Research Study 1: Lottia asmi

Fig. 1.  Heavily eroded black turban-shell Tegula funebralis bearing limpets Lottia asmi

Courtesy Dave Cowles, Walla Walla University, WA

The limpet Lottia asmi is a small species (6mm shell length) commonly found on the shells of black turban snails Tegula (Chlorostoma) funebralis (Fig. 1), but also occasionally free-living in small rocky tidepools. Shell colours vary in relation to habitat occupied. An early description of the behaviour of L. asmi in Moss Beach, California reveals that an individual Tegula may host one, two, or sometimes up to 8 limpets, but the limpets seldom remain more than a few hours on any given host shell.  Indeed, when removed from a host Tegula the limpets preferentially seek out a new host, orienting most likely in response to chemical emanations from the snails.

Research Study 2: Lottia asmi

A study on Lottia asmi collected from Pigeon Point in San Mateo County, California reveals that their location on a “host”-snail Tegula funebralis is not static.  At any given time there may be 1-4 limpets on a single snail, and they move from snail to snail quite readily, especially at night. An individual limpet may change hosts 5-20 times over a 4wk period.  Interestingly, some snails are more popular and are ridden more than others, sometimes by the same cohort of limpets.  In fact, these cohorts sometimes move together as units from snail to snail.  The author suggests that L. asmi is a “carbonate-associated” species, and may be found not just on snails but also on other carbonate substrata, such as coralline algae.  If an individual limpet becomes stranded on rock for several months, its shell colour and gross morphology change significantly.

Research Study 1: Lottia digitalis

Fig. 2.   Relationship between shell length and shell height in limpets Lottia digitalis on two habitats, rocks and goose barnacles Pollicipes polymerus

Fig. 3.  Relationship between limpets Lottia digitalis and the scutal-plate size of goose barnacles Pollicipes polymerus on which the limpets live.  There appears to be no reason for the line to be curvilinear rather than straight

Fig. 1.  Pollicipes-type Lottia digitalis showing how well they are visually comouflaged. There are at leasst three limpets visible, and one winkle

Another type of habitat fidelity is exhibited by ribbed limpets Lottia digitalis.  Two forms are found in Oregon, one living on sandstone rock, the other on goose barnacles Pollicipes polymerus.  Individuals on Pollicipes are whiter in colour than their rock-inhabiting conspecifics and this base coloration, combined with black fleckings, may camouflage the limpets from predatory shore-birds and fishes Fig. 1.  Interestingly, the two types of limpets differ in their relative and absolute growth rates.  Not only does the Pollicipes-type limpet increase in height relatively faster than the rock type as it adds to its shell length (see Fig. 2), but growth of the barnacle form matches growth of its host¹ (see Fig. 3).  Despite the indication in Fig. 3 that large limpets (15-17mm shell length) frequent goose barnacles, the author notes in the paper that sizes greater than 14mm are rarely found on the barnacles, suggesting that larger limpets are forced to move from their barnacle hosts to the rock face when they reach a certain “over-grown” size.  The author notes that the limpets routinely crawl off the barnacles at high tide to feed on algae on the rock face, and return later.  If so, they must return to the same goose barnacle as they left, or at least to one equivalent in size to the one they left; otherwise, the data² in the graph would not hold true. As for the relatively taller sizes of limpets on Pollicipes, the author suggests that the protection offered the limpets from wave impact may allow taller sizes to be attained, but the significance³ of this is unclear.

NOTE¹ the author discusses the implication of the curved line (second-degree polynomial regression fit) to the biology of the relationship of shell length of limpets to scutal length of the barnacles, but it is unclear why the relationship would be expected to scale non-linearly. It is also unclear why the author chooses to log the data in the graph (above Right). There is no reason for it and it adds another level of transformation to what should be a simple arithmetic (and more easily interpretable) relationship

NOTE² this part of the study should be corroborated by another researcher, as there are several assumptions to be addressed. One of these is that limpets must stay put while they grow or, if they do move from barnacle to barnacle (which seems likely), they must move between ones of similar sizes. Another is that limpets returning from feeding expeditions to the rock must find their original barnacle or, at least, one similar in size

NOTE³ other limpet species grow relatively taller when inhabiting dry rock surfaces (as compared with under algal canopies), a result, it is thought, of the mantle muscles contracting around the circumference of the shell to maintain as tight a seal as possible on the substratum, thus pulling the shell in as it grows, and leading to a more conical shape

Research Study 2: Lottia digitalis

Fig. 1.  A light-coloured Lottia digitalis crawling on a cluster of goose barnacles Pollicipes polymerus.

As noted in the foregoing Research Study, rock-inhabiting Lottia digitalis tend to be darker coloured than Pollicipes-inhabiting conspecifics. In a follow-up study the same author investigates whether the colour polymorphism in Lottia digitalis in Oregon is genetically based.  When light-coloured Lottia digitalis are removed from goose barnacles and dark-coloured ones from bare rock, and the shell edges chipped off and allowed to grow back, the new growth of shells accords generally with the original colour – hence, suggesting that the polymorphism is genetically based.  Furthermore, both light and dark forms are observed in their early recruitment stages (2mm shell length) to be present on the rock face .  At about 3-4mm length the light-coloured individuals tend to gravitate towards Pollicipes, while the dark-coloured individuals stay on the rocks.  Based on the data in the previous Research Study, are we to believe that the individual shown here in Fig. 1 has 100% fidelity to a certain-sized barnacle, and never moves to other barnacles of different sizes?

Research Study 3: Lottia digitalis

Fig. 1.  An assay of shell colour of limpets Lottia digitalis living on goose barnacles Pollicipes polymerus and on rock confirms that light-coloured L. digitalis favour goose-barnacle habitats, and dark-coloured ones favour rock habitats

A complementary investigation to the foregoing Research Study, also in Oregon¹, uses tagged² limpets in reciprocal-translocation experiments to test habitat fidelity of the two types of Lottia digitalis. The two types are quite easily separated by shell colour (see Fig. 1). To obtain these colour data, the investigator collects 168 L. digitalis from Pollicipes polymerus habitat and 168 from adjacent rock habitat, and ranks them in 10 colour-categories from 1 (light colour) to 10 (dark colour). Light-colour shells correspond with the Pollicipes habitat and dark with the rock habitat.  Results of the habitat-fidelity experiments show that 58% of limpets translocated 15cm from Pollicipes to adjacent bare rock find their way back to their original barnacle habitat within 12d (mean = 2.4d).  In comparison, 100% of limpets originally on rock and moved to Pollicipes return to their rock habitat within 12d (mean = 1.2d).  The habitat relocation is remarkable in view of the relative areas of each habitat type. The Pollicipes cluster occupies only 13% of the total experimental area³, whereas bare rock represents the remaining 87%. 

NOTE¹  other experiments in Oregon on habitat fidelity in L. digitalis following reciprocal transplants show essentially the same results (see Research Study 2 above).  Giesel 1970 Evolution 24: 98

NOTE²  the study involves 336 tagged limpets (numbered plastic tags attached with cyanoacrylate-ester glue)

NOTE³  to understand this, imagine a clump of goose barnacles of 222 cm² total area, with the translocation distance extending 15cm out from its circumference.  The doughnut-shaped translocation area is 1498 cm² in size, or 87% of the total

Research Study 4: Lottia digitalis

Fig. 1.  Only a slight colour change (darker shell edge) in Lottia digitalis moved from Policipes to rock in the lab and kept for 10mos

The mechanism of colour change in Lottia digitalis is addressed in a California study in which limpets are transferred from shell plates of Pollicipes to a rock habitat set up in the laboratory.  After 10mo feeding on the new substratum (foods are presumably diatoms and microalgae), the new shell growth is 75% dark-coloured, suggesting to the authors that the colour change owes to different algae being consumed in the two habitats and, thus, is not genetically based.  The reciprocal translocation was unfortunately not done because of difficulty in keeping Pollicipes alive in the lab.

NOTE  researchers in California should note that recent genetic evidence confirms a 1978 suggestion based on habitat partitioning that L. digitalis is actually two species: a northern L. digitalis and a southern L. austrodigitalis. Where the species occur together, the former occurs lower in the intertidal region (favouring a barnacle habitat), while the latter is more abundant higher in the intertidal region on rocks.  The line of separation latitudinally (zone of sympatry) is at Pigeon Point, California, just south of San Francisco.  In the 3-decade interval between the two studies it appears that L. austrodigitalis has moved northwards from the Monterey Peninsula, displacing L. digitalis on its way. Of relevance to Research Studies 1-4 is that both species exhibit two microhabitat morphs, a gooseneck-barnacle morph in the mid-intertidal zone and a rock morph in the high intertidal zone.  The 2007 authors report no evidence of hybridisation between the two species.  Because the northwards shift in distribution of L. austrodigitalis corresponds with a shift in seawater temperature, the same authors suggest that the moving transition zone between the two species may be useful for tracking global warming.  Murphy 1978 Biol Bull 155: 193; Crummett & Eernisse 2007 Mar Biol 152: 1. 

NOTE  pigments that form the shell colour in molluscs are mostly porphyrins, metabolically derived from their foods

Research Study 1: Lottia pelta

Fig. 1.  Lottia pelta on sea palms Postelsia palmaeformis

Fig. 2. Nicely rounded Lottia pelta on rock habitat

Shell shape of Lottia pelta in California is also responsive to habitat.  In comparison with individuals inhabiting open rock surfaces,  ones living on the stipes of sea palms Postelsia palmaeformis have laterally compressed and elevated shells, possibly making a better fit with their narrow, rounded habitat (Figs. 1 & 2).

Research Study 2: Lottia pelta

Fig. 1.  Dark-coloured Lottia pelta on the holdfast of the brown alga Lessoniopsis littoralis.  One wonders the extent to which shell colour in this species owes to algal growths

Shell colours of Lottia pelta on Monterey Peninsula, California vary markedly with habitat.  Brown shells are associated with high intertidal occupation of barren rocks, green or black shells with areas under rockweed Silvetia compressia, and black shells within mussel beds.  The presence of feeding scars on the stipe or holdfast of Silvetia suggests that the limpet may remain for some time in one spot (Fig. 1).

Research Study 3: Lottia pelta

Fig. 1.  This photograph representes an educated guess of the types of habitat provided to the limpets: light-coloured barnacles and dark-coloured mussels. Lottia digitalis prefers the light-coloured substratum, while L. pelta prefers the dark-coloured one

To determine if Lottia pelta and L. digitalis prefer to inhabit substrata with colours matching their own specific shell colours, 20 x 20cm arenas (bounded by copper-paint strips) are set up on the shore in Barkley Sound, British Columbia with half of the area of each arena containing dark-coloured mussels Mytilus trossulus and dark rock, and half containing light-coloured barnacles Pollicipes polymerus and Semibalanus cariosus, and light rock (Fig. 1).  Twenty individuals of each species are randomly placed on the two habitats, left for 5d, and then their substratum preferences tallied.  The results show a preference by both species in accordance with shell colours, with 73% of L. digitalis having moved onto or stayed on the light coloured barnacle/rock habitats, and 84% of the L. pelta having moved onto or stayed on the dark coloured mussel/rock habitats.

NOTE  everything is fine with this design...except, why test the two species together?  Was some sort of interspecific interaction expected?  If so, why?  A tidier protocol would have been to test each species separately

Research Study 4: Lottia pelta

Fig.1: Ecophenotypes of Lottia pelta based upon habitats occupied

Fig.2: Transitional ecophenotypes of Lottia pelta showing evidence of habitat switching

The limpet Lottia pelta occupies different kinds of habitats, and has different shell shapes, textures, and colours specific to these habitats (see Fig.1).  The colours are derived from their foods.  On rock, for example, a diet of red or green microalgae and diatoms produces light coloration; on mussels, the limpets consume the periostracum of the mussels and this produces dark coloration; finally, on Egregia, a diet of the brown-alga's tissues produces a dark coloration.  The different colorations are presumed to be adaptive because, in matching the colour of their different habitats, the limpets may gain a measure of camouflage protection.  Interestingly, when a limpet moves from one habitat to another a record of the change is visible in the shell.  The photos in Fig. 2 show some examples of shell shapes and colours for L. pelta from various areas of the California coast that document such changes.  Individuals on rock have a rugose shell texture and a more or less circular aperture.  Ones living on mussels (Mytilus) have smoother, darker shells, with shell and aperture shapes somewhat constrained by the topography of the mussel-shell habitat.  Finally, individuals living on the brown alga Egregia tend to have more elongated apertures with parallel sides (see Fig.1 bottom photos).   The authors note that these morphs or “ecophenotypes” are so distinct that early molluscan taxonomists frequently described them as distinct species or subspecies. 

Research Study 5: Lottia pelta

Fig. 1.  Types of spatial movements tested in the study

The habitat-histories of the limpets in the previous Research Study suggests that the direction of movement is mostly from “unstable” to “stable” habitats, i.e., from Mytilus or Egregia to rock, and this brings up several questions.  Does a limpet on an “unstable” habitat, such as a dying and detached Egregia or Mytilus, sense that its perch is deteriorating, and then crawl off or drop off onto a “stable” rock habitat?  Or is it that the larvae preferentially settle onto "unstable" habitats and then later seek out stable ones as a regular part of the life cycle?  And, if so, what is the advantage of this?  In fact, such “bailing out” has been described for several species of limpets in studies at the Scripps Institution of Oceanography, La Jolla, California, in both field and laboratory tests. In short, if a limpet attached to a solid substratum underwater, like a tile, is rotated at 0.1-1.0 revolutions per second (RPS), it releases its attachment and falls off.  Only rotation in the horizontal plane is effective. Vertical rotation, yawing motion, stopping and starting, or rotating lights around an attached limpet are not effective stimuli (see Fig. 1).  The response is RPS-dependent, with virtually 100% of adult L. gigantea bailing out within 5sec at 0.5-1.0 RPS. At higher revolutions, such as 2 RPS, it takes almost twice as many revolutions for bail-out to occur, but the time is about the same because of the doubled RPS.  Small owl limpets (< 19mm shell length) cling on for slightly longer than larger ones.  Field experiments in which small rocks with attached limpets are rotated underwater at 0.5 RPS, show that most species bail out within 14sec (L. gigantea, L. digitalis, L. pelta, and L. scabra).  An exception is Lottia limatula that clings on more tightly than other limpet species.  If this behaviour is found to be universal, it would be an interesting project to pursue. The survival strategy of such bail-out behavour to gastropods inhabiting small rocks being tossed about in waves is obvious, even though they risk damage or death by being washed up on the beach or into deeper water.  Although not specifically tested, the authors suggest that bailing out is an adaptive response that allows an individual to abandon a dislodged substratum, such as a mussel, for a more stable one.  They observe that on release of its grip, a limpet falls, then clings immediately to the first object contacting its foot. The study is surprisingly interesting, and shows how persistence, and commitment to a topic, can yield unexpected rewards.

NOTE  yawing is familiar in space-shuttle terminology, but may need more explanation than the drawing provides. Hold out your hand with palm down and point it first to the left, then to the right, then back to the left, and so on, maintaining its horizontal orientation. This is yawing

NOTE  limpets, in general, have greater difficulty in righting themselves from upside-down positions than other gastropods.  Casual observation of L. persona, for example, suggests that an upside-down individual requires something to butt up against its foot in order to re-attach