Torsion in Patella caerulea (Mollusca, Patellogastropoda): ontogenetic process, timing, and mechanisms

Abstract. Torsion is a process in gastropod ontogenesis where the visceral body portion rotates 180° relative to the head/foot region. We investigated this process in the limpet Patella caerulea by using light microscopy of living larvae, as well as scanning electron microscopy (SEM) of larvae fixed during the torsion process. The completion of the 180° twist takes considerably less time in larvae of Patella caerulea than previously described for other basal gastropod species. At a rearing temperature of 20–22°C, individuals complete ontogenetic torsion in ?2 h. Furthermore, the whole process is monophasic, i.e., carried out at a constant speed, without any evidence of distinct ‘fast” or ‘slow” phases. Both larval shell muscles—the main and the accessory larval retractor—are already fully contractile before the onset of torsion. During the torsion process both retractors perform cramp‐like contractions at ～30 s intervals, which are followed by hydraulic movements of the foot. However, retraction into the embryonic shell occurs only after torsion is completed. The formation of the larval operculum is entirely in‐dependent from ontogenetic torsion and starts before the onset of rotation, as does the mineralization of the embryonic shell. The reported variability regarding the timing (mono‐ versus biphasic; duration) of torsion in basal gastropod species precludes any attempt to interpret these data phylogenetically. The present findings indicate that the torsion process in Patella caerulea, and probably generally in basal gastropods, is primarily caused by contraction of the larval shell muscles in combination with hydraulic activities. In contrast, the adult shell musculature, which is independently formed after torsion is completed, does not contribute to ontogenetic torsion in any way. Thus, fossil data relying on muscle scars of adult shell muscles alone appear inappropriate to prove torted or untorted conditions in early Paleozoic univalved molluses. Therefore, we argue that paleontological studies dealing with gastropod phylogeny require data other than those based on fossilized attachment sites of adult shell muscles.     

Die Torsion der Gastropoda - ein biomechanischer Prozeß

The recently proposed biomechanical model of gastropod torsion (edlinger 1988 a, b) is rejected on various reasons. First, the assumed original conditions in Polyplacophora and Tryblidiida as well as the constructed original condition in the Gastropoda do not emst in reality. Secondly, the mollus-can musculature is a very dynamic structure (continuous assembly and disassembl) so that biomechanic rocesses are of much minor importance than assumed by Edlinger . Thirdly, the biomechanical model resented does not explain the change of the relative position of the lateral (= visceral) nerve cords which are surrounding the dorsoventral muscles in Polyplacophora and Tryblidiida (also Scaphopoda and Bivalvia), but are situated between the muscles in Cephalooda and Gastropoda. The consequences of these considerations to the early evolution of Gastropoda are briefly outlined.     

Ontogenetic torsion in two basal gastropods occurs without shell attachments for larval retractor muscles

Results of this study on two species of vetigastropods contradict the long-standing hypothesis, originally proposed by Garstang (1929), that the larval retractor muscles power the morphogenetic movement of ontogenetic torsion in all basal gastropods. In the trochid Calliostoma ligatum and the keyhole limpet Diodora aspera, the main and accessory larval retractor muscles failed to establish attachments onto the protoconch (larval shell) when the antibiotics streptomycin sulfate and penicillin G were added to cultures soon after fertilization. Defects in protoconch mineralization were also observed. Despite these abnormalities, developing larvae of these species accomplished complete or almost complete ontogenetic torsion, a process in which the head and foot rotate by 180 degrees relative to the protoconch and visceral mass. Analysis by using phalloidin-fluorophore conjugate and transmission electron microscopy showed that myofilaments differentiated within myocytes of the larval retractor muscles and adherens-like junctions formed between muscle and mantle epithelial cells in both normal and abnormal larvae. However, in abnormal larvae, apical microvilli of mantle cells that were connected to the base of the larval retractor muscles failed to associate with an extracellular matrix that normally anchors the microvilli to the mineralized protoconch. If morphogenesis among extant, basal gastropods preserves the original developmental alteration that created gastropod torsion, as proposed by Garstang (1929), then the alteration involved something other than the larval retractor muscles. Alternatively, the developmental process of torsion has evolved subsequent to its origin in at least some basal gastropod clades so that the original alteration is no longer preserved in these clades.     

Larval muscle contraction fails to produce torsion in a trochoidean gastropod.

The causes and effects of ontogenetic torsion in gastropods have been debated intensely for more than a century (1-19). Occurring rapidly and very early in development, torsion figures prominently in shaping both the larval and adult body plans. We show that mechanical explanations of the ontogenetic event that invoke contraction of larval retractor muscles are inadequate to explain the observed consequences in some gastropods. The classic mechanical explanation of Crofts (4, 5) and subsequent refinements of her explanation have been based on species with rigid larval shell properties (18, 19) that cannot be extrapolated to all gastropods. We present visual evidence of the lack of rigidity of the uncalcified larval shell in a basal trochid gastropod, Margarites pupillus (Gould), and provide photographic confirmation of our prediction that larval retractor muscle contraction is insufficient to produce more than local deformation or dimpling at the site of muscle insertion. These findings do not refute muscular contraction as a primary cause of ontogenetic torsion in gastropods that calcify their larval shells prior to the onset of torsion, nor do they refute the monophyly of torsion. They do, however, suggest that torsion may be a loosely constrained developmental process with multiple pathways to the more constrained end result (20, 21).     

Development of asymmetry in the caenogastropods Amphissa columbiana and Euspira lewisii

Abstract. The asymmetry displayed by the body plan of gastropods has been directly or indirectly attributed to an evolutionary process called torsion. Torsion is defined as a rotation of 180° between the cephalopodium (head and foot) and visceropallium (visceral organs, mantle, mantle cavity, and shell). During development, the displacement of anatomical components occurs during a process called "ontogenetic torsion." Although ontogenetic torsion is central to theories of gastropod evolution, surprisingly few studies have documented actual tissue movements during the development of asymmetry in gastropods. We investigated the development of the mantle cavity and pleurovisceral nerve connective (visceral nerve loop) in the caenogastropods Amphissa columbiana and Euspira lewisii , because displacements of both of these structures are interpreted as major consequences of torsion. Scanning electron micrographs, histological sections, and immunofluorescence images showed that the developing vis-ceropallium twists by 90° relative to the cephalopodium, the mantle cavity initially forms on the right side, and displacements of the visceral nerve loop become evident on the left side before the right side. A developmental stage in which the mantle cavity is confined to the right side has also been reported in members of the Vetigastropoda and Heterobranchia. We suggest that further comparative studies should test the hypothesis that early development throughout the Gastropoda converges on an embryonic organization in which the mantle cavity and anus are located laterally, despite clade-specific differences in developmental patterns both before and after this stage.     

Early differentiating neuron in larval abalone (Haliotis kamtschatkana) reveals the relationship between ontogenetic torsion and crossing of the pleurovisceral nerve cords

Crossing of the pleurovisceral nerve cords in gastropods has supported the view that gastropods evolved by 180 degrees rotation between the ventral and dorsal body regions. Indeed, a rotation of this type occurs as a dramatic morphogenetic movement ("ontogenetic torsion") during the development of basal gastropods. According to a long-standing hypothesis, ontogenetic torsion in basal gastropods preserves an ancient developmental aberration that generated the contorted gastropod body plan. It follows from this reasoning that crossing of the pleurovisceral nerve cords during gastropod development should be mechanically coupled to ontogenetic torsion. The predicted mechanical coupling can now be examined because of the discovery of an early differentiating neuron in Haliotis kamtschatkana (Vetigastropoda) that expresses 5-hydroxytryptamine-like immunoreactivity. The neuron appeared to delineate the trajectory of the pleurovisceral nerve cords beginning before ontogenetic torsion. Before torsion, the neuronal soma is embedded in mantle epithelium at the ventral midline and two neurites extend anteriorly toward the apical sensory organ. Contrary to expectation, the two neurites of this cell did not cross-over during ontogenetic torsion because the soma of this mantle neuron shifted in the same direction as the rotating head and foot. Full crossing of the pleurovisceral nerve cords occurred gradually during later development as the mantle cavity deepened and expanded leftward. These results are consistent with a generalization emerging from comparative studies indicating a conserved developmental stage for gastropods in which the mantle cavity is localized to one side, despite a fully "post-torsional" orientation for other body components. Developmental morphology before this stage is much more variable among different gastropod clades.     

Sequential developmental programmes for retractor muscles of a caenogastropod: reappraisal of evolutionary homologues

Evolutionary changes in the development of shell-attached retractor muscles in gastropods are of fundamental importance to theories about the early evolution and subsequent diversification of this molluscan class. Development of the shell-attached retractor muscle (columellar muscle) in a caenogastropod has been studied at the ultrastructural level to test the hypothesis of homology with the post-torsional left retractor muscle (larval velar retractor) in vetigastropod larvae. The vetigastropod muscle has been implicated in the generation of ontogenetic torsion, a morphogenetic twist between body regions that is important to theories about early gastropod evolution. Two shell-attached retractor muscles develop sequentially in the caenogastropod, Polinices lewisii, which is a pattern that has been also identified in previous ultrastructural studies on a vetigastropod and several nudibranch gastropods. The pattern may be a basal and conserved characteristic of gastropods. I found that the first-formed retractor in larvae of P. lewisii is comparable to the larval velar retractor that exists at the time of ontogenetic torsion in the vetigastropod, Haliotis kamtschatkana. However, the post-metamorphic columellar muscle of P. lewisii is derived exclusively from part of the second-formed muscle, which is comparable to the second-formed pedal muscle system in the vetigastropod. I conclude that the post-metamorphic columellar muscle of P. lewisii, is not homologous to the larval velar retractor of the vetigastropod, H. kamtschatkana.     

The musculature of Draculiciteria tessalata (Chaetonotida, Paucitubulatina): implications for the evolution of dorsoventral muscles in Gastrotricha

The muscular system of the marine interstitial gastrotrich Draculiciteria tessalata (Chaetonotida, Paucitubulatina) was analyzed with fluorescent phalloidin. Muscles in circular, longitudinal, helicoidal and dorsoventral orientations were found. Circular muscles were present as discreet rings on the pharynx only. Five pairs of longitudinal muscles were found in dorsal, lateral and ventral positions. One of the two pairs of lateral muscles is newly described for the species. Helicoidal muscles, external to the circular muscles and some longitudinal bands, spiraled around the pharynx and anterior portion of the intestine. Two pairs of segmentally-arranged dorsoventral muscles were also present. Lateral dorsoventral muscles extended from the base of the pharynx to the anterior part of the caudal furca. Medial dorsoventral muscles extended from the pharyngeal-intestinal junction into each ramus of the caudal furca. A hypothesis on the evolution of dorsoventral muscles in D. tessalata is proposed which includes a splitting of circular muscles into separate somatic and splanchnic components with a further displacement of both muscle sets into a dorsoventral orientation.     

Major muscle systems in the larval caenogastropod,Ilyanassa obsoleta,display different patterns of development

This study describes the anatomical and developmental aspects of muscular development from the early embryo to competent larval stage in the gastropod Ilyanassa obsoleta. Staining of F‐actin revealed differential spatial and temporal patterns of several muscles. In particular, two major muscles, the larval retractor and pedal retractor muscles originate independently and display distinct developmental patterns similar to observations in other gastropod species. Additionally, together with the larval retractor muscle, the accessory larval muscle developed in the embryo at the trochophore stage. Therefore, both these muscles develop prior to ontogenetic torsion. The pedal retractor muscle marked the most abundant growth in the mid veliger stage. Also during the middle stage, the metapodial retractor muscle and opercular retractor muscle grew concurrently with development of the foot. We show evidence that juvenile muscles, such as the buccal mass muscle and siphon muscle develop initially during the late veliger stage. Collectively, these findings substantiate that larval myogenesis involves a complex sequence of events that appear evolutionary conserved within the gastropods, and set the stage for future studies using this model species to address issues concerning the evolution and eventual fates of larval musculature in molluscs. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Pelagiella exigua,an early Cambrian stem gastropod with chaetae: lophotrochozoan heritage and conchiferan novelty

Exceptionally well-preserved impressions of two bundles of bristles protrude from the apertures of small, spiral shells of Pelagiella exigua, recovered from the Kinzers Formation (Cambrian, Stage 4, ‘Olenellus Zone’, c. 512 Ma) of Pennsylvania. These impressions are inferred to represent clusters of chitinous chaetae, comparable to those borne by annelid parapodia and some larval brachiopods. They provide an affirmative test in the early metazoan fossil record of the inference, from phylogenetic analyses of living taxa, that chitinous chaetae are a shared early attribute of the Lophotrochozoa. Shells of Pelagiella exhibit logarithmic spiral growth, microstructural fabrics, distinctive external sculptures and muscle scars characteristic of molluscs. Hence, Pelagiella has been regarded as a stem mollusc, a helcionelloid expressing partial torsion, an untorted paragastropod, or a fully torted basal member of the gastropod crown group. The inference that its chaeta-bearing appendages were anterior–lateral, based on their probable functions, prompts a new reconstruction of the anatomy of Pelagiella, with a mainly anterior mantle cavity. Under this hypothesis, two lateral–dorsal grooves, uniquely preserved in Pelagiella atlantoides, are interpreted as sites of attachment for a long left ctenidium and a short one, anteriorly on the right. The orientation of Pelagiella and the asymmetry of its gills, comparable to features of several living vetigastropods, nominate it as the earliest fossil mollusc known to exhibit evidence of the developmental torsion characteristic of gastropods. This key adaptation facilitated an evolutionary radiation, slow at first and rapid during the Ordovician, that gave rise to the remarkable diversification of the Gastropoda.     

CLSM analysis of the phalloidin-stained muscle system in Nerilla antennata, Nerillidium sp. and Trochonerilla mobilis (Polychaeta; Nerillidae)

The entire muscle system of Nerilla antennata, Nerillidium sp. and Trochonerilla mobilis was three-dimensionally reconstructed from whole mounts. In juvenile and adult specimens the F-actin musculature subset was stained with FITC-conjugated phalloidin and visualized with a confocal laser scanning microscope (cLSM). The muscle system shows the following major organization: 1) circular muscles are totally absent in the body wall; 2) the longitudinal muscles are confined in two ventral and two dorsal thick bundles; 3) additional longitudinal muscles are located in the ventro- and dorsomedian axis; 4) three segmental pairs of ventral oblique muscles elongate into the periphery: the main dorsoventral muscles that run along the body side posterior and dorsally and the anterior and posterior oblique parapodial muscles, which contribute to the ventral chaetal sacs; 5) one segmental pair of dorsal oblique parapodial muscles, contributing to the dorsal chaetal sacs; 6) five to seven small dorsoventral muscles per segment; and 7) complex head and pharyngeal musculature. These results support the belief that absence of circular muscles in the polychaete body wall is much more widely distributed than is currently presumed.     

Initiation,calcification, and form of larval “archaeogastropod” shells

The coiled shell of gastropods begins as a cap-shaped lens of organic and calcified material that covers the posterior dorsal side of the larva. During development the cap enlarges to cover the larval visceral mass. Marginal growth then produces the characteristic coiled shell. One model of the initiation of shell coiling in “archaeogastropods” requires that the shell remains flexible and uncalcified until after torsion, and that muscle contraction during torsion deforms the shell. We describe early shell calcification and tested this requirement of the model for the patellogastropod limpets Tectura scutum and Lottia digitalis, the trochids Calliostoma ligatum and Margarites pupillus and the abalone Haliotis kamtschatkana. We determined the stage of initial calcification by staining larvae with the fluorescent calcium marker calcein and observing them with bright field, crossed polarizing filter, and fluorescence microscopy. In T. scutum the earliest observable shell was calcified and calcium was sometimes detected even before the initial shell was visible. Larvae of the other species deposited a noncalcified matrix that was subsequently calcified, and in C. ligatum and M. pupillus this initial calcification was distinctly spotty. Shells of both patellogastropods and the abalone were demonstrably rigid prior to torsion while the shells of the trochids were not. These results suggest that shell coiling in patellogastropods and abalone is not initiated by contraction of the larval retractor muscle during torsion; in trochids this mechanism is possible. However, analysis of camera lucida drawings of pre- and post-torsional shells of T. scutum and C. ligatum did not detect shell shape changes during torsion. J. Morphol. 235:77–89, 1998. © 1998 Wiley-Liss, Inc.     

The species flocks of lacustrine gastropods: <Emphasis Type="Italic">Tylomelania</Emphasis> on Sulawesi as models in speciation and adaptive radiation

Endemic radiations provide splendid opportunities for studies in evolutionary biology. Species flocks in ancient lakes, such as in Tanganyika, Malawi or Baikal, have featured prominently in evolutionary biology, viewing these “evolutionary theatres” as hotspots of diversification. However, following a century of neglect, the endemic evolution of limnic cerithioidean gastropods in the two central lake systems on the Indonesian island of Sulawesi (i.e. Lake Poso and the lakes of the Malili system, e.g. Danau Matano, Mahalona and Towuti) also provide instructive model cases for the study of speciation mechanisms, adaptive radiation and annidation (i.e. niche exploitation). We here discuss the evolutionary and taxonomic implications of the lacustrine species flocks in Tylomelania from these lakes in Sulawesi as an exceptional endemic assemblage of morphologically distinct viviparous pachychilid gastropods. This first comprehensive compilation of data on both ancient lake systems, Poso and Malili, offers a new perspective on ecological differentiation in this radiation. Presented here within the framework of the theory of evolutionary ecology it provides a research program for acquiring a synthetical perspective that includes morphology, molecular genetics, ecology and biogeography. In this context, it will be possible to compare the species flocks of these truly “Darwinian snails” on Sulawesi with the long enigmatic, so-called thalassoid (i.e. marine-like) gastropod radiation in East African’s Lake Tanganyika.     

COMPUTER-SIMULATED SHELL SIZE AND SHAPE VARIATION IN THE CARIBBEAN LAND SNAIL GENUS CERION: A TEST OF GEOMETRICAL CONSTRAINTS

A computer graphical model of gastropod shell form is used to test a hypothesis of geometric constraint proposed to explain the disjunct distribution of shell forms observed in Cerion, a species-rich and geometrically varied genus of terrestrial gastropods. The mapping of computer-simulated forms into a morphospace of Cerion shells produces a continuum of sizes and shapes. Therefore, the absence of particular shell forms is not explained by geometric constraints. Two proposed modes of shell morphogenesis at extreme ranges in size (“dwarfs” and “giants”) previously were thought to be exclusive routes to the construction of high-spired (“smokestack”) forms. The present study shows that there are, in fact, multiple routes of transformation. In addition, these routes are geometrically reversible and interconnect the members of the shell-form continuum. Thus, the possible pathways followed during the course of evolution within this genus cannot be determined until an adequate phylogenetic hypothesis has been proposed.     

THE ORGANISATION OF THE PEDAL MUSCULATURE AND ITS CONNECTION TO THE DORSOVENTRAL MUSCULATURE IN SCAPHOPODA

Scaphopoda possess one or two pairs of dorsoventral muscles.At the level of the diaphragm between the intestinal and theperianal sinus these muscles divide into a latero-dorsal portionand a medio-ventral portion. The entire medio-ventral portionand the ventral parts of the latero-dorsal portion form thepedal longitudinal musculature. This is the general patternin both orders, Dentaliida and Gadilida. In Gadilida exceptthe family Entalinidae, these muscle portions are reduced comparedto an additional pair of central pedal retractor muscles. Themusculature of the pedal wall is four-layered: outer circularmuscles, two layers of helical muscles, and inner longitudinalmuscles. Because of differences in the organization of the musculature,the foot consists of three functional units: a pedal base, amiddle piece, and an anchoring organ. In the dentalud foot,all of the longitudinal muscles are in contact with the pedalwall. In the Entalinidae, three or four pairs of central retractormuscles become free of the pedal wall at the base of the middlepiece. In all other Gadilida the two central retractors arecontinuous from the dorsoventral muscles into the anchoringorgan. The elongation of the foot is purely hydraulic in Gadilida.In Dentaliida, however, the principles of hydraulic and muscular-hydrostatsare combined. (Received 17 July 1991; accepted 28 October 1991)     

Functional morphology of amplexus (clasping) in spinicaudatan clam shrimps (Crustacea,Branchiopoda) and its evolution in bivalved branchiopods: A video‐based analysis

Male clam shrimps (Crustacea: Branchiopoda: Laevicaudata, Spinicaudata, and Cyclestherida) have their first one or two trunk limb pairs modified as “claspers,” which are used to hold the female during mating and mate guarding. Clasper morphology has traditionally been important for clam shrimp taxonomy and classification, but little is known about how the males actually use the claspers during amplexus (clasping). Homologies of the various clasper parts (“movable finger,” “large palp,” “palm,” “gripping area,” and “small palp”) have long been discussed between the three clam shrimp taxa, and studies have shown that only some structures are homologous while others are convergent (“partial homology”). We studied the clasper functionality in four spinicaudatan species using video recordings and scanning electron microscopy, and compared our results with other clam shrimp groups. General mating behavior and carapace morphology was also studied. Generally, spinicaudatan and laevicaudatan claspers function similarly despite some parts being nonhomologous. We mapped clasper morphology and functionality aspects on a branchiopod phylogeny. We suggest that the claspers of the three groups were adapted from an original, simpler clasper, each for a “stronger” grip on the female's carapace margin: 1) Spinicaudata have two clasper pairs bearing an elongated apical club/gripping area with one setal type; 2); Cyclestherida have one clasper pair with clusters of molariform setae on the gripping area and at the movable finger apex; and 3) Laevicaudata have one clasper pair, but have incorporated an additional limb portion into the clasper palm and bear a diverse set of setae. J. Morphol. 278:523–546, 2017. © 2017 Wiley Periodicals, Inc.     

Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan

More than a century of speculation about the evolutionary originof the contorted gastropod body plan has been inspired by adultanatomy and by long-standing developmental observations. Theresult has been a concept of gastropod torsion that I call the"rotation hypothesis." Under the rotation hypothesis, gastropodsoriginated when all components of the visceropallium (shell,mantle, mantle cavity with contained structures, and viscera)rotated by 180° relative to the head and foot. This evolutionaryrotation is echoed during early development of patellogastropodsand vetigastropods and occurs to some extent during developmentof more derived clades. However, comparative developmental dataon ontogenetic torsion are minimal and I argue that the rotationhypothesis is a tautological argument. More recent studies onrepresentatives from 3 major clades of gastropods suggest thatthe highly conserved aspect of gastropod development is notsynchronous rotation of all components of the visceropalliumrelative to the head and foot but rather a state of anatomicalorganization in which the developing mantle cavity is on theright but the shell coil is posterior (endogastric orientation).This conserved state of developmental anatomy has inspired analternative hypothesis for the evolutionary origin of the gastropodbody plan, the "asymmetry hypothesis." Under the asymmetry hypothesis,the gastropod mantle cavity originated from 1 side only of abilateral set of mantle cavities. The asymmetry hypothesis doesnot require a saltation event to explain the origin of gastropods,nor does it require that the ancient molluscan precursor ofgastropods carried the shell coil over the head (exogastricorientation).     

Functional morphology of the pedicellariae of the asteroid Marthasterias glacialis (Echinodermata)

Summary Marthasterias glacialis bears two kinds of pedicellariae. The straight pedicellariae are single and occur everywhere on the asteroid body surface except in the ambulacral groove. The crossed pedicellariae are clustered on mobile structures (the rosettes) build around marginal and abactinal spines.Basically, each pedicellaria has a head and a stalk. A skeleton occurs only in the pedicellarial head. It consists of two valves and a basal piece. Muscular bundles are anchored on these skeletal ossicles. The straight pedicellariae have two pairs of adductor muscles (the inner and the outer adductors) and one pair of abductor muscles, these latter being weakly developed. Longitudinal muscle fibers occur all along the stalk of straight pedicellariae. The crossed pedicellariae have two pairs of adductor muscles (the distal and the proximal adductors) and two pairs of abductor muscles (the distal and the proximal abductors). The proximal adductors of crossed pedicellariae are homologous to the stalk muscles of straight pedicellariae.The pedicellariae are able to react to direct and indirect tactile stimuli. There is a great deal of individual variation among pedicellarial responses. Moreover, the reactions occur at random and lack coordination. The seemingly aberrant behavior of the pedicellariae is interpreted as a preventive activity that protects the asteroid body surface against unwanted materials and organisms.     

Does variation in morphology correspond with variation in habitat use in freshwater gastropods?

We tested if variations (i.e., breadth) in morphology and habitat use vary predictably among six aquatic gastropod species that were collected across Indiana and Illinois, USA. We predicted that interspecific morphological variation would positively covary with variation in habitat use among species. We used geometric morphometrics (Procrustes technique and relative warp analysis) to quantify morphology and multivariate analyses (PCA) to quantify habitat. Increased morphological breadth did not vary predictably with increased habitat breadth. However, we found that life history traits correspond with patterns in morphological and habitat breadth for these six aquatic gastropods. Pulmonate gastropods (use lungs for respiration) that lack an operculum cover exhibited decreased morphological breadth compared to coenogastropods (use gills for respiration). This pattern may ultimately be a function of behavioral adaptations in freshwater gastropods. Gastropods that are capable of breathing air or using other behavioral modifications such as burrowing to escape predators may not require high morphological breadth. Conversely, selection may favor higher morphological breadth in gastropods with gills that also do not move out of the water column to escape predators.