Dear List,

I'm passing along Greg Herbert's responses to my questions about the
Urosalinx tampaensis riding on the back of the Limulus caught in the act.

By the way, are the drills cheating or are they the world's fastest snails?

David Kirsh
Durham, NC

----------
From: [log in to unmask]
Date: Fri, 15 Dec 2000 12:40:04 -0800 (PST)
To: David Kirsh <[log in to unmask]>
Subject: Re: wet and wild


Dear David,

SINCE I STUDY THESE CRITTERS AND AM VERY INTERESTED IN THEIR EVOLUTION AND
ECOLOGY, I'VE DECIDED TO WRITE YOU A SMALL TREATISE TO ANSWER SOME OF THE
QUESTIONS YOU MENTIONED IN THE CC E-MAIL.

FIRST, UROSALPINX TAMPAENSIS IS REALLY AN ABERRANT EUPLEURA.  DON'T GO BY
THE
NUMBER OF VARICES.  TYPICAL WESTERN ATLANTIC EUPLEURA HAVE 4.  UROSALPINX
HAS 0
OR 1.  IF YOU TAKE A CLOSE LOOK AT EUPLEURA TAMPAENSIS, YOU'LL SEE THAT IT
HAS 2
OR (RARELY) 3 VARICES; ALSO, THE SHELL SCULPTURE IS UNQUESTIONABLY THAT OF A
EUPLEURA.  THE SPIRE WHORLS OF A EUPLEURA CAUDATA AND A EUPLEURA TAMPAENSIS
ARE
VERY DIFFICULT TO DISTINGUISH, AND NEITHER LOOK AT ALL LIKE UROSALPINX
CINEREUS.
 THE REDUCED NUMBER OF VARICES IN EUPLEURA TAMPAENSIS OCCURRED IN THE FOSSIL
HISTORY OF THIS GROUP AND REPRESENTS, POSSIBLY, EARLY MATURATION RELATIVE TO
AN
ANCESTRAL CONDITION.  IT DOESN'T HAVE 4 VARICES BECAUSE IT DOESN'T LIVE LONG
ENOUGH TO MAKE 4.

> Your comments seem to parallel the abstract I forwarded on the symbiotic
> (commensal is more accurate?) relationship between Thais h. floridana and
> blue crabs/hermit crabs.

'SYMBIOSIS' IS OFTEN USED IN A VERY GENERAL SENSE BY SCIENTISTS, USUALLY
BECAUSE
VERY LITTLE IS KNOWN ABOUT THE INTERACTIONS BETWEEN THE SPECIES IN QUESTION.
STRICTLY SPEAKING, A SYMBIOSIS MAY REFER TO COMMENSALISM, PHORESY, OR
MUTUALISM.
 IN 'COMMENSALISM' NUTRIENT SOURCES ARE SHARED.  IN 'PHORESY' ONE PARTNER IS
BORNE OR CARRIED BY ANOTHER.  IN 'MUTUALISM' ONE PARTNER BENEFITS ANOTHER IN
SOME WAY.  THESE RELATIONSHIPS CAN BE FURTHER SUBDIVIDED INTO 'FACULTATIVE'
(WHERE ONE PARTNER CAN COMPLETE ITS LIFE HISTORY IN THE ABSENCE OF THE
OTHER)
AND 'OBLIGATE'(WHERE ONE PARTNER REQUIRES PHYSICAL CONTACT WITH THE OTHER
THROUGHOUT MOST OR ALL OF ITS LIFE HISTORY).  SO I WOULD SAY THAT IN THE
EUPLEURA/LIMULUS CASE, WE PROBABLY HAVE FACULTATIVE PHORESY, COMMENSALISM,
AND
MUTUALISM.

> As usual, a little information raises yet more questions. The article deals
> with blue crabs and hermit crabs not Limulus as passive transport. And
> Thais, Cantharus and Boonea species as mounters, not Urosalpinx. So, both
> gastropods who "mount" and their mounting targets are generalized across
> taxonomic categories.
> Doesn't this suggest that this behavior likely evolved before the divergence
> of muricid and buccinid ancestors, etc.?

NO.  IF THIS BEHAVIOR IS ADAPTIVE, THEN IT COULD HAVE EVOLVED INDEPENDENTLY
IN
MULTIPLE LINEAGES BY A PROCESS OF NATURAL SELECTION.  BECAUSE BUCCINIDS AND
MURICIDS ARE SO DIVERSE ECOLOGICALLY, EVOLUTIONARY CONVERGENCE SEEMS THE
MORE
LIKELY EXPLANATION.  MY GUESS IS THAT THERE ARE MULTIPLE TARGETS OF
SELECTION
HERE.  FIRST, IT'S A BETTER STRATEGY TO LAY EGG CAPSULES IN TWO OR MORE
OYSTER
PATCHES THAN ONE SINCE SOME CATASTROPHE MAY STRIKE THE FIRST PATCH.  SECOND,
DISPERSAL INCREASES GENETIC DIVERSITY AND DECREASES THE LIKELIHOOD OF
PROBLEMS
ASSOCIATED WITH INBREEDING.  THIRD, THIS TRANSPORTATION SERVICE MAY ONLY BE
SOUGHT OUT WHEN FOOD RESOURCES ON ONE OYSTER PATCH ARE EXHAUSTED OR NEARLY
EXHAUSTED.

> From the information in the abstract (snails dismounting quickly in the
> presence of oysters), isn't it likely that the need for transport to a new
> food source outweighs the need for food on the transporter itself as a
> primary reason for the genesis of this behavior?

YOU MAY BE RIGHT ABOUT THIS ONE.  IT WOULD BE INTERESTING TO TEST WHETHER
DRILLS
ARE ACTUALLY ATTRACTED BY ANY CHEMICAL CUES GIVEN OFF BY LIMULUS.  IT
WOULD BE RELATIVELY EASY TO TEST.  HOWEVER, COMPETITION FOR FOOD AMONG
DRILLS IS
PROBABLY STRONG.  MANY OF THE SMALLER DRILLS, SUCH AS EUPLEURA, SEEM TO BE
INCAPABLE OF EATING LARGER OYSTERS AND STARVE AND CEASE GROWTH IN THE
ABSENCE OF
OYSTER SPAT.  BARNACLES AREN'T TOO COMMON IN THE OYSTER FLATS I'VE STUDIED
BUT
ARE A KNOWN DELICACY FOR DRILLS.  MAYBE THE DRILLS ARE ATTRACTED BY THE
SCENT OF
BARNACLES ON THE CRAB CARAPACE?  I WOULD THINK THAT IF ALL OF THE SPAT IN
THE
AREA HAD BEEN EATEN, THE DRILLS THAT COULD FIND ALTERNATIVE FOOD RESOURCES
(EITHER ON THE CRAB ITSELF OR IN ANOTHER ESTUARY MADE POSSIBLE BY CRAB
TRANSPORT) WOULD HAVE A COMPETETIVE EDGE OVER OTHER DRILLS IN TERMS OF LONG
TERM
GROWTH AND REPRODUCTIVE SUCCESS.  BIGGER DRILLS LAY MORE EGG CAPSULES AND
HAVE
MORE EMBRYOS PER EGG CAPSULE.

GREG