The iridescence of abalone nacre is simply a physical characteristic of the shell; it plays no functional role in the animal’s life.  However, like pearls in pearl oysters, an abalone will form beautiful blister pearls in response to parasitic organisms, such as worms, clams, and sponges, which burrow into its shell from the outside.  Shell repairs of this sort must negatively affect growth because of the extra allocation of energy required. Trial studies at the Bamfield Marine Sciences Centre, British Columbia employing plastic and other inserts in the shells of red abalone Haliotis rufescens from California show that it is indeed possible to create raised blister pearls for use in jewellry making. Implanted shell courtesy Peter Fankboner, SFU

External surface of shell of H. rufescens
with plastic implant for pearl production. Note
also the erosion caused by burrowing organisms 0.4X

Internal surface of same shell showing
blister pearl in process of formation 0.4X

Research study 3

An example of negative effects of shell-inhabiting parasites on growth of abalone is presented in a study at an abalone farm in Cayucos, California.  Here, a sabellid polychaete Terebrasabella heterouncinata infests the leading edge of shells of Haliotis rufescens and seriously affects subsequent growth. Apparently, the shells become much more porous and brittle than in uninfected individuals, and new growth of the shell is directed downwards, forming a lip.  The respiratory pores of the abalone may also become clogged.  The effect is more severe in older, slower-growing abalone than in younger, faster-growing ones.  A young abalone can quickly encapsulate a bunch of worms and extend its shell beyond them.  Fortunately, the sabellid produces a crawling larva, not a free-swimming one, and infested abalone in a hatchery can be quarantined.  The authors note that this is the first report of a sabellid inhabiting the shells of abalone.  Oakes & Fields 1996 Aquaculture 140: 139.

NOTE more information on this worm is available elsewhere in the ODYSSEY: LEARN ABOUT TUBEWORMS: PARASITIC TUBEWORMS

Research study 4

The colours of abalone shells come from bilin-type pigments incorporated from algal foods into the proteinaceous outer covering, or periostracum.  Studies on diet and shell colour in several abalone species in Santa Barbara, California confirm that shell colours are red on diets of red algae, and pale green/green-white/blue-black/blue-green/white-turquoise on diets of kelps Macrocystis spp. and Nereocystis luetkeana. Moreover, for one population of red abalone Haliotis rufescens, colour banding-patterns correlate well with seasonal change of diet from red algae to brown kelp N. luetkeana.  Thus, during spring-autumn (Jun-Sep) when kelps are seasonally plentiful, growth produces a wide band of white-coloured shell.  During winter (Nov-Apr), when kelps undergo seasonal die-off, the shells exhibit a narrower red band from diets of red algae.  The specimen shown here has a large initial red band because of a 4-yr absence of brown algae N. luetkeana from its Morro Bay, California habitat.  The author suggests that such annual banding patterns may provide useful information for fisheries managers on age and growth rates in red abalone.  Olsen 1968 Veliger 11: 135; Olsen 1968 Biol Bull 134: 139.

NOTE  more information on shell structure in abalones, and use of the shell in physical defense and possibly camouflaging can be found elsewhere in this learn-about section: PREDATORS & DEFENSE: PHYSICAL DEFENSES

NOTE  mainly H. rufescens, H. sorenseni, H. corrugata, and H. cracherodii, but other species are included in the study

A comparison of growth of juveniles of 3 species of abalones in La Jolla, California shows clear differences in temperature optima corresponding to the average depths at which each species lives.  Thus, the green abalone Haliotis fulgens, the most shallow-dwelling species (0-5m depth), grows best at 24-27^oC.  The next deepest of the three, the pink abalone H. corrugata (1-20m), grows best around 21^oC, and the deepest species, the red abalone H. rufescens (10-25m), grows best around 15-18^oC. Leighton 1974 Fish Bull 72: 1137.

A study on growth of red abalone Haliotis rufescens in Santa Rosa Island, California (see map) by researchers at the California Department of Fish and Game show that maximum shell length of 175-200mm is reached within 8-10yr  (see graph).  The authors note that elsewhere in California the species may reach 300mm shell length.  Haaker et al. 1998 J Shellf Res 17: 747.

NOTE  data from 2,145 tagged individuals are collected over a 5-yr period from 1978-82.  Some years are combined owing to lack of significant differences

Black turban shells Chlorostoma (Tegula) funebralis feed on algae in the mid- to low-intertidal zone.  Counts of annual growth lines, which are quite easy to see on shells that are not damaged, indicate a lifespan of 30yr or more.  Growth rates vary considerably among west-coast populations. For example, 2cm-diameter individuals are estimated to be 6yr old in central Oregon (Sunset Bay)and 15yr old in northern Washington (Mukkaw Bay). In British Columbia Tegula apparently grows even more slowly, lives longer, and attains larger body size.  These differences are unlikely to be related to latitudinal temperature differences; rather, more to differences in food types, food availability, and intertidal position.  Oregon observations from Darby 1964 Veliger 6(Suppl): 6, Frank 1965 Growth 29: 395, and Frank 1975 Mar Biol 31: 181; Washington data from Paine 1969 Ecology 50: 950. Photo of Chlorostoma funebralis courtesy Gary McDonald, Long Marine Laboratory, Santa Cruz.

NOTE  two of these studies related shell diameter to annual growth lines on the shell (Darby 1964, Paine 1969), while the third interpreted growth increments from “marked-and-recaptured” individuals using growth models (Frank 1965)

Often the shell spires of Chlorostoma funebralis are heavily eroded (see photo in preceding Research Study).  A fungus Pharcidia balani is implicated in the shell erosion and to offset this the snail secretes a secondary layer from within.  The normal shell is comprised of the usual 2 molluscan layers: an outer prismatic layer consisting of elongated crystals of calcium carbonate (photo on far Left) representing about 10-20% of the shell thickness, and an inner nacreous layer consisting of plate-like crystals representing the remainder (the photo on far Right shows the transition between the two layers).  Studies at the Bodega Marine Laboratory, California indicate that the repair material repeats the composition of the original shell, so at a repair zone there may be 4 layers: outer prismatic/outer nacre and inner prismatic/inner nacre.  Apparently, however, the repair material has a different physical structure than the original crystalline form, namely, a cross-lamellar form.  The author does not discuss why the original shell material is not deposited in this harder form, but perhaps it is more energy costly to do so.  Geller 1982 Veliger 25: 155.

NOTE another form of calcium-carbonate crystal that is harder in structure. Scale bars in figures =10um

The frequency of shell repair in an intertidal gastropod like Chlorostoma funebralis can be an indicator of fitness.  Thus, a “survivor” snail will exhibit many repair marks, while a less fit snail may be, shall we say, dead.  Collections of C. funebralis and Nucella ostrina from various types of habitats in Bodega Bay, California reveal that the frequency of shell repair varies, not surprisingly, between habitats.  In surge channels, for example, 39% of Tegula (of 450 specimens collected) show repair marks, while in mussel beds only 4% (177 specimens) show repair marks.  Comparative data for Nucella (434 specimens) are: 15% with marks in surge channels, 9% in mussel beds, and 5% in tidepools.  The author speculates that the more intense wave action in surge channels and the fact that the channels represent a handy conduit for predatory crabs and fishes make this type of habitat a generally more risky one for snails.  Geller 1983 Veliger 26: 113.