The effect of coil phenotypes and genotypes on the fecundity and viability of Partula suturalis and Lymnaea stagnalis: implications for the evolution of sinistral snails

Why are sinistral snails so rare? Two main hypotheses are that selection acts against the establishment of new coiling morphs, because dextral and sinistral snails have trouble mating, or else a developmental constraint prevents the establishment of sinistrals. We therefore used an isolate of the snail Lymnaea stagnalis, in which sinistrals are rare, and populations of Partula suturalis, in which sinistrals are common, as well as a mathematical model, to understand the circumstances by which new morphs evolve. The main finding is that the sinistral genotype is associated with reduced egg viability in L. stagnalis, but in P. suturalis individuals of sinistral and dextral genotype appear equally fecund, implying a lack of a constraint. As positive frequency‐dependent selection against the rare chiral morph in P. suturalis also operates over a narrow range (< 3%), the results suggest a model for chiral evolution in snails in which weak positive frequency‐dependent selection may be overcome by a negative frequency‐dependent selection, such as reproductive character displacement. In snails, there is not always a developmental constraint. As the direction of cleavage, and thus the directional asymmetry of the entire body, does not generally vary in other Spiralia (annelids, echiurans, vestimentiferans, sipunculids and nemerteans), it remains an open question as to whether this is because of a constraint and/or because most taxa do not have a conspicuous external asymmetry (like a shell) upon which selection can act.     

Sinistral snail shells in the sea: developmental causes and consequences

The paucity of sinistral (left-coiling) relative to dextral (right-coiling) species of gastropods in the marine realm is an enigma. In Conus , one of the most diverse marine animal genera, sinistral shell coiling has evolved as a species-wide character only once. Fossils of this species, Conus adversarius , are found in Upper Pliocene and lowermost Pleistocene deposits in the southeastern USA. Conus adversarius had nonplanktonic larval development; this may have been a critical factor for the early establishment of the species, as well as sinistral marine species in other clades. Notably, most specimens of aberrantly sinistral modern Conus are derived from typically dextral species that have nonplanktonic development. If C. adversarius was reproductively isolated from dextral conspecifics, then this species may provide an example of nearly instantaneous sympatric speciation in the fossil record. Furthermore, the common and widespread – while geologically short-lived – fossil shells of C. adversarius show large amounts of variability in form and this variation may be related, at least in part, to a pleiotropic effect associated with the reversed coiling direction of this species.     

Enantiomorphs differ in shape in opposite directions between populations

Abstract Development is left–right reversed between dextral and sinistral morphs of snails. In sympatry, they share the same gene pool, including polygenes for shell shape. Nevertheless, their shell shapes are not the mirror images of each other. This triggered a debate between hypotheses that argue either for a developmental constraint or for zygotic pleiotropic effects of the polarity gene. We found that dextrals can be wider or narrower than sinistrals depending on the population, contrary to the prediction of invariable deviation under a developmental constraint. If the pleiotropy is solely responsible instead, the mean shape of each morph should change, depending on the frequency of polarity genotype. Our simulations of this mean shape change under zygotic pleiotropy, however, show that the direction of interchiral difference remains the same regardless of genotype frequency. Our results suggest the presence of genetic variation among populations that changes the maternal or zygotic pleiotropic effect of the polarity gene.     

Multiple reversals of chirality in the land snail genus Albinaria (Gastropoda,Clausiliidae)

Reversed chirality has frequently evolved in snails, although the vast majority coils dextrally. However, there are often sinistral species within a dextral genus or almost exclusively sinistral families, such as the Clausiliidae. Some populations of the predominantly sinistral clausiliid genus Albinaria, in the southern Greek mainland, coil dextrally. The origin, evolution and distribution of the dextral Albinaria are puzzling, and as there is no reliable phylogenetic reconstruction for this speciose genus, it remains unclear how many times a shift in chirality has really occurred. In this study, our aim was to elucidate the evolutionary pathways of dextrality in Albinaria. We undertook a molecular phylogenetic analysis of two mtDNA (16S and COI) and one nDNA marker (ITS1) and included dextral and sinistral representatives found in syntopy or not. Both mtDNA and nDNA tree topologies imply that dextrals did not evolve as a monophyletic lineage. Instead, dextral lineages have evolved from sinistral ancestors multiple times independently. The fragmented population structure in Albinaria facilitates genetic drift and contributes to fixation of the opposite chirality and overcoming of the mating disadvantage of left–right reversal. Stochastic phenomena and biogeographical barriers have trapped those reversals in a limited geographical area.     

Population structure and coil dimorphism in a tropical land snail

Tree snails of the subgenus Amphidromus s. str. are unusual because of the chiral dimorphism that exists in many species, with clockwise (dextrally) and counter-clockwise (sinistrally) coiled individuals co-occurring in the same population. Given that mating in snails is normally impeded when the two partners have opposite coil, positive frequency-dependent selection should prevent such dimorphism from persisting. We test the hypothesis that a strong population structure with little movement between tree-based demes may result in the fixation of coiling morphs at a very small spatial scale, but apparent dimorphism at all larger scales. To do so, we describe the spatial structure in a Malaysian population of A. inversus (Müller, 1774) with 36% dextrals. We marked almost 700 juvenile and adult snails in a piece of forest consisting of 92 separate trees, and recorded dispersal and the proportions of dextrals and sinistrals in all trees over a 7-day period. We observed frequent movement between trees (155 events), and found that no trees had snail populations with proportions of dextrals and sinistrals that were significantly different from random. Upon recapture 1 year later, almost two-thirds of the snails had moved away from their original tree. We conclude that population structure alone cannot stabilise the coil dimorphism in Amphidromus.     

Evolution of a dextral lineage by left–right reversal in Cristataria (Gastropoda,Pulmonata, Clausiliidae)

It has long been debated whether mirror image‐like similarity in shell morphology between enantiomorphic pairs of dextral and sinistral taxa represents their sister relationship, or each of them is closer related to other congeners with the same coiling direction. The obligate rock‐dwelling genus Cristataria Vest, 1867 of the eastern Mediterranean region belongs to the Alopiinae subfamily of door snails (Clausiliidae). Cristataria and a few other genera of this subfamily include enantiomorphic pairs that are conchologically very similar to each other. Dextral C. colbeauiana (Pfeiffer, 1861) and its sinistral counterpart of such an enantiomorphic pair occur nearby one another in southern Turkey. However, the latter has been classified either as the sinistral subspecies C. colbeauiana inversa Szekeres, 1998 or as a form of sinistral C. leprevieri (Pallary, 1922). To examine the phylogenetic relationship of this enantiomorphic pair, we carried out molecular phylogenetic analysis of all the Turkish and two other Cristataria taxa based on both mitochondrial and nuclear DNA markers. Our results show that dextral C. colbeauiana and its sinistral counterpart are closest related to one another. This supports the classification of this enantiomorphic pair as dextral C. colbeauiana colbeauiana and sinistral C. colbeauiana inversa. Our results also reveal that these taxa and C. intersita Németh & Szekeres, 1995, sharing a characteristic collar behind the aperture of the shell, represent a monophyletic lineage. By contrast, the Cristataria species of non‐collared shells belong to another clade.     

Internal selection against the evolution of left-right reversal

Among metazoan species, left-right reversals in primary asymmetry have rarely gone to fixation. This suggests that a general mechanism suppresses the evolution of polarity reversal. Most metazoans appear externally symmetric and reproduce by external fertilization or copulation with genitalia located in the midline. Thus, reversal should generate little exogenous disadvantage when interacting with the external environment or in mating with the common wild-type. Accordingly, an endogenously caused fitness reduction may be responsible for the general absence of reversed species. However, how this selection operates is little understood. Phenotypic changes associated with reversal are usually inseparable from zygotic pleiotropy. By exploiting hermaphroditism and the maternal inheritance of left-right polarity, we generated dextral and sinistral snails that share the same zygotic genotype. Before hatching, these sinistrals developed lethal morphological anomalies more frequently than dextrals. Their shell shape at maturity differed from the mirror image of the dextral shell. These interchiral differences demonstrate pleiotropy in maternal effects of the polarity or linked genes. Variation in interchiral differences between parental crosses suggests the presence of epistatic variation in relative performance of sinistrals. Our results show that internal selection operates against polarity reversal, and we suggest that this is due to changes in blastomere configuration.     

Familial Sinistrals Avoid Exact Numbers

We report data from an internet questionnaire of sixty number trivia. Participants were asked for the number of cups in their house, the number of cities they know and 58 other quantities. We compare the answers of familial sinistrals – individuals who are left-handed themselves or have a left-handed close blood-relative – with those of pure familial dextrals – right-handed individuals who reported only having right-handed close blood-relatives. We show that familial sinistrals use rounder numbers than pure familial dextrals in the survey responses. Round numbers in the decimal system are those that are multiples of powers of 10 or of half or a quarter of a power of 10. Roundness is a gradient concept, e.g. 100 is rounder than 50 or 200. We show that very round number like 100 and 1000 are used with 25% greater likelihood by familial sinistrals than by pure familial dextrals, while pure familial dextrals are more likely to use less round numbers such as 25, 60, and 200. We then use Sigurd’s (1988, Language in Society) index of the roundness of a number and report that familial sinistrals’ responses are significantly rounder on average than those of pure familial dextrals. To explain the difference, we propose that the cognitive effort of using exact numbers is greater for the familial sinistral group because their language and number systems tend to be more distributed over both hemispheres of the brain. Our data support the view that exact and approximate quantities are processed by two separate cognitive systems. Specifically, our behavioral data corroborates the view that the evolutionarily older, approximate number system is present in both hemispheres of the brain, while the exact number system tends to be localized in only one hemisphere.     

The developmental genetics of dextrality and sinistrality in the gastropodLymnaea peregra

Summary The genetics of body asymmetry inLymnaea peregra follows a maternal mode of inheritance involving a single locus with dextrality being dominant to sinistrality. Maternal inheritance implies that all members of a brood have the same phenotype, however, some broods contain a few individuals of opposite coil. One purpose of this paper is to explain the origin of these anomalous individuals. Genetic analyses of sinistral broods with a few dextral individuals have led to the development of a cross-over model, with the anomalous dextrals originating as a consequence of crossing over either during meiosis or mitosis in the female germ line. The crossover either reconstitutes the dextral gene from previously dissociated parts, or creates a dextral gene by means of a position effect. The probability of a crossover event depends upon the appropriate combination of complementary sinistral chromosomes. Each crossover event has the potential of creating a unique dextral gene. Genetic analyses of dextral broods containing a few sinistral individuals have demonstrated that different dextral genes vary in penetrance.The dextral gene produces a product during oogenesis which influences the pattern of cleavage in the embryo; this cleavage pattern is translated into the appropriate body asymmetry. The other purpose of this paper is to provide an assay for this gene product. Cytoplasm from dextral eggs injected into uncleaved sinistral eggs causes these eggs to cleave in a dextral pattern. Cytoplasm from sinistral eggs has no effect on the cleavage pattern of dextral eggs. While the dextral gene product is made during oogenesis, it does not function in controlling cleavage until just before this process begins.     

Molecular phylogeny of the land snail genus Alopia (Gastropoda: Clausiliidae) reveals multiple inversions of chirality

Whereas the vast majority of gastropods possess dextral shell and body organization, members of the Clausiliidae family are almost exclusively sinistral. Within this group a unique feature of the alpine genus Alopia is the comparable representation of sinistral and dextral taxa, and the existence of enantiomorph taxon pairs that appear to differ only in their chirality. We carried out a molecular phylogenetic study, using mitochondrial cytochrome c oxidase subunit I (COI) gene sequences, in order to find out whether chiral inversions are more frequent in this genus than in other genera of land snails. Our results revealed multiple independent inversions in the evolutionary history of Alopia and a close genetic relationship between members of the enantiomorph pairs. The inferred COI phylogeny also provided valuable clues for the taxonomic division and zoogeographical evaluation of Alopia species. The high number of inverse forms indicates unstable fixation of the coiling direction. This deficiency and the availability of enantiomorph pairs may make Alopia species attractive experimental models for genetic studies aimed at elucidating the molecular basis of chiral stability. © 2013 The Linnean Society of London     

Phylogenetic reconstruction and shell evolution of the Diplommatinidae (Gastropoda: Caenogastropoda)

The fascinating and often unlikely shell shapes in the terrestrial micromollusc family Diplommatinidae (Gastropoda: Caenogastropoda) provide a particularly attractive set of multiple morphological traits to investigate evolutionary patterns of shape variation. Here, a molecular phylogenetic reconstruction, based on five genes and 2700 bp, was undertaken for this family, integrated with ancestral state reconstruction and phylogenetic PCA of discrete and quantitative traits, respectively. We found strong support for the Diplommatininae as a monophyletic group, separating the Cochlostomatidae into a separate family. Five main clades appear within the Diplommatininae, corresponding with both coiling direction and biogeographic patterns. A Belau clade (A) with highly diverse (but always sinistral) morphology comprised Hungerfordia, Palaina, and some Diplommatina. Arinia (dextral) and Opisthostoma (sinistroid) are sister groups in clade B. Clade C and D solely contain sinistral Diplommatina that are robust and little ornamented (clade C) or slender and sculptured (clade D). Clade E is dextral but biogeographically diverse with species from all sampled regions save the Caroline Islands. Adelopoma, Diplommatina, Palaina, and Hungerfordia require revision to allow taxonomy to reflect phylogeny, whereas Opisthostoma is clearly monophyletic. Ancestral state reconstruction suggests a sinistral origin for the Diplommatinidae, with three reversals to dextrality.     

Dextral and sinistral <Emphasis Type="Italic">Amphidromus inversus</Emphasis> (Gastropoda: Pulmonata: Camaenidae) produce dextral sperm

Coiling direction in pulmonate gastropods is determined by a single gene via a maternal effect, which causes cytoskeletal dynamics in the early embryo of dextral gastropods to be the mirror image of the same in sinistral ones. We note that pulmonate gastropod spermatids also go through a helical twisting during their maturation. Moreover, we suspect that the coiling direction of the helical elements of the spermatozoa may affect their behaviour in the female reproductive tract, giving rise to the possibility that sperm chirality plays a role in the maintenance of whole-body chiral dimorphism in the tropical arboreal gastropod Amphidromus inversus (Müller, 1774). For these reasons, we investigated whether there is a relationship between a gastropod’s body chirality and the chirality of the spermatozoa it produces. We found that spermatozoa in A. inversus are always dextrally coiled, regardless of the coiling direction of the animal itself. However, a partial review of the literature on sperm morphology in the Pulmonata revealed that chiral dimorphism does exist in certain species, apparently without any relationship with the coiling direction of the body. Though our study shows that body and sperm chirality follows independent developmental pathways, it gives rise to several questions that may be relevant to the understanding of the chirality of spermatid ultrastructure and spermatozoan motility and sexual selection.     

Coiling dimorphism within a genetic type of the planktonic foraminifer Globorotalia truncatulinoides

The coiling direction—sinistral (left-coiled) or dextral (right-coiled)—of planktonic foraminiferal shells is a classical proxy used to assess past environmental changes and to understand their evolutionary patterns. Globorotalia truncatulinoides is composed of five different genetic types (I to V), each with a specific biogeographic distribution. So far, type II is the only type within which both coiling types have been frequently found. Here we examine the coiling direction as a dimorphic variation in a single genetic type, and show that the two forms with different coiling directions differ in ecology. The studied 140 left- and 137 right-coiled specimens from eight depth layers at four stations in the Sargasso Sea all belonged to Type II, based on the phylogenetic analyses of the ITS (internal transcribed spacer) region of ribosomal DNA. The distributions of left- and right-coiled Type II (Type II-L and II-R) differed strongly: Type II-L dominated between 400 and 200 m depth in the central water mass, whereas Type II-R was common in the top 200 m of the subtropical gyre. The left- and right-coiled forms are thus associated with different water masses. However, the vertical distribution along a temperature gradient indicates that their coiling direction is not determined by temperature, and might have some genetic basis. Our molecular phylogenetic analyses showed that the two forms probably share the same gene pool through their dispersals between different water masses. Moreover, the lineage of G. truncatulinoides is young (< 3 Ma), which might not be long enough to evolve into two lineages with opposite coiling direction. We discuss the practical and theoretical usefulness of a simple genetic model for coil dimorphism, in order to build testable hypotheses regarding the evolutionary history of planktonic foraminifera at the population level.     

Response of hermit crabs to sinistral shells

Shell rotating behavior of the hermit crabPagurus geminus was investigated. In preliminary observations, hermit crabs motivated to change shells rotated presented shells, filled with sand, in a way that dislodged the inside material. In order to determine if this behavior is stereotyped, or flexible and dependent on shell type, hermit crabs were tested with ordinary dextral shells ofLatirulus nagasakiensis and sinistral shells ofAntiplanes contraria. Sinistral shells are not normally encountered by hermit crabs. Their rotation of the dextral shell to the left was adequate for sand discharge. Sinistral shells were rotated in both directions. Analysis of recorded videotapes showed that variation in rotation direction could be attributed to variation in the position of the crab relative to the shell. When the crab faced the shell aperture from the inner lip, it rotated the sinistral shell to the right, and to opposite direction when it faced from the outer lip side. The crab always pushed the upper side of the horizontally laid shell, regardless of shell type or its own position.     

Curious chiral cases of caddisfly larvae: handed behavior, asymmetric forms, evolutionary history

Studies of right-left asymmetries have yielded valuable insights into the mechanisms of both development and evolution. Larvae from several groups of caddisflies (Trichoptera) build portable asymmetrical cases within which they live. In nearly all species that build spiral-walled tubular cases, the direction of wall coiling is random (equal numbers of dextral and sinistral cases within species) whereas in all species that build helicospiral, snail-like cases the direction of coiling is exclusively dextral. Asymmetrical tubes result from handed behavior, and ～20% of larvae removed from a spiral-walled, tubular case build a replacement case of opposite chirality. So handed behavior (and hence direction of tube-wall spiraling) is likely learned rather than determined genetically. Asymmetrical larval cases appear to have evolved at least seven times in the Trichoptera, five times as spiral-walled tubes and twice as snail-like helicospiral cases. Helicospiral cases may reduce vulnerability to predation by mimicking snail shells, whereas spiral arrangements of vegetation fragments in tube walls may be more robust mechanically than other arrangements, but experimental evidence is lacking. Within one family (Phryganeidae), one or perhaps two species exhibit an excess of sinistral-walled cases, suggesting that genes that bias handed behavior in a particular direction evolved after handed behaviors already existed (genetic assimilation).     

Postradiation changes in the brain asymmetry and higher mental functions of right- and left-handed subjects (the sequelae of the accident at the Chernobyl Atomic Electric Power Station)

The present study was aimed at comparative electrophysiological, clinical, and neurophysiological assessment of the brain functional state in dextrals (50 men) and sinistrals (5 men), who participated in the Chernobyl clean. The patients were observed over the course of 10 years (from 1990 to 1999). The results of examination of healthy persons (20 dextrals and 10 sinistrals) were used as a control. The clinical examination revealed the earlier manifestation and more severe development of paroxysmal and epileptic seizures in sinistrals than in dextrals. Electrophysiological study showed a progressive decrease in interhemispheric asymmetry of the EEG coherence characteristic for a healthy brain to inversion of its sign. These changes were more pronounced in sinistrals. In the remote terms after radiation, the interhemispheric EEG coherence decreased over the whole cortex, especially, in the frontal and central areas in both groups of patients. Neurophysiological study revealed a progressive impairment of voluntary motor activity and tactile sensibility, especially, in the left hand. These defects were more expressed in sinistrals than in dextrals. The results of complex and longitudinal examination suggest that the observed changes in brain asymmetry and interhemispheric interaction can be not only a result of a dysfunction of subcortical limbic-reticular and mediobasal brain structures but also a result of the white matter damage including corpus callosum. More expressed impairments in sinistrals than in dextrals can be explained by specific morphofunctional organization of the brain in persons with different sensor and motor asymmetries.     

The effects of shell size and coil orientation on reproduction in female hermit crabs, Clibanarius vittatus

We investigated the effects of shell coil orientation and shell size on reproduction in field populations of the hermit crab, Clibanarius vittatus. Females were collected in the intertidal in Beaufort, NC. Shell parameters were measured and size (cephalothorax length) and reproductive status were determined for 70 females occupying Busycon shells. Crabs were categorized as berried (eggs on the pleopods), mature ovaries, or non-reproductive (no eggs). For berried females, the number of eggs was recorded. By offering a separate group of females access to empty shells, it was possible to calculate optimal shell size and the deficit in shell size for field-collected animals.Females that were berried were in shells closer to the optimal shell size than females with mature ovaries, both for shell weight and shell volume. And females with mature ovaries were in shells that were closer to the optimal size than females that were non-reproductive. For both categories of females without eggs on the pleopods, the majority of females were in shells that were too big (in weight and internal volume). While the percentage of berried females did not differ between dextral (Busycon carica) and sinistral (Busycon sinistrum) shells, the non-reproductive females had a much smaller deficit in volume in sinistral shells compared to dextral shells. For berried females, there was no relationship between the magnitude of their shell deficit and the number of eggs carried. Our results suggest that reproduction is inhibited when females occupy shells sufficiently greater than the optimal shell size.     

High gene flow despite opposite chirality in hybrid zones between enantiomorphic door snails

We studied differentiation and geneflow patterns between enantiomorphic door‐snail species in two hybrid zones in the Bucegi Mountains (Romania) to investigate the effects of intrinsic barriers (complications in copulation) and extrinsic selection by environmental factors. A mitochondrial gene tree confirmed the historical separation of the examined populations into the dextral Alopia livida and the sinistral Alopia straminicollis in accordance with the morphological classification, but also indicated gene flow between the species. By contrast, a network based on amplified fragment length polymorphisms (AFLP) markers revealed local groups of populations as units independent of their species affiliation. Admixture analyses based on AFLP data showed that the genomes of most individuals in the hybrid zones are composed of parts of the genomes of both parental taxa. The introgression patterns of a notable fraction of the examined markers deviated from neutral introgression. However, the patterns of most non‐neutral markers were not concordant between the two hybrid zones. There was also no concordance between non‐neutral markers in the two genomic clines and markers that were correlated with environmental variables or markers that were correlated with the proportion of dextral individuals in the populations. Neither extrinsic selection by environmental factors nor intrinsic barriers resulting from positive frequency‐dependent selection of the prevailing coiling direction were sufficient to maintain the distinctness of A. straminicollis and A. livida. Despite being historically separated units, we conclude that these taxa now merge where they come into contact.     

Fine Mapping of the Pond Snail Left-Right Asymmetry (Chirality) Locus Using RAD-Seq and Fibre-FISH

The left-right asymmetry of snails, including the direction of shell coiling, is determined by the delayed effect of a maternal gene on the chiral twist that takes place during early embryonic cell divisions. Yet, despite being a well-established classical problem, the identity of the gene and the means by which left-right asymmetry is established in snails remain unknown. We here demonstrate the power of new genomic approaches for identification of the chirality gene, “D”. First, heterozygous (Dd) pond snails Lymnaea stagnalis were self-fertilised or backcrossed, and the genotype of more than six thousand offspring inferred, either dextral (DD/Dd) or sinistral (dd). Then, twenty of the offspring were used for Restriction-site-Associated DNA Sequencing (RAD-Seq) to identify anonymous molecular markers that are linked to the chirality locus. A local genetic map was constructed by genotyping three flanking markers in over three thousand snails. The three markers lie either side of the chirality locus, with one very tightly linked (<0.1 cM). Finally, bacterial artificial chromosomes (BACs) were isolated that contained the three loci. Fluorescent in situ hybridization (FISH) of pachytene cells showed that the three BACs tightly cluster on the same bivalent chromosome. Fibre-FISH identified a region of greater that ∼0.4 Mb between two BAC clone markers that must contain D. This work therefore establishes the resources for molecular identification of the chirality gene and the variation that underpins sinistral and dextral coiling. More generally, the results also show that combining genomic technologies, such as RAD-Seq and high resolution FISH, is a robust approach for mapping key loci in non-model systems.     

Possible functions of Dpp in gastropod shell formation and shell coiling

We examined dpp expression patterns in the pulmonate snail Lymnaea stagnalis and analyzed the functions of dpp using the Dpp signal inhibitor dorsomorphin in order to understand developmental mechanisms and evolution of shell formation in gastropods. The dpp gene is expressed in the right half of the circular area around the shell gland at the trochophore stage and at the right-hand side of the mantle at the veliger stage in the dextral snails. Two types of shell malformations were observed when the Dpp signals were inhibited by dorsomorphin. When the embryos were treated with dorsomorphin at the 2-cell and blastula stages before the shell gland is formed, the juvenile shells grew imperfectly and were not mineralized. On the other hand, when treated at the trochophore and veliger stage after the shell gland formation, juvenile shells grew to show a cone-like form rather than a normal coiled form. These results indicated that dpp plays important roles in the formation and coiling of the shell in this gastropod species.