DEVELOPMENTAL INTEGRATION RELATED TO BUOYANCY CONTROL IN NAUTILOIDS: EVIDENCE FROM UNUSUAL SEPTAL APPROXIMATION AND ONTOGENETIC ALLOMETRIES IN A JURASSIC SPECIES

Abstract:  The meaning of modifications in septal spacing that often coincide with maturity in extant Nautilus and fossil nautiloids, and also in ammonoids, remains controversial. In the Callovian nautilid species Paracenoceras marocense Miller and Collinson, 1952 , the extent of decrease in septal spacing and the exceptional number of approximated septa are correlated with an unusual positive ontogenetic allometry in whorl-width expansion. This allometric growth implies that the threshold weight of the animal, requiring the formation of a new chamber to maintain near-neutral buoyancy, was reached for an increasingly shorter angular length of shell added to the aperture. Thus, the available space for the newly forming chamber behind the advancing body was reduced accordingly. Ontogenetic modifications in septal spacing are linked to relative growth of the animal. The flexibility in the mechanisms of buoyancy regulation would be expected to have been a critical factor affecting the possible set of ontogenetic trajectories in chambered cephalopods and thus the realm of variation upon which selection could act.     

The Evolution and Development of Cephalopod Chambers and Their Shape

The Ammonoidea is a group of extinct cephalopods ideal to study evolution through deep time. The evolution of the planispiral shell and complexly folded septa in ammonoids has been thought to have increased the functional surface area of the chambers permitting enhanced metabolic functions such as: chamber emptying, rate of mineralization and increased growth rates throughout ontogeny. Using nano-computed tomography and synchrotron radiation based micro-computed tomography, we present the first study of ontogenetic changes in surface area to volume ratios in the phragmocone chambers of several phylogenetically distant ammonoids and extant cephalopods. Contrary to the initial hypothesis, ammonoids do not possess a persistently high relative chamber surface area. Instead, the functional surface area of the chambers is higher in earliest ontogeny when compared to Spirula spirula. The higher the functional surface area the quicker the potential emptying rate of the chamber; quicker chamber emptying rates would theoretically permit faster growth. This is supported by the persistently higher siphuncular surface area to chamber volume ratio we collected for the ammonite Amauroceras sp. compared to either S. spirula or nautilids. We demonstrate that the curvature of the surface of the chamber increases with greater septal complexity increasing the potential refilling rates. We further show a unique relationship between ammonoid chamber shape and size that does not exist in S. spirula or nautilids. This view of chamber function also has implications for the evolution of the internal shell of coleoids, relating this event to the decoupling of soft-body growth and shell growth.     

How to recognize in situ fossil cephalopods: evidence from experiments with modern Nautilus

Field and flume experiments with modern Nautilus pompilius establish two prerequisites to recognize in situ preservation of fossil cephalopod shells (soft parts were within body chamber in situ at the time of fossilization): occurrence of the upper jaw within the body chamber and the position of jaws within the body chamber. Morphology of shells and jaws in modern and fossil nautiloids is so similar that these prerequisites can be applied for fossil nautiloids and provide implications for ammonoids. The upper jaws of Nautilus start to move at a water velocity of > 0.2 m/s, when the shells are reoriented with the aperture downstream; jaws are therefore unlikely to be secondarily deposited near the shell aperture by bottom currents. The lower jaws, moved at the velocity of > 0.1 m/s, can be deposited around the shell aperture by weak current (0.1–0.2 m/s in velocity), but never enter the inside of body chamber. Neither jaw is likely to be separately and selectively displaced from the inside of the body chamber through scavenging of the soft parts by burrowing infaunal animals. An upper jaw preserved inside the body chamber, together with a lower jaw, is thus a reliable indicator of in situ preservation; a sole lower jaw preserved around the shell aperture is likely to be secondarily deposited. Sedimentary structures inferring rapid burial events and jaw size are useful as additional evidence. Smaller jaws were more likely to be displaced from the body chamber by scavenging by infaunal animals after in situ burial, so that smaller jaws preserved within the body chamber suggest less scavenging. These findings are crucial to interpreting the taphonomic history and palaeo-ecology of fossil cephalopods.     

Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.     

The bearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda

Yochelson, E. L., Flower, R. H. & Webers, G. F.: The hearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda.
Knightoconus , a new genus of the Hypseloconidae (Mollusca: Monoplacophora) from rocks of early Franconian age in Antarctica, is multiseptate. The multiple septa are a criticàl feature to be expected in a form ancestral to cephalopods. Fossil cephalopods, however, invariably have a siphuncle as well as septa; some gastropods, some hyolithids, and some monoplacophorans also have septa but lack a siphuncle. Therefore, only the siphuncle can be considered a unique and particularly significant feature of the cephalopod shell. Hypothetical reconstructions of molluscan anatomy support the notion that cephalopods may have been derived directly from a hypseloconid having a high, slightly curved, multiseptate, bilaterally symmetrical shell, by the subsequent development of a siphuncle.     

Cephalopod embryonic shells as a tool to reconstruct reproductive strategies in extinct taxa

An exhaustive study of existing data on the relationship between egg size and maximum size of embryonic shells in 42 species of extant cephalopods demonstrated that these values are approximately equal regardless of taxonomy and shell morphology. Egg size is also approximately equal to mantle length of hatchlings in 45 cephalopod species with rudimentary shells. Paired data on the size of the initial chamber versus embryonic shell in 235 species of Ammonoidea, 46 Bactritida, 13 Nautilida, 22 Orthocerida, 8 Tarphycerida, 4 Oncocerida, 1 Belemnoidea, 4 Sepiida and 1 Spirulida demonstrated that, although there is a positive relationship between these parameters in some taxa, initial chamber size cannot be used to predict egg size in extinct cephalopods; the size of the embryonic shell may be more appropriate for this task. The evolution of reproductive strategies in cephalopods in the geological past was marked by an increasing significance of small‐egged taxa, as is also seen in simultaneously evolving fish taxa.     

An unusual trochiform gastropod from the upper Ordovician of Sweden

An unusual trochiform gastropod, Semizona bella gen. et sp. nov., is described from the Boda Limestone carbonate mounds (upper Ordovician, Ashgill) of central Sweden. A second species, S. glindmeyeri (Rohr, 1996), is recognised from the Ordovician (Whiterockian) of Nevada. The shell shape and the strongly prosocline tangential aperture of Semizona suggest that balancing of the shell on the head-foot mass was accomplished by tilting of the axis of coiling of the shell to about 65 degrees with 10–30 degrees of regulatory detorsion. The rounded aperture allowed straight contraction of the retractor muscles, suggesting clamping behaviour often associated with a sedate, grazing snail. This agrees with the environmental setting, which suggests a hard substrate with rich microbial growth. Besides clamping, the subsutural nodes and thick shell were probably effective against predation; repaired injuries indicate failed predatory attacks. Semizona shows morphological similarities with some pleurotomariin vetigastropods, and with the family Pseudophoridae Miller, 1889.     

Sexual dimorphism in shells of Cochlostoma septemspirale (Caenogastropoda, Cyclophoroidea, Diplommatinidae, Cochlostomatinae)

Sexual dimorphisms in shell-bearing snails expressed by characteristic traits of their respective shells would offer the possibility for a lot of studies about gender distribution in populations, species, etc. In this study, the seven main shell characters of the snail Cochlostoma septemspirale were measured in both sexes: (1) height and (2) width of the shell, (3) height and (4) width of the aperture, (5) width of the last whorl, (6) rib density on the last whorl, and (7) intensity of the reddish or brown pigments forming three bands over the shell. The variation of size and shape was explored with statistical methods adapted to principal components analysis (PCA) and linear discriminant analysis (LDA). In particular, we applied some multivariate morphometric tools for the analysis of ratios that have been developed only recently, that is, the PCA ratio spectrum, allometry ratio spectrum, and LDA ratio extractor. The overall separation of the two sexes was tested with LDA cross validation.The results show that there is a sexual dimorphism in the size and shape of shells. Females are more slender than males and are characterised by larger size, a slightly reduced aperture height but larger shell height and whorl width. Therefore they have a considerable larger shell volume (about one fifth) in the part above the aperture. Furthermore, the last whorl of females is slightly less strongly pigmented and mean rib density slightly higher. All characters overlap quite considerably between sexes. However, by using cross validation based on the 5 continuous shell characters more than 90% of the shells can be correctly assigned to each sex.     

Segmental growth in planulate ammonites: inferences on costal function

Checa, Antonio & Westermann, Gerd E. G. 1989 01 15: Segmental growth in planulate ammonites: inferences on costal function. Lethaia , Vol. 22, pp. 95–100. Oslo. ISSN 0024–1164.
In planulate Ammonitina, the directions of costae and their parts coincide with the growth lines, both being accreted in parallel and subtriangular increments, i.e. segments. This implies that the growing margin was consistently corrugated (plicated) parallel to the edge of the peristome. The adventral separation of the ribs caused by the coiling increased with whorl involution and was compensated for by costal division into primaries and secondaries. We propose that costation of the shallow-water, nekto-benthic plandates reduced the stiffness and increased effective thickness of their growing peristome. Ribs functioned (primarily or secondarily) as a protection for the immature, fragile aperture against predators and impacts.□ Ammonite shell, Jurassic, morphology, growth lines, ornamentation, costae .     

Mode of life and soft body shape of heteromorph ammonites

Ebel, K. 1992 04 15: Mode of life and soft body shape of heteromorph ammonites. Lethaia , Vol. 25, pp. 179–193. Oslo. ISSN 0024–1164.
Using the idea of a benthic mode of life for ammonites, based on a gastropod-like shell position, it is possible to reconstruct the development of all heteromorph ammonites by regarding single growth stages and the presumable acting forces. The reconstruction of shell formation, particularly the final shell position of heteromorphs with a hook, indicates that the soft body of the ammonite animal was considerably larger than comparison with the present-day Nautilus would suggest.     

Irregular calcareous sheets preserved in a conch of the Cretaceous ammonoid Hypophylloceras from Hokkaido,Japan

The mantle tissue is essential for understanding the diverse ecology and shell morphology of ammonoid cephalopods. Here, we report on irregular calcareous sheets in a well-preserved shell of a Late Cretaceous phylloceratid ammonoid Hypophylloceras subramosum from Hokkaido, Japan, and their significance for repairing the conch through the mantle inside the body chamber. The sheets are composed of nacreous layers arranged parallel to the irregularly distorted outer whorl surface. The nacreous sheets formed earlier are unevenly distributed and attached to the outer shell wall locally, whereas the last formed sheet covers a wide area of the outer shell wall. The absence of any interruption of ribbing around the irregular area suggests that these sheets were secreted inside the body chamber from the inner mantle. Gross morphological and X-ray computed tomography observations revealed that the spacing of septal formation was not affected by this event. The complex structure of the irregular sheets suggests a highly flexible mantle inside the body chamber.     

Description of the early ontogenic part of the Tentaculitids, with implications for classification

Ontogenic development and classification of tentaculitids at high systematic levels are reevaluated in the light of new findings on shell structure and morphology of larval parts, and these features are here regarded as being of primary importance for taxonomy. Class Tentaculita Bouček, 1964 is subdivided into two subclasses, of which subclass Chionioconarida Farsan, 1994 is distinguished by a tubular larval process closed at the apex and covered with microrings. The process is differentiated into a prolarval, metalarval and epilarval part, of which the latter coincides with metamorphosis. Morphology of the larval parts suggests that metamorphosis proceeds in two different manners, giving rise to superorders within this subclass. Within superorder Trompetoconarida Farsan, 1994 a bilaterally symmetrical larval cone develops with an aperture oblique to the long axis of the conch; following metamorphosis the conch becomes radially symmetrical and the aperture perpendicular to the axis; secondary shell, septa and pseudopunctae develop in the adult phase, and the structure of the shell is lamellar. In contrast, within the second superorder, Lirioconarida Farsan, 1994 the epilarval tube develops into a larval bulb with no changes in symmetry and position of the aperture; secondary shell, septa and pseudopuncta are absent. The microstructure of the shell is lamellar in the larval part whereas in postlarval parts it is either sigmoidal or lamellar. The subclass Dacryoconarida Fisher, 1962 possesses a subspherical, tear- or drop-like embryonic chamber which may have a caudal process. The microstructure of the embryonic chamber is variable within this group, being lamellar in some taxa whereas in others, a single layer of shell is present. The postembryonic parts of the lamellar forms possess nacreous or sigmoidal structures.     

A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan, and an inferred secondary occupant

Morris, P. J., Linsley, R. M. & Cottrell. J. F. 1991 01 15: A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan. and an inferred sceondary occupant. Lethaia , Vol. 24. pp. 55–67. Oslo. ISSN 0024–1164.
The high-spired gastropod Palaeozygopleuru (Loxonematacea) of the Hamilton Group (Middle Devonian) in New York State is often entrusted by the trepostomatous bryozoan Leptotrypella (Hetero-trypidae). The form of this enerustation leads us to infer a secondary occupant that dwelled in empty shells of Palaeozygopleura while the bryozoan grew upon them. Encrusted specimens usually have an open aperture, thin or no enerustation on the apertural side of the gastropod shell. and thick encrustation on the abapertural side. While the aperture is invariably open. the columellar and parictal lips of the aperture, and the apertural face of the first whorl are usually encrusted. While the gastropod is alive, this area rests upon the dorsal surface of the foot and the remainder of the shell rests upon the substratc. These encrustation patterns suggest that an occupant of the shell. other than the gastropexl. prevented overgrowth of the aperture and oriented the shell aperture-down. This allowed ahapertural growth of the bryozoan. The presence of thin encrustation on the apertural side of the shell is problematical. It requires either the presence of a secondary occupant capable of holding a thinly encrusted shell off the substrate, or the ability of the juvenile bryozoan colony to extend onto portions of the shell that were in contact with the substrate. The presence of a secondary occupant. such as a sipunculan worm. capable of lifting the shell, but usually resting it aperture-down on the substrate seems the most suitable explanation for the nature of the encrustation. Gastropoda. bryozoa. symbiosis. Devoniun. hermit crab, Sipuncula     

Soft‐tissue imprints in fossil and Recent cephalopod septa and septum formation

Several soft‐tissue imprints and attachment sites have been discovered on the inside of the shell wall and on the apertural side of the septum of various fossil and Recent ectocochleate cephalopods. In addition to the scars of the cephalic retractors, steinkerns of the body chambers of bactritoids and some ammonoids from the Moroccan and the German Emsian (Early Devonian) display various kinds of striations; some of these striations are restricted to the mural part of the septum, some start at the suture and terminate at the anterior limit of the annular elevation. Several of these features were also discovered in specimens of Mesozoic and Recent nautilids. These structures are here interpreted as imprints of muscle fibre bundles of the posterior and especially the septal mantle, blood vessels as well as the septal furrow. Most of these structures were not found in ammonoids younger than Middle Devonian. We suggest that newly formed, not yet mineralized (or only slightly), septa were more tightly stayed between the more numerous lobes and saddles in more strongly folded septa of more derived ammonoids and that the higher tension in these septa did not permit soft‐parts to leave imprints on the organic preseptum. It is conceivable that this permitted more derived ammonoids to replace the chamber liquid faster by gas and consequently, new chambers could be used earlier than in other ectocochleate cephalopods, perhaps this process began even prior to mineralization. This would have allowed faster growth rates in derived ammonoids.     

Shell shape and habitat use in the North‐west Pacific land snail Mandarina polita from Hahajima,Ogasawara: current adaptation or ghost of species past?

The endemic land snail genus Mandarina of the Ogasawara Islands provides an excellent model system to investigate adaptive radiation. Previously, it has been shown that coexisting species of the islands segregate by microhabitat, so that they are either predominantly found on the ground in relatively wet and sheltered sites, dry and exposed sites, or else are arboreal. Moreover, shell morphology correlates with microhabitat, so that species in wet and sheltered sites tend to have high-spired shells with a high aperture, and those in dry and exposed sites tend to have relatively low-spired shells with a wide aperture. We have now found that on Hahajima, Mandarina polita have variable shell morphology, and there is a correlation between morphology and the depth of leaf litter, as well as the presence/absence of other terrestrial species. Specifically, when high-spired terrestrial Mandarina ponderosa is present, M. polita tend to be low-spired and have a large aperture, indicative of character displacement. When M. ponderosa is absent, the shell shape of M. polita is much more variable, the overall spire is higher, individuals are found in deeper litter, and there is a strong correlation between litter depth and spire height. We argue that these patterns are due to local adaptation, but it remains possible that they are an artefact due to the 'ghost of species past'.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 149–159.     

Finite element analysis of a double membrane tube (DMS-tube) and its implication for gastropod shell morphology

Molluscan shells, including those of Gastropoda, are formed by accretionary growth at the mantle edge. The mantle is a thin membrane of skirt-like shape, which extends minutely beyond the aperture, and its edge adds a shell increment to the aperture margin so that each increment copies a configuration of the mantle edge at that time. Thus, regulation of shell morphogeny is almost equivalent to the factors which control the mantle form at the moment of shell growth. Form of the mantle skirt is considered to be kept in a state of balance between the force of its internal stress and forces acting on it such as fluid pressure or muscle contraction. The expansion behavior of the mantle skirt has been numerically analyzed by using an elastic model (DMS-tube), which represents the fundamental structure of the mantle tissue as a double membrane structure with internal springs (DMS). Four characteristic expansion patterns of the DMS-tube have been detected: (1) general outward expansion; (2) developing a ridge-like fold on an initial longitudinal protrusion of the tube edge; (3) drastic shift of the expanded state from a uniformly curved to an elliptical shape in outline, owing to the existence of a fixed boundary condition on the tube wall; and (4) constricted protrusion on the open region of the shell wall surrounding the DMS-tube. These results have the potential for answering the following questions relating to the morphogenesis of gastropod shells. How does the mantle skirt usually make contact with the inner surface of the shell wall so as to ensure continuous accretion of shell materials to the aperture margin? What is the cause of spiral ridges? Why do open coiling or minimally overlapping shells have generally circular apertures, while shells with apertures overlapped by whorls have non-uniformly curved apertural lips? What is the cause of long closed spines and why do they always appear on spiral ridges?     

Recent advances in heteromorph ammonoid palaeobiology

Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, heteromorphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, tentacles, a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontogeny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inflicted by crustaceans, fish, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro- and mesoplankton. The Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinction of ammonoids. Ammonoids likely persisted after this event for 40–500 thousand years and are exclusively represented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.     

Depth inferences from vertically imbedded cephalopods

Raup, D. M.: Depth inferences from vertically imbedded cephalopods.
Vertically imbedded cephalopods are not uncommon in the fossil record. Experiments with modern Nautilus show that after death the shell cannot maintain a vertical orientation on the sea floor unless the depth is less than about 10 m. At greater depths, hydrostatic pressure causes flooding of the phragmocone sufficient to make the shell fall over. The depth limit applies whether the shell floats after death or not. Calculations made on data from six ammonite species indicate that the same depth limit can be applied to fossil coiled cephalopods. Vertical preservation can occur in deeper water only if the shell is vertically oriented upon impact and if the sediment is such as to trap the shell in that position.     

Control of gastropod shell form via apertural growth rates

This paper discusses and corrects ideas in Løvtrup and Løvtrup (J. Morphol. 197:53–62, '88) on how differential growth rates around the aperture cause the gastropod shell to coil at particular angles. The relationship between position relative to the shell apex and growth rate is derived. This lets us understand what information on relative growth rates around the aperture is sufficient to determine the shape of the logarithmic spiral that these growth rates generate. I argue that differential growth rates could not be physiologically controlled precisely enough to regulate apical angle; they passively follow, not actively direct, shell shape.     

A multivariate study of geographic variation in the whelk Dicathais

An analysis has been made of the variation in shell shape and shell characteristics of 889 Australian and New Zealand specimens of the genus Dicathais, using multivariate techniques. Shell measurements taken were: the overall length, length of spire, length of aperture, and width of aperture. Weight of the shell plus the preserved animal was also recorded. The sculpture of the shell, thickness of the lip, and the presence or absence of a reddish or purplish colouration or banding on the inside of the lip, were assessed qualitatively.Principal component analyses of the size measurements for each site showed that the first principal component, which accounted for greater than 95% of the variation at each site, was associated with variation in the ‘size’ of the animal. Canonical analysis of the size measurements showed a cline in shell shape from the animals on the western side of Australia to those on the eastern side of Australia and New Zealand. The resulting canonical variates were associated with variation in the ‘shape’ of the shell. Principal component analyses of the between-group matrix and of the within-group matrix of the size measurements showed that the site means exhibited a similar pattern of dispersion to that of the animals within each site.Canonical analysis of the shell characteristics showed that variation along the first canonical axis was largely produced by shell sculpture, while variation along the second resulted from differences in colouration/banding.The generalized variances of the correlation matrices for the size measurements showed that groups with similar shell shape were associated with the presence of granite substrata and/or mussel beds or, alternatively, with limestone substrata, but canonical correlation analysis of the relationship between the size measurements and shell characteristics showed that no consistent trend was evident over all sites.A subjective examination of the structure of the radula of 84 animals showed that two distinct morphological forms were present, but that they were not correlated either with sex or any of the named shell forms or site groupings.An analysis of the growth curves of 27 animals of the two forms from the eastern and western coasts of Australia, held in the laboratory, was carried out. The eastern coast form showed a loss of sculpturing and a change in shell shape when kept under west coast conditions and on a mussel diet.Water temperature, diet, substratum, and degree of exposure to wave action were all found to show associations with variations in either shell shape or shell characteristics. It is suggested that the selective force of the habitat which produces changes in shell shape and shell characteristics of the animals at any site is a complex of factors, many of which are interrelated. The genetic basis for the development of shell shape and production of the shell characteristics in Dicathais may be similar to that found in Nucella lapillus (L.) in the Northern Hemisphere.These data suggest that the Dicathais found at the sites studied in this investigation are all part of the same ‘population’, the shell shape and shell characteristics of the adult populations being determined both by selection and phenotypic expressions caused by the selective force of the habitat at each site. It is concluded that the genus consists of a single highly variable species.The value of the application of multivariate analyses to this type of study is shown to lie in the way in which the techniques provide an overall picture of the variation within sites and of the variation between sites.