title for limpet section of the Odyssey
   
  Foods, feeding, & growth
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Radula structure & function

  This section includes an explanation of radula structure & function.  Information on FOODS OF DIFFERENT SPECIES is presented in another section.
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Research study 1
 

photograph of radula of limpet Lottia personadrawing of head end of a limpet to show action of radula scraping on algaeLimpets use a radula  to scrape algae from rocks.  The radula consists of evenly spaced horizontal rows of cusps or teeth on a supporting ribbon, each row comprising 2-6 cusps. Rows in some species number up to 200 or more.  The cusps and ribbon are contained in a posterior-oriented radular sac, within which the radular components are produced, and anteriorly the ribbon wraps around a cartilaginous structure called the odontophore.  The ribbon, shown in side view in the photograph, is comprised of chitin and protein. It is firmly fixed to the odontophore but can slide along and over it, much as a human fingernail slides over the underlying tissues, as cusps are worn at the front and produced at the back in thel secretory sac.  Several muscle groups attach to the odontophore (not shown in the drawing) and cause the radula to be extended out in a rolling action, then withdrawn to scrape up food.  The cusps have a backwards orientation for maximum effectiveness.  After being  scraped up, the food passes into the esophagus and thence to the stomach and digestive gland.  Drawing adapted from Padilla 1985 Mar Biol 90: 103. 

NOTE  more information on radular feeding in gastropods is found elsewhere in the ODYSSEY: LEARN ABOUT ABALONES: NUTRITION & GROWTH: FEEDING

 
Research study 2
 

The cusps are hardened with a combination of silicon and crystalline iron oxide.  The latter chemical imparts the typically black coloration to the cusps.  Unlike in chitons, where the cusps are hardened with another oxide of iron known as magnetite and exhibit a small magnetic moment, photograph of radula of limpet Lottia personalimpet cusps are non-magnetic.  Both types have a similar hardness of 5-6 on the Mohs scale of hardness, slightly softer than quartz.  The discovery of goethite in the radulae of owl limpets Lottia gigantea, dunce-cap limpets Acmaea mitra, and other species represents the first known example of precipitation of this mineral in marine invertebrates.  The author of the discovery suggests that the iron may come from iron-rich algal foods or from dissolution of ingested iron-containing sediment particles in the gut.  Lowenstam 1962 Science 137: 279.

NOTE the mineral involved in hardening limpet radulae is goethite: Fe2O3 . H2O

 
Research study 3
 

The hard radulae of limpets and other molluscs scraping across the substratum create feeding noises that can be heard and recorded by suitable microphone and amplifier equipment.  Feeding noises of limpets can be recorded and related to type of food, time of day, tidal level, and so on. For photograph of unidentified radula scrapings on shaleexamples of such data for a variety of snails including several limpets see Kitting 1979 Oecologia 40: 1.

NOTE  the author uses a waterproofed contact microphone with preamplifier and standard tape recorder, but notes that many feeding sounds (such as limpets, turban shells, and chitons) can actually be heard via a surgical rubber tube or stethoscope tube held in the ear, as long as the test subject is out of water or in water less than about 1cm deep.  Indeed, on a quiet night at low tide the human ear can easily pick up a limpet’s scratching noises as it feeds.  Other transducer devices used to record radular scrapings include piezo-electric transducers (e.g., from old phonograph players) coupled with amplifiers that can be glued directly to the shell.  For information on such devices see Boyden & Zeldis 1979 Estuar Coastal Mar Sci 9: 759

Unidentified radula scrapings on shale.
The barnacles are Balanus glandula

 
Research study 4
  photograph of limpet Lottia personagraph showing radula lengths of different species relative to shell lengthLimpet radulae are relatively long, matching the length of shell in species such as Lottia pelta and L. scutum, or up to 2-times shell length in L. persona.  This must represent a balance between rate of use and rate of production, but there is little published information on it for west-coast species.  Why L. persona should have such an extraordinarily long radula relative to shell length is not known.  In comparison, chitons have relatively short radulae, yet mobilise and incorporate iron into their radulae (for hardness) in ways similar to limpets.  Perhaps the preference of chitons for fleshy algae and their (presumably) lower metabolic rates and slower feeding rates account for some of the difference.  Dehnel 1978 Can J Zool 56: 643.
 
Research study 5
 

photograph of dunce-cap limpet Acmaea mitra bearing coating of coralline algaephotograph of close view of radula of dunce-cap limpet Acmaea mitra showing morphology of cuspsHow resistant are different forms of algae to limpet radulae? This is tested at Friday Harbor Laboratories, Washington with radulae of Acmaea mitra and Lottia scutum mounted in such a way that alga thalli can be pulled against the cusps and the forces required to do this measured with a force-transducer system. Three types of algae are tested, each with a different thallus form: the thin-bladed red alga  Mazzaella splendens, the thick-bladed brown alga Hedophyllum sessile, and the red encrusting coralline alga Pseudolithophyllum sp., with the expectation that resistance of the algal thalli to the radulae will follow this order. 

The radula of A. mitra, a specialist feeder on coralline algae, has 6 cusps per row and each cusp has a clearance angle of 0-10o (the exact angle depends upon the amount of wear on the cusp; photograph upper Right) and a rake angle of 50o (see drawing lower Right). In comparison, the radula of L. scutum, a generalist feeder, has 4 cusps per row, a clearancephotograph of close view of radula of limpet Lottia scutum showing cusp morphology angle of about 50o, and a rake angle of about 20o (see photo middle Left).

drawing a action of radula cusp of a limpet as it scrapes along the substratumSurprisingly, the results follow the inverse order, suggesting that thallus form, contrary to predictions made by earlier functional-group models, is actually a poor predictor of structural resistance to the limpet radulae (see histogram lower Left). Note that the encrusting coralline alga Pseudolithophyllum is readily abraded by the cusps of both limpet species, while both the fleshy brown Hedophyllum and the corticated red Mazzaella are more easily scraped by cusps of A. mitra than by cusps of L. scutum. The author explains the results as follows. The crystals of calcite (calcium carbonate) that make up the tissue graph of force required to remove algal tissues by different species of limpetof coralline algae are quite brittle and tend to break under applied forces.  In comparison, fleshy algae are rubbery and stretch under applied forces, thus requiring more energy to break apart the tissues. Padilla 1985 Mar Biol 90: 103.

NOTE  previously known as Iridaea cordata

NOTE  this clearance angle actually provides for self-sharpening of the cusps

 
Research study 6
 

photograph of limpet Lottia personaIt has long been known that “radula fraction”, that is, the ratio of radula length to shell length in a limpet species, is greater in individuals living higher in the intertidal zone than in ones living lower down.  The explanation is that the high-level individuals use their radulae less than low-level ones because they are covered by the tides for shorter periods.  This abundance of radula can be thought of as an investment to be drawn on under improved feeding conditions. Use of shell length as a unit of radula fraction, however, has inherent problems in that shell length is known to decrease with increasing intertidal height in some species owing to desiccation stress.  The explanation for this is that in clamping more firmly to dry rock the foot muscles pull in on the shell and growth in such a habitat leads to a taller but smaller-diameter shell. This would increase radula fraction without a change in absolute radula length.  A study at the Bamfield Marine Sciences Centre, British Columbia on limpets Lottia persona and Lottia digitalis has identified a correlate of radula length that is not independently affected by intertidal height, namely, dry mass of soft tissues.  graph of radula lengths of limpets Lottia persona and Lottia digitalis vs. mass of soft tissues at high and low tide levels

The plot on the Right shows significantly longer radulae in higher-level L. persona, but not in higher-level L. digitalis, when a correlate of dry mas of soft tissues is used. Comparative radula lengths at a common dry-tissue mass of 60mg, are 31 and 28mm for high and low L. persona, respectively, and 17 and 17mm for L. digitalis, respectively.photograph of cluster of limpets Lottia digitalis Why the difference in the 2 species?  Comparison of radula fractions in these and other species of limpets, including some tropical homing ones, show that intertidal height-related differences in radula fraction are accentuated in species with less intertidal-height habitat fidelity.  This makes sense, as the more an individual stays in the same spot on a rock as, for example, in a homing species, the more likely the radula fraction will reflect the extent of day-to-day feeding opportunity of that individual.  Daily movements of Lottia persona tend to be more in the horizontal plane, while those of Lottia digitalis tend to be in both vertical and horizontal planes.  Thus, while L. persona mostly sticks to one intertidal level, L. digitalis moves vertically in response to tides and wave splash, exhibits dispersal in response to crowding, and undertakes seasonal vertical migrations. Taylor et al. 1989 Veliger 32: 274. Photos courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC. 

NOTE  a stepwise regression method is used to determine the best-fit correlate out of the following parameters: shell length, shell width, shell height, shell volume, and dry mass of soft tissues, using both linear and logarithmic values

NOTE  vertical separation in the populations of both species is about 1.6m.  Regression statistics for logY = loga + blogX for high and low L. persona are a = 0.87 (a is given in log form), b = 0.35, and a = 0.89 b = 0.31, respectively, and for L. digitalis, 0.72 and 0.29 (for high) and 0.74 and 0.28 (for low)

 
Research study 7
 

close view of radula of an owl limpet Lottia gigantea showing structureFurther information on the mineralization of radular cusps in limpets is provided from research on Lottia gigantea at Scripps Institution of Oceanography, La Jolla, California.  The radula of  a large individual of this species has about 150 transverse rows (250 maximum) and is comprised of iron, silicon, and calcium incorporated into a base scaffolding of chitin/protein.  As in chitons, these component materials are secreted from a layer of columnar epithelial cells that is sited dorsally to the cusps.  Iron is transported as crystalline granules of ferritin, has the form of goethite (alpha-FeOOH), becomes morphologically evident with histological staining not until about Row 15 counting from the back of the radula, and is largely complete by about Row 30.  In a similar pattern, silicon is incorporated into the cusps in the form of opal (SiO2 . nH2O).  Both components have about the same hardness on the Mohs scale,  5.5-6, making them slightly softer than pure quartz (7) and softer than the hardening agent in chiton radulae (magnetite: 7).  Rinkevich 1993 Mar Biol 117: 269.

NOTE  the author provides values for concentrations of calcium and opal at selected levels in the radulae, but only relative to the concentration of iron.  Thus, nowhere in the study are absolute values or percentage concentrations of any of these minerals given

 
Research study 8
 


Radulae of limpets Lottia spp. and Acmaea mitra are quite different from those of keyhole limpets Diodora aspera and Megathura crenulata. The former, known as a doccoglossan1 (“beam” “tongue” L.) type, consists as we have seen of rows of small similarly shaped, shovel-like cusps that are scraped along rocky or algal substrata to excavate material that is then ingested. In contrast, keyhole limpets have a rhipidoglossan (“fan” “tongue” L.) radula, which consists of rows of cusps of quite different appearance (see photographs on the Left; note the brush- and rake-like morphologies of the Megathura radula). Note in the accompanying diagram that a Lottia-type radula has only 2 types of cusps, marginal and lateral, of similar morphology, while a Megathura-type radula has 5 tooth morphologies, including the distinctive brush-like marginal and central rachidian cusps. Instead of excavating, this latter type of radula is good for brushing across the substratum and collecting loose organic debris. photo-schematic comparing doccoglossan-type and rhipidoglossan-type radulae using Lottia persona and Megathura crenulata as examples

A group of scientists from Germany and Slovenia investigate radulae of specimens of Megathura crenulata imported from California, incorporating numerous scanning and analytical techniques to provide us with all the information on structure and functioning of rhipidoglossan radulae than we could ever need…and more. Apart from the different tooth morphologies noted above, doccoglossate radulae are less mobile owing to fewer radula muscles and firmer bonding of the cusps to the radular membrane. As described by the authors, these radulae function as “chains of shovels” for scraping hardened macroalgae, but are unsuitable for collecting microalgae from substrata. With their mineral (mainly iron) components, doccoglossate-limpet cusps have tensile strength that is among the highest recorded for any biological material. In comparison, rhipidoglossan radulae have cusps that are more narrow at their bases, are more loosely attached to the membrane, and are relatively long and more flexible. The teeth lack mineralising iron compounds and are thus softer. Additionally, there are more types of muscle two views of the major lateral cusps in the radula of a giant keyhole limpet Megathura crenulatabundles attaching the radula to the odontophore cartilages in the buccal mass; hence, providing greater diversity of movement. Although the rhipidoglassan radula is used in related keyhole-limpet species mainly for sweeping up microalgae from rough substrata, the radula of Megathura crenulata is even more specialised, as its diet also includes red and brown macroalgae, softer-bodied tunicates, and even some calcified coralline algae. Note in the photograph on the Left that the major lateral cusps of Megathura have similar curved morphology to those of a Lottia-type radula and these cusps, although composed only of softer chitin/protein without iron mineralisation, may be the cusp-type that is involved in eating harder algae. As described by the authors, the variety of “tools” represented by M. crenulata’s radula (hook-like shovels, knives, and brooms) combines the sweeping-behaviour “know-how” of the general rhipidoglossan radulae with the high scraping efficiency of a doccoglossate radula. Ukmar-Godec et al. 2015 J Struct Biol 192: 392; photograph below Left courtesy NOAA; photograph below Right courtesy photoandy.

NOTE1 this word is normally spelled "docoglossan", but is used here in the form adopted by the authors. Perhaps they know best. The radula types discussed here represent only 2 out of at least 7 types used in the literature to describe molluscan radulae

NOTE2 these include light- and electron-microscopical, spectrophotometric, micro-tomographic, X-ray scattering, and amino-acid-analysis techniques

 
NOTE3 this species is the largest fissurellid species known, reaching up to 13cm in length. It is indigenous to the west coast of North America

photograph of giant keyhole limpet Megathura crenulata photograph of the head of a giant keyhole limpet Megathura crenulata

Giant keyhole limpet Megathura crenulata crawling amongst a bed of coralline alga.
The mantle can be moved higher or lower on the shell and its colours are highly variable. The part of the shell visible here is brownish in colour and through the whiteish hole can be seen the ventilatory siphon. Deeper within will be the paired ctenidia and anus

This view shows the mantle covering the head and foot and the spacious mantle cavity. The muscular buccal mass surrounds the mouth (the latter not visible here) and the head bears the 2 large tentacles. Just visible to the right of the right-hand tentacle is the right eye. This, like other limpets, is not image-resolving

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