Current Research:

                                                                
                                                         

Energetic Implications of Living in a Wave-swept Habitat
    We have been working to understand the energetic implications of "wave" force and period.  To this end, I have constructed a "gastropullomatic" which mimics waves in a laboratory setting.  Period, force, and the angle at which the force is exerted can all be manipulated.  Two motors are attached via clamps to a metal rod arcing over a seawater bath, causing two pegs to rotate in separate circles.  The peg can be moved along a bar attached to the motor to control the diameter of the circle.  Two springs are attached, one to each peg.  The other ends of the springs are attached, via a length of wire, to a hook affixed to the shell of the experimental animal.  Each animal is placed in the seawater bath, which is contained by a water jacket connected to a recirculating chiller to maintain a constant temperature.

    "Wave" period is manipulated by controlling the time for each motor to complete one revolution.  For example, during a period of 10 sec each motor would compete its revolution in 5 sec, representing a wave washing over the animal and back  again.  "Wave" force is manipulated by using springs with varying tensions and placing the pegs attaching them to the motors at varying positions along the bar.  Thus, if a particular spring would exert a 5 N force when it is stretched 3 cm, the peg on the bar would be set such that the diameter of the circle it traveled (and thus the distance the spring would be stretched) is 3 cm.  The angle at which the force acted can be manipulated by sliding the motors along the metal arc to the desired angle then clamping them firmly in place.

    This experimental device has been used to investigate the effect of "wave" force and period on aerobic energy expenditure and tenacity in limpets, and on anaerobic energy expenditure in the Elephant snail Scutus antipodes.  We found that oxygen consumption increases with increasing "wave" force and decreasing "wave" period in the limpet Lottia pelta and that aerobic energy expenditure in moderate "waves" is similar to that of a crawling limpet.  We have also found that tenacity decreases with increasing force and period in the limpet Acmaea mitra.
 
   Three grad students working in my lab, Chris May, Mike Thimgan, and Scott Cowan have investigated different aspects of this project.

Energetics of Gastropod Swimming
        We have recently begun experiments investigating the energetics of gastropod swimming.  There are three general forms of swimming in gastropods: parapodial flapping, lateral bending, and dorso-ventral undulating.  These experiments started when one of my Masters students, Sandra Caldwell, investigated the energetics of lateral bending in the Lion Nudibranch Melibe leonina.  
       More recently, and with Dr. Tom Carefoot and Dr. Steve Pennings, I have investigated the energy expended during parapodial flapping and compared it to energy expended during crawling in the sea hare Aplysia brasiliana.  We found that although swimming and crawling require similar amounts of aerobic energy, swimming cost of transport is much lower than crawling cost of transport probably due to the fast, directional nature of swimming in this species.  During prolonged swimming bouts, A. brasiliana decreased parapodial beat frequency by approximately 10% after 4 h while swimming velocity decreased by 30%.  This demonstrates a loss of flapping efficiency with prolonged swimming and we observed that the parapodia did not overlap as completely during a stroke as the animals tired.  
        I recently investigated dorso-ventral undulation in the nudibranch Tritonia diomedea with Beth Moore, a former WWU undergraduate and REU student at Shannon Point Marine Center.  In this case, we videotaped swimming animals and found that the number of body flexions per swim bout was dependent upon animal size, with smaller animals completing significantly more flexions per swim bout. The rate at which T. diomedea flexes its body was also dependent upon animal size, with smaller animals showing a faster flexion rate. Larger animals exhibited significantly farther distances traveled per individual flexion. Total swim time (49 ± 8 sec) was independent of animal size, as well as total swim distance (166 ± 43 cm).  Swim speed (124 ± 27 m h-1) was also independent of animal size, as was crawling speed (8  ± 2 m h-1).  To measure energy expenditure during swimming and crawling, T. diomedea were placed in respirometers and oxygen consumption was monitored continuously during resting, crawling, and swimming activity states.  Resting oxygen consumption for an average 400g animal was 5.6 ml O2 h-1, crawling oxygen consumption was 7.8 ml O2 h-1, and swimming oxygen consumption was 17.6 ml O2 h-1.  Cost of transport (COT) was estimated by dividing mass-specific oxygen consumption of a moving animal by its speed.  Swimming COTnet for smaller individuals was significantly higher than for larger individuals.  Swimming and crawling COTnet for an average 400g animal were similar to other swimming and crawling invertebrates, although comparisons with other swimming opisthobranchs are still inconclusive due to the non-directional swimming nature of T. diomedea.

Swimming of Epibiont-encrusted scallops
    Two local species of scallops, Chlamys hastata and Chlamys rubida, are regularly found heavily encrusted with the sponges Mycale adhaerens and Myxilla incrustans.  It is thought that this relationship is a mutualism since both scallops and sponges are less vulnerable to predation when they coexist.  However, the energetic implications of carrying sponges while the scallops are swimming are unknown.  Scallops are also found encrusted by balanoid barnacles, which are potentially more detrimental than sponges.  We investigated the impacts of sponge and barnacle epibionts on scallop swimming by 1) videotaping scallops swimming with and without epibiont encrustation to determine whether epibionts affect the distance the  scallops are able to swim, 2) measuring oxygen uptake to determine aerobic energy expenditure while swimming with and without epibiont encrustation, 3) measuring anaerobic metabolite buildup after swimming with and without epibiont encrustation to determine anaerobic energy expenditure during swimming, 4) determining morphological changes in shell dimensions and muscle mass with increased scallop size and epibiont encrustation, and 5) measuring the drag experienced by scallops encrusted with different epibionts.  Briefly, we found that 1) there were no apparent changes in swimming behavior with sponges although other animals (especially barnacles) have dramatic effects, decreasing the distance and height of swimming, 2) there were no differences in aerobic energy expenditure when scallops swam with and without sponge and barnacle encrustation, 3) scallops swimming with sponges produced the same amount of anaerobic metabolites as scallops swimming without sponges, but barnacle encrustation increased anaerobic energy expenditure, 4) scallop morphometry is not affected by epibiont encrustation but Chlamys do show intraspecific ontogenetic changes similar to other scallop species, and 5) sponge encrustation does not alter the drag experienced by swimming scallops but other epibionts such as barnacles and bryozoans do.  Thus, sponges do not appear to affect the swimming capability of scallops but barnacles increase drag, increase anaerobic energy expenditure, and decrease swimming distance and height.  This work was conducted with Brian Bingham at the Shannon Point Marine Center.
 

Swimming in the file shell Limaria fragilis
    Swimming has been developed in only a few orders of Bivalves, including scallops (Pectinidae) and file shells (Family Limidae).  The mechanics and energetics of scallop swimming have been well studied.  Scallops exhibit a range of swimming abilities, from short-burst, "zig-zag" swimmers such as Chlamys to long-range, gliding swimmers such as Amusium and Placopecten.  Swimming ability in scallops is related to shell morphometry and the ability to produce power, and these vary amongst scallop species.  These also change intraspecifically with size since scallops must produce proportionately more power as they grow to compensate for increased mass.  For example, shells of long-swimming pectinids are comparatively thin  and relative thinning of the shell occurs as these scallops grow, making lift easier to generate with increasing mass.  As well, shell width is usually positively allometric with shell height in swimming pectinids (that is, shells become relatively broader as scallops grow), which increases the aspect ratio and makes swimming more efficient by decreasing drag.  Muscle mass is also positively allometric with shell height, increasing the relative power that can be generated by larger scallops.  In all scallops, swimming lasts a relatively short time (a few minutes) and is powered, to a large degree, anaerobically.  Scallops swim to escape predation, to position themselves in better habitat, and to migrate.
     Swimming in Limidae is less well understood.  File shells, like scallops, swim by quickly adducting their valves and producing posterior propulsive jets of water.  This causes them to swim with their hinge hindmost.  The energetics of swimming in file shells have been well characterized.  Limaria fragilis is capable of swimming for extended periods (several minutes) and swimming is mostly aerobic, but with a substantial anaerobic contribution from arginine phosphate, octopine dehydrogenase, and ATP stores.  Their cost of transport is substantially higher than in other swimming invertebrates, possibly due to the cost of carrying a shell, no matter how light, and to swimming with the extensive mantle tentacles protruding into the water.  File shells typically have long tentacles that extend from the mantle and can not be withdrawn into the shell.  During swimming, these tentacles are capable of producing thrust by participating in a rowing motion and the thin shell is thought to decrease the sinking rate of the animal when it stops valve-clapping.
    More recently, the behavior, morphometry and mechanics of swimming in Limaria fragilis were characterized and compared to the better understood scallops.  Absolute swimming speed (cm sec-1) increased linearly with increasing shell height, although relative swimming speed (body lengths sec-1) was independent of shell height.  The increase in absolute swimming speed was due to an increase in the distance covered during each valve clap as clap distance (cm clap-1) also increased linearly with shell height while clapping frequency (claps sec-1) was independent of animal size.  Limaria fragilis displayed a variety of morphological changes related to size.  Shell length was negatively allometric with shell height indicating the shell became proportionately slimmer in larger animals.  Dry shell mass was negatively allometric with shell height, while both dry adductor muscle mass and dry mantle + tentacle mass were positively allometric.  Autotomy of mantle tentacles significantly decreased clap distance by 13% without affecting clapping frequency or swimming speed.
  These studies were conducted at Heron Island Research Station on Australia's Great Barrier Reef with Dr. John Baldwin and Dr. John Elias.


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