Columellar Muscle Research Articles

Replacing mechanical protection with colorful faces-twice: parallel evolution of the non-operculate marine worm-snail genera Thylacodes (Guettard, 1770) and Cayo n. gen. (Gastropoda: Vermetidae).

Vermetid worm-snails are sessile and irregularly coiled marine mollusks common in warmer nearshore and coral reef environments that are subject to high predation pressures by fish. Often cryptic, some have evolved sturdy shells or long columellar muscles allowing quick withdrawal into better protected parts of the shell tube, and most have variously developed opercula that protect and seal the shell aperture trapdoor-like. Members of Thylacodes (previously: Serpulorbis) lack such opercular protection. Its species often show polychromatic head-foot coloration, and some have aposematic coloration likely directed at fish predators. A new polychromatic species, Thylacodes bermudensis n. sp., is described from Bermuda and compared morphologically and by DNA barcode markers to the likewise polychromatic western Atlantic species T. decussatus (Gmelin, 1791). Operculum loss, previously assumed to be an autapomorphy of Thylacodes, is shown to have occurred convergently in a second clade of the family, for which a new genus Cayo n. gen. and four new western Atlantic species are introduced: C. margarita n. sp. (type species; with type locality in the Florida Keys), C. galbinus n. sp., C. refulgens n. sp., and C. brunneimaculatus n. sp. (the last three with type locality in the Belizean reef) (all new taxa authored by Bieler, Collins, Golding & Rawlings). Cayo n. gen. differs from Thylacodes in morphology (e.g., a protoconch that is wider than tall), behavior (including deep shell entrenchment into the substratum), reproductive biology (fewer egg capsules and eggs per female; an obliquely attached egg capsule stalk), and in some species, a luminous, "neon-like", head-foot coloration. Comparative investigation of the eusperm and parasperm ultrastructure also revealed differences, with a laterally flattened eusperm acrosome observed in two species of Cayo n. gen. and a spiral keel on the eusperm nucleus in one, the latter feature currently unique within the family. A molecular phylogenetic analysis based on mitochondrial and nuclear rRNA gene sequences (12SrRNA, trnV, 16SrRNA, 28SrRNA) strongly supports the independent evolution of the two non-operculate lineages of vermetids. Thylacodes forms a sister grouping to a clade comprising Petaloconchus, Eualetes, and Cupolaconcha, whereas Cayo n. gen is strongly allied with the small-operculate species Vermetus triquetrus and V. bieleri. COI barcode markers provide support for the species-level status of the new taxa. Aspects of predator avoidance/deterrence are discussed for these non-operculate vermetids, which appear to involve warning coloration, aggressive behavior when approached by fish, and deployment of mucous feeding nets that have been shown, for one vermetid in a prior study, to contain bioactive metabolites avoided by fish. As such, non-operculate vermetids show characteristics similar to nudibranch slugs for which the evolution of warning coloration and chemical defenses has been explored previously.

Molecular investigation and description of Iberozospeum n. gen., including the description of one new species (Eupulmonata, Ellobioidea, Carychiidae)

The subterranean realm of the Cantabrian-Pyrenean region of northern Spain harbours a rich diversity of Zospeum. Due to their tiny size and the difficulty of finding them alive, scarce animal material has been available for scientific investigation. Recent investigations of Zospeum shells have provided valuable, but limited insights towards our understanding of the evolutionary processes occurring within this taxon in northern Spain. In an integrative study, we investigate 57 populations of Zospeum from northern Spanish caves using two mitochondrial (COI and 16S) and two nuclear markers (H3 and 5.8 S rRNA + ITS2). Revealed is a separate radiation of the northern Spanish species for which the new genus, Iberozospeum, is proposed. The independent radiation of Dinaric Zospeum from that of northern Spain justifies the designation of Iberozospeum n. gen. Morphological evidence is provided via histological analysis of Iberozospeum vasconicum and SEM analyses of radulae of eastern Alpine, Dinaric and Iberian species. Important differences in morphological structure and character states are presented, including the first view of the sexually mature female and the presence of the giant albumen gland in an individual of the subterranean, troglobitic Carychiidae. Significant differences are revealed in superficial crystallographic structure of the columellar lamellae, the morphology of the columellar muscle and in the radula. Radular ribbon length, ribbon broadness, straightness of the ribbon base and cusp configuration are distinctive in the Iberian species. One new species is described corroborated by genetic and morphological characters.

Inferring octopodoid and gastropod behavior from their Plio-Pleistocene cowrie prey (Gastropoda: Cypraeidae)

Predation is an evolutionary force shaping sea floor communities, with the record of drilling predation being particularly useful to study predatory behavior on short and long timescales. Most predatory drill holes are caused by gastropods, but octopods within Octopodoidea also produce characteristic drill holes, yet remain severely understudied in deep time. Consequently, studies comparing octopodoid versus gastropod drill holes from the same prey group are exceedingly rare. Cypraeid gastropod (cowries) are commonly drilled by octopodoids today, but nothing is known about drill holes produced by octopodoids and gastropod in fossil cypraeids. We studied 2406 Plio-Pleistocene fossil cowries from southern Florida, USA. Drilling by octopodoids was common (13.4% of specimens), whereas gastropod drill holes were rare (0.2%). There is strong site selectivity: most octopodoid drill holes are found in the ventral-left-posterior and ventral-left-middle shell regions. These parts are not the thinnest regions of the shell, but represent the location where the columellar muscle anchors the soft tissue to the shell interior. We suggest that the octopodoids sought to paralyze and relax this muscle via the drill hole to minimize the time to consumption. Specimens with multiple octopodoid drill holes (24.1% of drilled specimens) have a higher proportion of drill holes located outside the ventral-left-posterior and ventral-left-middle regions. Thus, drilling outside these regions appears less effective. The few gastropod drill holes occur on the dorsal side. A meta-analysis shows that octopodoids produce significantly more drill holes per drilled molluscan shell than do gastropod drillers. We found mixed evidence for size selectivity: whereas the size of drilled specimens does not significantly differ from non-drilled specimens, drill-hole size is significantly larger in larger cowries, which may imply they were drilled by larger octopodoid individuals. We suggest that the molluscan fossil record should be leveraged to further study the evolution of octopodoid predation and behavior.

Histology Atlas and Systematic Approach to Postmortem Examination of the Queen Conch Lobatus gigas

The queen conch Lobatus gigas is the second largest commercial fishery in the Caribbean, and overharvesting has resulted in significant population declines. Depleted populations are at greater risk of stochastic events, including disease epidemics; however, disease diagnosis in L. gigas has been limited by the lack of standard procedure and histological reference material. This manual outlines a systematic technique for postmortemexamination and compiles a comprehensive histology atlas to facilitate research regarding the pathology of L. gigas. Methods for euthanasia, dissection, sampling, and fixation are described, which produced optimal presentation and preservation of tissues. The recommended approach includes anesthesia with magnesium sulphate, extraction from shell, and euthanasia by incision through ganglia posterior to the buccal mass, followed by exposure of all tissues using four incisions. Anatomy is described for both sexes and standard sample locations are established. Tissue fixation is optimal using Davidson's formula. A histological reference for 16 tissues, nine of which were described for the first time in this species, is presented, including the anus, columellar muscle, digestive gland, esophagus, foot, ganglia and nerves, gill, gonad, heart, hypobranchial gland, kidney, mantle, nephridial gland, osphradium, small intestine and rectum. This manual is the first necropsy guide and histology atlas for L. gigas, a baseline resource for researchers monitoring health and disease in the species.

Morphological and ecological adaptation of limpet-shaped top shells (Gastropoda: Trochidae: Fossarininae) to wave-swept rock reef habitats.

Flattening of coiled shells has occurred in several gastropod lineages, while the evolutionary process of shell flattening is little known. The subfamily Fossarininae of the top shell family (Trochidae) is unique, because it includes four genera at various stages of shell flattening. Broderipia and Roya, have zygomorphic shells that has lost coiling, while the sister genera, Fossarina and Synaptocochlea, have respectively turbiniform and auriform shells. Therefore, comparisons of biology, habitats and detailed morphology among these four genera may help us to detect selection pressure driving shell flattening and loss of coiling. Although Broderipia has recently been identified as living symbiotically in the pits of sea urchins, the habitats and biology of the other three Fossarininae species, especially Roya are poorly known. After an extensive search on rocky shores of the Japanese Archipelago, we found live Roya eximia snails on intertidal/subtidal rock surfaces exposed to strong waves. Roya snails crept on the bare rock surface to graze periphyton at low tide, and fled into vacant barnacle shells at high tide. Comparison of the morphology of soft bodies in Fossarininae revealed that the columellar muscle of flattened species has been drastically elongated and arranged in posterior semi-outer edge of the flattened shell as observed in true limpets. The flattering and loss of coiling of the shell in Roya caused acquisition of a zygomorphic flat body, retraction of coiled visceral mass, and expansion of the foot sole. All of these changes improved tolerance against strong waves and the ability to cling to rock surfaces, and thus enabled a lifestyle utilizing both wave-swept rock surfaces and the inside of vacant barnacle shells.

Localization of Octopus Drill Holes on Cowries

Abstract: Octopus localize their drill holes on prey but it remains unclear how this is accomplished. Asymmetrical prey such as snails and crabs would provide cues to the octopus but the nominally symmetrical cowry presents a unique drill hole localization scenario. To investigate how octopus drill cowries we collected their shells from octopus middens in Bonaire and surveyed drilled specimens in shell collections. Cowry drill holes were predominantly localized to the ventral-posterior-lateral region of the shell, the attachment point of the columellar muscle. Cowries with multiple drill holes suggest a trial-and-error learning process leading to localization. Further studies with naive octopuses are proposed to determine the role of learning in this feeding behavior.

Observations and Experiments on the Biology and Life History of Riseriellus occultus (Heteronemertea: Lineidae).

Studies on the biology and life history of nemerteans are scarce, mostly because these animals are nocturnal. In order to broaden the knowledge base on the life history of nemerteans as a prerequisite for comparative analyses, we studied a population of Riseriellus occultus (Heteronemertea: Lineidae) inhabiting the rocky intertidal in southern Brittany near Concarneau (France) for more than 10 years. Our studies show that R. occultus is an iteroparous, perennial species exclusively inhabiting rocky shore crevices that result from onionskin weathering of the granite. From September through October R. occultus reproduces by external fertilization and develops via a planktonic pilidium larva, which, under laboratory conditions, metamorphoses after about six weeks. Adults of R. occultus are nocturnal macrophagous predators that preferentially feed on the gastropods Gibbula umbilicalis and Patella species, but also consume the bivalve Mytilus edulis. Since R. occultus devours the snail inside the shell, we fixed individuals while feeding, and serially sectioned them. Reconstruction of the sections shows that R. occultus swallows the entire soft body and finally detaches the columellar muscle from the shell. Estimates on the density of R. occultus inside the rock crevices provide evidence for clustered distribution and locally high abundance on the rocky shore. These data strongly suggest that R. occultus affects the structure of the rocky shore gastropod community. Although our data are still fragmentary with respect to the ecology of this species and its role in the local food web, our knowledge has grown to such extent that R. occultus can now be regarded as one of the few well characterized nemertean species.

The extinct Nerineoidea and Acteonelloidea (Heterobranchia, Gastropoda): a palaeobiological approach

ABSTRACT Based on the morphology of the aperture and internal plaits or folds, the Nerineoidea Zittel, 1873 are subdivided into seven families: the Pseudonerineidae Pchelintsev, 1965, Ceritellidae Wenz, 1940, Nerinellidae Pchelintsev, 1960, Eunerineidae n. fam. (new family that includes the Nerineidae Zittel, 1873 (pars) and the Diptyxidae Bouchet & Rocroi, 2005) the Ptygmatididae Pchelintsev, 1960, Nerineidae and Itieriidae Cossmann, 1896. The internal plaits of the Nerineoidea had different functions: the columellar plaits subdivided the columellar muscle into strands, which allowed portions of the foot to be moved individually. A tubelike space delimited by the parietal and palatal plaits is compared with the pallial caecum or posterior mantle chamber of heterobranchs and served the respiration. Waste from both mantle chambers was expelled through a subsutural notch, which is the common feature of the Nerineoidea. The Nerineoidea were shell draggers and probably deposit feeders with a semi- infaunal mo...

Comparative Morphology ofDorsanum miranandBullia granulosafrom Morocco (Mollusca: Caenogastropoda: Nassariidae)

ABSTRACT The anatomy and taxonomy of two western African nassariids are explored, based on samples collected in Morocco. The species are Dorsanum miran, the type species of the genus, and Bullia granulosa, a characteristic member of Bullia. Both possess the typical morphological and anatomical features of the family, including a pair of metapodial tentacles, a well-developed proboscis, elongated odontophore with fusion of cartilages, and highly concentrated central nervous system. Both species have in common the socket-like heads, bifid columellar muscles, and reduction of the gland of Leiblein. D. miran has well-developed eyes, cement gland, and preputial protection at the penis tip. B. granulosa lacks eyes, has multiplicity of some buccal mass muscles (transverse muscles and main dorsal tensor muscles — m2), and a thick-walled and broad anterior oesophagus. The characters are discussed in the light of present knowledge concerning caenogastropod taxonomy.

Morphology, ultrastructure and contractile properties of muscles responsible for superior tentacle movements of the snail

Bending, twitching and quivering are different types of tentacle movements observed during olfactory orientation of the snail. Three recently discovered special muscles, spanning along the length of superior tentacles from the tip to the base, seem to be responsible for the execution of these movements. In this study we have investigated the ultrastructure, contractile properties and protein composition of these muscles. Our ultrastructural studies show that smooth muscle fibers are loosely embedded in a collagen matrix and they are coupled with long sarcolemma protrusions. The muscle fibers apparently lack organized SR and transverse tubular system. Instead subsarcolemmal vesicles and mitochondria have been shown to be possible Ca2+ pools for contraction. It was shown that external Ca2+ is required for contraction elicited by high (40 mM) K+ or 10-4 M ACh. Caffeine (5 mM) induced contraction in Ca2+-free solution suggesting the presence of a substantial intracellular Ca2+ pool. High-resolution electrophoretic analysis of columellar and tentacular muscles did not reveal differences in major contractile proteins, such as actin, myosin and paramyosin. Differences were observed however in several bands representing presumably regulatory enzymes. It is concluded that, the ultrastructural, biochemical and contractile properties of the string muscles support their special physiological function.

Ultrastructural features of the columellar muscle and contractile protein analyses in different muscle groups of Megalobulimus abbreviatus (Gastropoda, Pulmonata)

In Megalobulimus abbreviatus, the ultrastructural features and the contractile proteins of columellar, pharyngeal and foot retractor muscles were studied. These muscles are formed from muscular fascicles distributed in different planes that are separated by connective tissue rich in collagen fibrils. These cells contain thick and thin filaments, the latter being attached to dense bodies, lysosomes, sarcoplasmic reticulum, caveolae, mitochondria and glycogen granules. Three types of muscle cells were distinguished: T1 cells displayed the largest amount of glycogen and an intermediate number of mitochondria, suggesting the highest anaerobic metabolism; T2 cells had the largest number of mitochondria and less glycogen, which suggests an aerobic metabolism; T3 cells showed intermediate glycogen volumes, suggesting an intermediate anaerobic metabolism. The myofilaments in the pedal muscle contained paramyosin measuring between 40 and 80 nm in diameter. Western Blot muscle analysis showed a 46-kDa band that corresponds to actin and a 220-kDa band that corresponds to myosin filaments. The thick filament used in the electrophoresis showed a protein band of 100 kDa in the muscles, which may correspond to paramyosin.

The Architecture of the Physid Musculature ofPhysa acutaDraparnaud, 1805 (Gastropoda: Physidae)

ABSTRACT The basommatophoran family Physidae possesses a remarkable but little known set of muscles called the “physid musculature”. Using Physa acuta as a model, this musculature was shown to be anatomically complex and associated in places with the columellar muscle. The physid musculature has two main components, the physid muscle sensu stricto and the fan muscle, both of which have previously been named but not examined in detail. The physid muscle s.s. is branched with the larger branches running to the neck, head and foot, and the smaller ones to the lung floor and mantle. The fan muscle is not branched. We propose that the physid musculature is responsible for a unique ability of physids to rapidly flick their shells from side to side—a reaction that frequently enables them to escape predation. We suggest that during this movement the lung floor, which is strengthened by several bands of muscle from both the physid musculature and the columellar muscle, serves as a pivot for the rotating visceral h...

<i>Niku-nuki:</i> a useful method for anatomical and DNA studies on shell-bearing

Often only one or a few individuals of rare species are collected. How do we treat them as intact voucher specimens? The shell of the whole individual in formalin or alcohol will corrode or fade. In order to dissect the soft parts, you must crack or dissolve the shell. Niku-nuki, a traditional method that has been used by Japanese malacologists overcomes this dilemma. It is also applicable to minute molluscs. The outline is: 1. Prepare boiling hot freshwater, a small beaker, forceps (with fi ne tips), a small syringe, a petri dish, and a stereomicroscope; 2. When the live animal in the beaker crawls on the bottom, pour boiling hot water over the animal, which is killed immediately. Some seconds later take the specimen out of the hot water, hold it with two fi ngers of one hand and hold the forceps with another hand; 3. Under the microscope, grab the foot with the forceps and pull carefully to just separate the columellar muscle from the shell; 4. Pull the foot again in a petri dish fi lled with cold water as under 3. With coiled gastropods, unscrew the specimen by approximately ¼ whorls. If it is diffi cult to move the soft parts, inject water into the aperture gently with the syringe. Repeat it several times, then you will get an empty shell and the complete soft parts. With this method, we can obtain intact shells and soft parts for multiple purposes such as conchological observation and gross anatomy. DNA can also be extracted from those soft parts because DNA is stable at high temperature. The boiled animal can be dehydrated in alcohol. We can prevent the negative effect of DNase (by heat) and magnesium (by washing in freshwater) on the DNA.

The systematic position of Scolodonta Döring, 1875 and Scolodontidae H. B. Baker, 1925 (Gastropoda: Pulmonata)

Scolodonta semperi (Döring, 1875) from Argentina is redescribed. The aulacopod foot, the branching of the penial retractor from the columellar muscle, the elongated kidney and the lack of corneous hooks inside the penis show that Scolodonta Döring, 1875 has to be classified in the Systrophiidae Thiele, 1926. Thus, the family has to be called Scolodontidae Baker, 1925. There are only minor differences between Scolodonta semperi and Systrophia lutea Tillier, 1980, the type species of Wayampia Tillier, 1980. Thus, Wayampia Tillier, 1980 becomes a synonym of Scolodonta Döring, 1875.

Neuroeffector Connections of Giant Multimodal Neurons in the African Snail Achatina Fulica

A new method of making preparations was used to analyse the neuroeffector connections of the paired giant neurons of the African snail Achatina fulica. These neurons were found to induce postsynaptic potentials in the muscles of the mantle, heart, the wall of the pulmonary cavity, and the muscular elements of the renal complex, the pericardium, the sexual apparatus, the walls of the cerebral arteries, the filaments of the columellar muscles, the wall of the abdomen, and the tentacle retractor muscles. Rhythmic neuron activity led to the development of marked facilitation and long-term potentiation of synaptic potentials. The possible significance of the multiple neuroeffector connections of giant neurons is discussed.

Synaptic potentials in common snail muscles during stimulation of command neurons.

Giant defensive behavior command neurons have extensive efferent outputs to the muscles of the body wall and mantle of the common snail. Neurons in the mediorostral zone of the parietal ganglia, i.e., neurons LPa2, LPa3, RPa2, and RPa3, send processes via most of the visceral nerves, the pedal nerves, and the cutaneous nerves of the pedal ganglia [1, 3]. Command neurons trigger contraction of the pneumostoma and a series of somatic muscles. In addition, these cells send synaptic potentials to the heart and the walls of the pulmonary cavity [2, 4]. We have observed that activation of command neurons in the parietal ganglia leads to the appearance of postsynaptic potentials (PSP) in many visceral muscles and in the walls of some internal organs. These data were obtained using a new modification of the semi-intact preparation in which virtually all neuroeffector connections are preserved [5]. Command neuron activity was recorded by standard microelectrode techniques. Postsynaptic potentials were recorded using macroelectrodes consisting of glass pipettes with tip diameters of 100‐250 μm. Macroelectrodes were attached to flexible polyethylene tubing and filled with physiological saline. Large muscles were studied by inserting the electrode into the muscle, while thin-walled organs were studied by recording potentials from the surface. Activation of command neurons LPa2 and RPa2, as well as RPa3, induced postsynaptic potentials in the heart, pericardium, the wall of the pulmonary cavity, the columellar muscle, the muscles of the mantle ridge and body wall, the walls of the kidney, the antenna tip retractors, and the cerebral artery. In all cases, postsynaptic potentials showed marked facilitation during rhythmic neuron discharges. The first postsynaptic potential following a prolonged period of rest was sometimes below the noise level. In these cases, the coefficient of facilitation was taken as the ratio to the second postsynaptic potential. The coefficient of facilitation during prolonged series of neuron discharges could reach values of the order of 20 or more (Fig. 1. A). Series of postsynaptic potentials in command neurons were accompanied by long-lasting potentiating effects, which then decreased with a time constant of 3‐4 min. Facilitation and potentiation of postsynaptic potentials occurred differently in different neurons. This is evidence for the absence of any common terminal switching. The latent periods of postsynaptic potentials varied slightly from preparation to preparation and depend on the recording site. The latent periods of postsynaptic potentials at different points can be compared from the results presented from one animal (Fig. 2). In this experiment, postsynaptic potentials were sequentially recorded from eight different points in conditions of stimulation of LPa2 and RPa2, as well as RPa3. After action potentials in neuron RPa3, the shortest-latency postsynaptic potentials arose in the spurs of the columellar muscle and the wall of the cerebral artery (about 110 msec), while the longest delays were recorded in the mantle (160 msec; Fig. 2, B). Comparison of postsynaptic potentials from different neurons but at the same point showed that postsynaptic potentials from neuron RPa2 appeared in the mantle muscles 25‐30 msec earlier than those from RPa3 (Fig. 2, C). Analysis of the latent periods of postsynaptic potentials at different points showed that in general, changes in potentials from one recording point to another corresponded to the probable changes in the length of the pathways for the neuronal action potentials. Over the range recorded, delays corresponded quite closely to data obtained by Wilgenburg and Milligan [4], who measured the latent periods between action potentials in cell B (terminology used at that time;

Effects of starvation on RNA : DNA ratio, glycogen content, and C : N ratio in columellar muscle of the Japanese turban shell Turbo (Batillus) cornutus (Gastropoda)

: The validity of using the RNA : DNA ratio, glycogen content, and C : N ratio in the columellar muscle as indicators of the nutritional condition in the Japanese turban shell Turbo cornutus was examined. These biochemical indices were compared between fed and starved animals. The fed animals were fed brown algae to satiation for 108 days as a control, whereas the starved animals were not fed for the first 75 days and then fed for next 33 days. All three indices declined during the starvation period, and the values for the starved group were significantly lower than those for the fed group during days 20–75. At day 108, after the re-feeding period, the indices for the starved group were found to have increased. Among the indices, only the RNA : DNA ratio for the starved males responded rapidly to starved conditions and became significantly lower even at day 4, and only the RNA : DNA ratio for the starved males and females recovered to those levels of the fed animals at day 108. The results indicated that the RNA : DNA ratio is the most rapid indicator of nutritional stress among the three indices. However, it is recommended that glycogen content and C : N ratio be used in addition to the RNA : DNA ratio for monitoring the health of T. cornutus as the RNA : DNA ratio showed large variations.

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 .

Histological characteristics of the foot and locomotory activity of Crepidula dilatata Lamarck (Gastropoda: Calyptraeidae) in relation to sex changes

Crepidula dilatata Lamarck individuals can be categorized into one of the following sexual states: immature (up to 8.5 mm shell length), male (7.5–19.0 mm), intersex (11.5–19.9 mm) or female (over 17.3 mm). Histological analysis of the gonads shows protandry in this species. The locomotory activity of these animals diminishes from active movement in the early stages, juveniles, and males to a strong sessile condition in females. This is related to reproductive mechanisms which involve copulation, benthic capsules and parental care. Histological analysis of the foot shows prevalence of three types of epithelial mucocytes in motile animals. A fourth type of mucocyte (M4) is present in higher abundance in sessile females and is associated with a greater adhesive capacity of the foot for the substrate. Predominance of smooth muscle bundles in the foot of younger animals will also facilitate locomotion. Older, sessile animals have thick columellar muscles, possibly associated with the need to lift the shell up for ventilation and feeding.

The Columellar Muscle of Prosobranch Gastropods: Morphological Zonation and Its Functional Implications

The arrangement of muscle and connective tissue fibers in the columellar muscle of three prosobranch gastropods, Calliostoma euglyptum, Littorina littorea, and Ilyanassa obsoleta, was analyzed. The columellar muscle in the three snails is a flat, sheet-like muscle that originates on the columella of the shell and inserts on the operculum. The muscle was found to comprise a densely packed three-dimensional array of muscular and connective tissue fibers, supporting the hypotheses of previous authors that it functions as a muscular hydrostat. The fiber arrangement is not uniform throughout the entire muscle; instead it includes three regions that grade into one another. Nearest its origin, the muscle consists primarily of longitudinal muscle fibers with few dorsoventral or transverse fibers, and no oblique fibers. The middle portion of the columellar muscle in the region posterior to the head consists of longitudinal, dorsoventral, and transverse fibers wrapped by two opposed layers of oblique fibers. The third region of the columellar muscle extends through the foot and includes longitudinal, transverse, and dorsoventral fibers with a layer of oblique fibers on the dorsal surface. A narrow band of spherical spaces divides the columellar muscle into dorsal and ventral halves in this region. The morphological zonation within the muscle suggests that different regions perform different functions. In addition, compared to a muscle that is circular in cross section, the elliptical crosssectional shape may increase the muscular force necessary to twist the foot relative to the shell and may decrease the resistance to buckling. Additional key words: muscular antagonism, muscular hydrostat, hydrostatic skeleton, Mollusca One familiar aspect of most coiled snails is their ability to retract the soft parts into the shell when disturbed. In coiled gastropods, the muscle responsible for retraction, the columellar muscle, originates on the columella of the shell and extends into the foot, where it inserts on the operculum. Although the function of the columellar muscle in retraction is straightforward, the mechanisms of elongation of the columellar muscle and protraction of the body from the shell are less obvious. Early studies of the anatomy of the gastropod foot and visceral mass noted that the longitudinal fibers of the columellar muscle were divided into bundles by muscle fibers oriented perpendicular to the axis of the muscle (Kiittler 1913; Weber 1926; Rotarides 1941). In a study of locomotion of Bullia digitalis, Trueman & Brown (1976) observed a similar three-dimensional array of muscle fibers in the columellar muscle and a Author for correspondence. E-mail: billkier.wilson @mhs.unc.edu hypothesized that such an arrangement could provide a mechanism for muscular antagonism. Later, Brown & Trueman (1982) briefly surveyed metapodia from five prosobranchs and two pulmonates and found a three-dimensional network of muscle fibers in the columellar muscles of each. Finally, Voltzow (1990) described a three-dimensional array of muscle fibers in the columellar muscle in the foot of Busycon contrarium and in the pedal retractor muscle of Haliotis kam-