particular ecosysytem are co-evolved,
but striking examples can be found in commensal species. Some species have
co-evolved to such a degree that both would die without the other.
 
Character displacement: Generally this is when two populations of the same
species occupy the same environment and compete for resources. Character
displacement occurs when one population modifies its behavior (and
physiology) so that it no longer competes with the other. This is known as
"resource partitioning" and is one major step in adaptive radiation. A great
example can be found here in Alaska with our resident and transient pods of
Orca (Killer) whales. Both populations share the same habitat, but one
population feeds almost exclusively on salmon while the other population
feeds only on other sea mammals. Character displacement also occurs when you
find resource partitioning between age classes of the same population.
 
Now for some molluscan examples of these evolutionary adaptations.
 
The most obvious example of molluscan Mullerian mimicry can be found in the
nudibranchs. Those striking patterns have the same meaning worldwide ("I'm
toxic or taste bad") I am not an expert on this group, but I am sure others
can name many divergent nudibranchs that display similar color patterns. I
don't know if the striking patterns on certain cone shells are unique
warning colors or follow the scheme of other toxic species There are
theories that state that at least some warning colors and/or patterns are
somewhat "hardwired" into animal species. That is, at some point in the
distant past, some toxic animal set the standard that red, yellow, and black
(for example) mean's "deadly" whether you're a nudibranch, polyclad, or
snake. The relative rarity of Mullerian mimicry in molluscs is probably due
to the fact that they are (as a whole) non-toxic.
 
Batesian mimicry is far more widespread amoungst mollusks. There are many
pulmonates that mimic bird feces and thus appear unpalatable to birds. I
don't know this for a fact, but I suspect the striking coloration of the
mantle (and shell) of Cypraeids is meant to fein toxicity (if not to
specifically look like toxic nudibranchs, sponges, etc). One finds Batesian
mimicry to usually be the case when a very edible species advertises itself
with bright colors. Some species are opportunistic and seem to live around
toxic species (that they resemble probably by coincidence rather than
design) when possible. An example is our local Siphonaria (Liriola?)
theristes. Around Juneau you can find dense populations in colonies of the
toxic sea cucumber Cucumeria vegae. To find them I run my fingers over a mat
of the cucumbers until I feel a shell (they look almost identical when out
of the water). In other areas I have found the Siphonaria where there are no
cucumbers. The similarity is undoubtedly just a fortunate accident.
 
A discussion about convergent evolution in mollusks could open a whole can
of worms (or Solenogasters?). As I stated above, no other area of
evolutionary biology is more misunderstood or subject to heated debate.
Convergence is obvious when talking of whale flippers and fish fins or bird
wings and bat wings. The distinctions become more blurred when dealing with
more closely related species. The Atlantic vs Pacific mollusks are an
excellent example. There are many species on the Pacific that have a
"sister" species in the Atlantic. This is probably due to the fact that the
ancestor of both crossed over when Panama was a natural canal between the
two oceans. In this case the similarities between two species are due to
derived characteristics. The other possibility is that both species
independantly evolved similar traits because both exploit a similar habitat
(or similar environmental pressures). I am sure that both scenarios occured.
Commonly, organisms we see today are a blend of derived and convergent
traits. For example, most Murcidea have spines and/or varices. So it would
be pointless to discuss convergence the spines of Murex pecten and Chicoreus
ramosus. Both have spines because both evolved from a spiny ancestor.
However, let's take the varices of a Typhis and the varices of Ceratostoma.
Do the varices imply a common ancestor WITH varices, or did both evolve from
a spiny murcid ancestor and subsequently evolve varices as an adaptation to
their environment? I am thinking of varices (at least in Murcidea) as
"webbed spines" more or less. The tendency for spinyness is derived, the
varices may be convergent. (Lest I be flamed for fatal flaws in my murcid
evolutionary history, I was mere stating a "for instance", not expressing
any specific knowledge of Murcidea phylogeny)
 
As I stated above, all species in existence are the product of adaptive
radiation. For specific examples of "Finch-like" cases, I am sure that there
are many other Conch-Lers out there who could cite thousands of island
pulmonate examples alone.
 
Co-evolution is an on-going process in any ecosystem. It is most noticable
in commensal relationships. Tridacna clams feed off of particular species of
dinoflagellate algaes (Zoothanthellae) that live within their tissues.
Without the dinos, the clam dies, without the clam, the algae dies. Both
have co-evolved to such a degree as to have created an obligate mutulaism.
Many bivalves are commensal in burrows of various crustacea. Many of these
crustacea are generalistic predators, yet somehow the bivalves have
co-evolved with the crustacean (probably by offering something in return?)
as to live relatively peacefully. Other great examples can be seen in
predator/prey relationships. Nudibranchs often feed on very toxic organisms.
While the nudibranch (predator) has evolved means of metabolising and even
using these toxins, the prey is also evolving ways to survive such as new
variations of toxins. For this reason we are now discovering that many
sessile organisms are equipped with an enormous and complicated battery of
new compounds that rival those found in the tropical jungles.
 
Finding good examples of character displacement in mollusks is outside of my
knowledge base. The one good example I can think of are juveniles of Tectura
scutum and Margarites beringensis. Both species (as adults) are mid to low
intertidal (at least in SE Alaska). However, the juveniles of both are
abundant in the high intertidal Fucus zone. This is resource partitioning at
its finest. It is counterproductive for an adult to outcomptete its own
offspring. By living in different zone on the same beach, the age classes
avoid intraspecific competition.
 
Well, I think I've said enough. Once again, I apologize for the length. I
hope I've answered some questions. Take care- Aaron
 
 
 
 
 
 
Aaron Baldwin
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