title for limpet section of the Odyssey
   
  Physiology
  Physiological topics relating to limpets include sensory, gas exchange, and osmotic regulation, dealt with in this section.
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  Sensory
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Research study 1
 

photograph of limpet Lottia scutum showing location of tentacles along the mantle edge
A study on Lottia scutum at Bodega Marine Laboratory, California explores the possible sensory functions of the many tentacles along the mantle edge.  The functions likely include food and predator sensing.  Each tentacle is crowned with a dense cap of stiff cilia and additional tufts are spread along the tentacle length.  Two types of sensory cells are evident on the tentacles, but the author is unable to assign functions to each based solely on ultrastructure.  In addition to the tentacle sensory cells, there are abundant photoreceptors on the mantle margin, and mechanoreceptors sensitive to touch and water currents.  Phillips 1977 Veliger 19: 266; Phillips 1979 Tissue & Cell 11: 623.

 
 
Gas exchange This section includes studies on limpets (Lottia spp.), keyhole limpets (Diadora aspera), and pulmonate "limpets" (Trimusculus reticulatus).
 
Research study 1
 

photograph of limpet Lottia pelta to show the direction of water flow through the mantle cavitydrawings of owl limpt Lottia gigantea with shell removed to show position of ctenidium and the water flow through the mantle cavityWhereas keyhole limpets have paired ctenidia, all “true” limpets (F. Acmaeidae) have a single ctenidium (the right one is lost in evolution).  The pattern of water flow for gas exchange for most west-coast species, including owl limpets Lottia gigantea, is into the mantle cavity from the left side, through the ctenidium, and out the right side (see drawing above Left, and photo). As the water flow exits the mantle cavity it receives discharge from the nephridiopore (urine) and anus. 

There are varying amounts of secondary flow through the pallial groove around the foot.  In most species, these tend to be cleansing flows rather than gas-exchanging ones.  drawing of limpet's pallial groove to show location of pallial gillsHowever, in L. gigantea there are many secondary, or pallial, gills arranged lappet-like in the pallial groove (see drawing lower Left). These are irrigated by weak water currents driven by cilia up and through the pallial gills (indicated by small arrows in the drawing).  As the shell is seldom clamped firmly down during low-tide periods, air spaces are likely present for aerial gas exchange.  The cilia may circulate a water film over the pallial gills and associated mantle surface.  The author notes that this is the first report of water currents for gas exchange in owl limpets.  Abbott 1956 The Nautilus 69: 79.

 
Research study 1.1
 

drawing from below of a pulmonate limpet Trimusculus reticulatusObservations on pulmonate “limpets” Trimusculus reticulatus by a researcher at Hopkins Marine Station, Pacific Grove, California suggest that gas exchange may be possible in both air and water.  The author describes a typical pulmonate pneumostome on the right side of the head that opens to allow air to circulate through the spacious mantle cavity (pallial groove; see drawing).  This cavity is lined by a thin epithelium, unciliated except for the channel leading to the exterior via the pneumostome.  An ability to repire both in air and water is suggested by the author’s witnessing the pneumostome opening widely to admit water in an immersed individual, and with the water visibly moving along the passage from pneumostome to mantle cavity.  However, the details of this behaviour are rather sketchy, and no mention is made of survival times of individuals continually immersed (see also Research Study 3.1 below).  Yonge 1958 J Malacol Soc Lond 33: 1.

NOTE  viewed from below with the animal on a glass slide

NOTE  this entire topic of gas exchange and survival in water would make a good research project for someone

 
Research study 2
 


drawings of limpets Lottia spp. to show ciliary currents through pallial groove and mantle cavityLottia paleacea lives exclusively on fronds of surfgrass Phyllospadix spp. in high-energy surf conditions.  Perhaps for this reason its shell fits tightly to the frond.  A notch is present on the right-hand side to accommodate water flow for gas exhange. Most other west-coast species have water-current patterns for gas exchange similar to that shown above for L. gigantea (see drawing). Lottia pelta differs from other species in not having cleansing currents in the pallial groove. Yonge 1962 Veliger 4: 119.

 
Research study 3
 

photograph of limpet Lottia digitalis

In Pacific Grove, California limpets Lottia digalis and L. scabra live sympatrically, but with the former species favouring more vertical rock surfaces and latter species inhabiting horizontal surfaces.  Measurements of rates of oxygen uptake of the 2 species at Hopkins Marine Station, California show that rates are higher in both when submerged than when air-exposed. The difference between the 2 conditions is most exaggerated for L. scabra than for L. digitalis.  Baldwin 1968 Veliger 11 (Suppl.): 79.

NOTE the graph shows cumulative oxygen consumption over a 2-h period

graph comparing oxygen uptake in limpets Lottia digitalis and L. scabra in and out of water
photograph of limpet Lottia digitalis
 
Research study 3.1
 

schematic showing location of air bubbles in subtidal cavesA researcher at Harvey Mudd College, Claremont, California describes an unusual habitat for air-breathing intertidal pulmonates Trimusculus reticulatus within air bubbles caught on the underside of a subtidal ledge.  The bubbles appear to be naturally formed from air suspended by wave action at high tide.  The situation is fine for subtidal gas exchange using a lung (mantle cavity), but comes with a cost.  Trimusculus is a suspension-feeder, and so must wait for the bubble to dissipate in order to feed.  However, given the risk of not being able to find new bubbles to survive, one wonders if the “habitat” might be happenstance and not one that can be permanently occupied.  The researcher assesses the “refill capacity” of the system by emptying air pockets and checking rate of regeneration, but a regularised protocol of checking is stymied by rough surf conditions.  On one occasion 9 SCUBA divers pass close to the ledge either to or from their dive site with no apparent effect on air-bubble size or number.  The study is short but interesting, and leaves the reader wanting more.  One immediate question is the extent to which the snail can utilise dissolved oxygen in the water as well as gaseous oxygen via its lung. Haddock photograph of pulmonate "limpet" Trimusculus reticulatus1989 The Veliger 32: 403. Photograph courtesy Gary McDonald, Long Marine Laboratory, Santa Cruz & CALPHOTOS.

NOTE  the author uses gas-chromatography to test whether the source of the air pockets was SCUBA activity, but the results are not conclusive.  The bubbles contain <0.5% CO2, while air contains about 0.04% and human exhalation about 3% 


Pulmonate "limpet" Trimusculus
reticulatus
encrusted with
tubeworms 2.5X

 
Research study 4
 


photograph of keyhole limpt Diodora asperaWater flow for gas exchange in keyhole limpets Diodora aspera is also driven by beating of cilia lining the mantle cavity.  The pattern of flow is in over the head and sensory tentacles, over the paired ctenidia for gas exchange, upwards past the kidney and reproductive openings, then past the anus and, finally, out the hole in top.  This pattern ensures that waste products are directed away from the limpet without contaminating the head and gills.  The pattern described here supports a long-held idea that sanitation is the primary factor involved in evolution of mantle-cavity design and water flow in molluscs. The hole in the shell of Diodora also tends to operate passively; that is, water drawings of keyhole limpets to show water currents through mantle cavity with hole open and pluggesflowing over it causes water to be “pulled” out of the mantle cavity, thus augmenting the gas-exhange flow described above. 

Interestingly, if the hole in Diodora is blocked either naturally or experimentally with a plug of clay there is no observable deleterious effect on the animal, giving rise to the suggestion that, rather than the hole being primarily for sanitation (allowing feces and urine to be released without contaminating the head and gills), it may be equally important in inducing passive flow.  Voltzow & Collin 1995 Invert Zool 114: 145.


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  there is actually no shortage of ideas relating to the function of the hole in Diodora’s shellThe fact that a keyhole limpet appears to maintain good health with its hole blocked suggests a function unrelated to sanitation or gas exchange - for example, reproduction.  Perhaps the hole in the shell increases fertilisation success by releasing the gametes well above the substratum surface where they would float for a longer time and have more chance of meeting gametes of the opposite sex

 
Research study 5
 

diagrammatic representation of design elements of the mantle cavity of a keyhole limpet Diodora asperadrawing showing flow pattern through the mantle cavity of a keyhole limpet Diodora aspera
















Water flow for gas exchange in a keyhole limpet Diodora aspera enters the mantle cavity on either side of the head, flows upwards past and through the ctenidia, and exits via the hole at the top of the shell. On its way out it picks up urinary and fecal wastes (see diagram on Right). Theoretically, the flow can be modeled on the Principle of Continuity that describes flow through pipes.  The Principle states that the volume-flow rate, or the product of the cross-sectional area and velocity, remains constant along the length of the pipe (see diagrammatic representation of the mantle cavity of Diodora on Left). Studies at Friday Harbor Laboratories, Washington using dyes to visualise flow patterns, and transport of neutrally buoyant particles to determine flow rates show, indeed, that water flow through the mantle cavity does accord with the Principle.The author comments that this information allows flow rates along any part of the flow path to be predicted.  Voltzow 2004 Am Malacol Bull 18: 155.

NOTE  the author also includes the tulip shell Fasciolaria hunteria in the study, a species not found on the west coast of North America

NOTE  flow volume is approximately 40 mm3 . sec-1 for an individual of 14g live mass

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  Osmotic regulation
 
Research study 1
 

photograph of cluster of limpets Lottia scutum in a tidepool
The limpet Lottia scutum tends to favour tidepools, where rainfall may create fluctuating salinity conditions.  Studies at the University of British Columbia show that the species is, in general, an osmoconformer. Hemolymph and urine concentrations of sodium, chloride, calcium, and magnesium are the same as in experimental  seawater media over a range of test salinities from 50 to 125%. Webber & Dehnel 1968 J Exp Zool 168: 327; Webber & Dehnel 1968 Comp Biochem Physiol 25: 49.

NOTE  where 100% = 32‰

 
Research study 2
 

photograph of limpet Lottia scutum upside-down, but not swelling because it is not immersed.  Note the feces accumulating from the anus on the right-hand sideLarge volume changes, up to 100% of the volume of soft tissues, are observed in limpets Lottia scutum when they are turned upside-down under water.  Experiments on specimens collected at Jordan River, British Columbia and Monterey, California using inulin and other dye markers confirm that the volume increase occurs by bulk movement of seawater into hemocoelic spaces, perhaps via the gut.  The function, if any, of the volume uptake is not known.  Webber 1970 Veliger 12: 417.

 

 

 

 

Lottia scutum upside-down, but not swelling because it is not immersed.
Note the feces accumulating from the anus on the right-hand side 2.2X


 
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