Comparative study of thermal reaction norms for development in ants

This is the first compilation of our research and data from the literature on the duration and thermal reaction norms for development in ants. Altogether, 97 regression lines characterizing the linear dependence of ant brood (egg, larval, prepupal and pupal) development on temperature were obtained for 33 species from 15 genera and three subfamilies. Significant positive correlations between the durations of different immature stages were revealed. We found differences between thermal reaction norms for development for various immature stages, some taxonomic groups, and southern and northern groups of species. All immature stages appeared to be shorter on average at 25°C in northern ants compared to southern species; this difference is insignificant only for eggs. Highly significant negative correlations were revealed between the temperature threshold for development (TTD) and the sum of degree‐days (SDD) for all immature stages. The latitudinal trends in intraspecific variation of thermal constants appeared to be opposite to those we observed at the interspecific level. At the latter, the coefficient of thermal sensitivity of development (i.e. the coefficient of linear regression of development rate on temperature) and TTD tends to decrease and SDD to increase from the south to the north. In contrast, at the intraspecific level, development became more temperature‐sensitive in northern populations, i.e. characterized by higher slopes of regression lines of development rate on temperature, and higher TTDs. The species of the genus Formica are characterized by the shortest and the most temperature‐sensitive immature development among all the ant species studied.     

Latitudinal variation in thermal reaction norms of post‐winter pupal development in two butterflies differing in phenological specialization

Latitudinal clines in thermal reaction norms of development are a common phenomenon in temperate insects. Populations from higher latitudes often develop faster throughout the range of relevant temperatures (i.e countergradient variation) because they must be able to complete their life cycle within a shorter seasonal time window compared to populations at lower latitudes. In the present study, we experimentally demonstrate that two species of butterflies Anthocharis cardamines (L.) and Pieris napi (L.) instead show a cogradient variation in thermal reaction norms of post‐winter pupal development so that lower latitude populations develop faster than higher latitude populations. The two species share host plants but differ in the degree of phenological specialization, as well as in the patterns of voltinism. We suggest that the pattern in A. cardamines, a univoltine phenological specialist feeding exclusively on flowers and seedpods, is the result of selection for matching to the phenological pattern of its local host plants. The other species, P. napi, is a phenological generalist feeding on the leaves of the hosts and it shows a latitudinal cline in voltinism. Because the latitudinal pattern in P. napi was an effect of slow development in a fraction of the pupae from the most northern population, we hypothesize that this population may include both bivoltine and univoltine genotypes. Consequently, although the two species both showed cogradient patterns in thermal reaction norms, it appears likely that this was for different reasons. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 981–991.     

Interspecific and intraspecific variation of the duration and thermal requirements for egg development in carabid beetles (Coleoptera,Carabidae) in the North-West of Russia

The thermal norms of egg development were studied in the ground beetles Amara communis, A. nitida, Carabus granulatus, Platynus assimilis, Poecilus versicolor, Pterostichus oblongopunctatus (spring breeding species), P. melanarius, and P. niger (autumn breeding species). The adults were collected in soil traps near Arkhangelsk, St. Petersburg, Moscow, and Bryansk in May–June. Females and males were kept in pairs at 20°C and 22 h light per day. The eggs laid by females were kept at constant temperatures (12, 14, 16, 18, 20, and 22 ± 0.1°C). The development time for each egg was determined accurate to 0.5 day. From the values of the individual development rate (a reciprocal of development time) at all the temperatures, the thermal constants for development were calculated: the coefficient of linear regression (CLR) of the development rate on temperature, the sum of degree-days, and the thermal threshold for development (TTD). The egg development time was found to vary significantly between the species, except for A. communis, A. nitida, and C. granulatus. The values of CLR (i.e., the slopes of the regression lines) and TTD varied rather distinctly between the species and populations that revealed differences in the thermal requirements for egg development. In the spring breeding species the mean egg development time was significantly shorter, and the CLR and TTD values were on average higher than those of the autumn breeding species. Intraspecific latitudinal variation of the development time and the thermal requirements for egg development was revealed. The eggs laid by beetles from the Arkhangelsk population developed faster than those of the same species from the southern populations at all experimental temperatures. These differences were not great but statistically significant. At the same time, the differences between the CLR and TTD values for the northern and southern populations of the same species were non-significant in most cases. Thus, the main ecophysiological adaptation of carabid beetles during their northward expansion was the shortening of development time within the entire range of favorable temperatures, while the CLR and TTD values only insignificantly changed.     

Photoperiod-temperature interaction-a new form of seasonal control of growth and development in insects and in particular a Carabid Beetle, <Emphasis Type="Italic">Amara communis</Emphasis> (Coleoptera: Carabidae)

Amara communis larvae were found to develop significantly faster and to have higher growth rate at short-day (12 h) as compared to long-day (22 h) photoperiods at all used temperatures (16, 18, 20, and 22°C). The coefficient of linear regression of larval development rate on temperature was significantly higher at the short day than at the long day. The thermal developmental thresholds appeared similar at both photoperiods. Body weight of young beetles reared under different photoperiods was almost the same. Thus, photoperiod does not simply accelerate or decelerate insect development, but modifies the thermal reaction norm. At short days, larval development becomes faster and more temperature-dependent, which provides a timely completion of development at the end of summer. The analysis of literature data has allowed us to find the photoperiodic modification of thermal requirements for development in 5 insect orders: Orthoptera, Heteroptera, Coleoptera, Lepidoptera, and Diptera. Modification may result in significant changes in the slope of the regression line, and hence the sum of degree-days, and in the thermal developmental threshold. Consequently, the thermal requirements for development in many insects gradually vary during summer under the effect of changing day-length, which may have adaptive significance. Thus, the photoperiodic modification of thermal reaction norms acts as a specific form of seasonal control of insect development.     

Patterns of intraspecific variation through ontogeny: a case study of the Cretaceous nautilid Eutrephoceras dekayi and modern Nautilus pompilius

The magnitude and ontogenetic patterns of intraspecific variation can provide important insights into the evolution and development of organisms. Understanding the intraspecific variation of organisms is also a key to correctly pursuing studies in major fields of palaeontology. However, intraspecific variation has been largely overlooked in ectocochleate cephalopods, particularly nautilids. Furthermore, little is known regarding the evolutionary pattern. Here, we present morphological data for the Cretaceous nautilid Eutrephoceras dekayi (Morton) and the modern nautilid Nautilus pompilius Linnaeus through ontogeny. The data are used to describe conch morphology and to elucidate the evolutionary patterns of intraspecific variation. We discovered a similar overall pattern of growth trajectories and the presence of morphological changes at hatching and maturity in both taxa. We also found that intraspecific variation is higher in earlier ontogeny than in later ontogeny in both taxa. The high variation in earlier ontogeny may imply increased flexibility in changing the timing of developmental events, which probably played an important role in nautilid evolution. We assume that the decrease in variation in later ontogeny reflects developmental constraints. Lastly, we compared the similarity/dissimilarity of ontogenetic patterns of variation between taxa. Results reveal that the similarity/dissimilarity of the ontogenetic pattern differs between E. dekayi and N. pompilius. We conclude that this shift in the ontogenetic pattern of variation may be rooted in changes in the developmental programme of nautilids through time. We propose that studying ontogenetic patterns of intraspecific variation can provide new insights into the evolution and development of organisms.     

Temperature-dependent plasticity of segment number in an arthropod species: the centipede Strigamia maritima

SUMMARY The evolution of arthropod segment number provides us with a paradox, because, whereas there is more than 20‐fold variation in this character overall, most classes and orders of arthropods are composed of species that lack any variation in the number of segments. So, what is the origin of the higher‐level variation? The centipede order Geophilomorpha is unusual because, with the exception of one of its families, all species exhibit intraspecific variation in segment number. Hence it provides an opportunity to investigate how segment number may change in a microevolutionary context. Here, we show that segment number can be directly altered by an environmental factor (temperature)—this is the first such demonstration for any arthropod. The direction of the effect is such that higher temperature during embryogenesis produces more segments. This potentially explains an intraspecific cline in the species concerned, Strigamia maritima, but it does not explain how such a cline is translated into the parallel interspecific pattern of lower‐latitude species having more segments. Given the plastic nature of the intraspecific variation, its link with interspecific differences may lie in selection acting on developmental reaction norms.     

Fertilization competence and sperm size variation in sperm-heteromorphic insects

Between species, variation in sperm size has been related to male–female coevolution and male–male competition. In contrast, variation within species is poorly understood. A particular case of intraspecific sperm-size variation occurs in sperm-heteromorphic species, where males produce distinct sperm morphotypes, usually only one of which is fertile. This allows to investigate sperm size variation under different selection regimes. Nonfertile morphotypes, whose role is aside from fertilization, may have other functions, and this may be reflected by changes in developmental processes and a different phenotype compared to fertile sperm. We show that the intraspecific coefficient of variation in sperm length is up to four times lower for fertile than nonfertile morphotypes across 150 sperm-heteromorphic species (70 butterfly, 71 moth, 9 diopsid fly species). This is in agreement with a previous study on 11 species in the Drosophila obscura group. Significantly lower variation in fertile than nonfertile sperm morphometry may result from fertilization-related selection for optimal sperm size, novel functions of nonfertile sperm, or from tighter control of fertile sperm development. More data are needed to clarify the consequences and adaptive significance of within-morph variation, and its consistent pattern across sperm-heteromorphic insects.Co-ordinating editor: Hurst     

Thermal plasticity of growth and development varies adaptively among alternative developmental pathways

Polyphenism, the expression of discrete alternative phenotypes, is often a consequence of a developmental switch. Physiological changes induced by a developmental switch potentially affect reaction norms, but the evolution and existence of alternative reaction norms remains poorly understood. Here, we demonstrate that, in the butterfly Pieris napi (Lepidoptera: Pieridae), thermal reaction norms of several life history traits vary adaptively among switch‐induced alternative developmental pathways of diapause and direct development. The switch was affected both by photoperiod and temperature, ambient temperature during late development having the potential to override earlier photoperiodic cues. Directly developing larvae had higher development and growth rates than diapausing ones across the studied thermal gradient. Reaction norm shapes also differed between the alternative developmental pathways, indicating pathway‐specific selection on thermal sensitivity. Relative mass increments decreased linearly with increasing temperature and were higher under direct development than diapause. Contrary to predictions, population phenology did not explain trait variation or thermal sensitivity, but our experimental design probably lacks power for finding subtle phenology effects. We demonstrate adaptive differentiation in thermal reaction norms among alternative phenotypes, and suggest that the consequences of an environmentally dependent developmental switch primarily drive the evolution of alternative thermal reaction norms in P. napi.     

Interspecific and intraspecific comparisons of ejaculates in the cotton bollworm <Emphasis Type="Italic">Helicoverpa armigera</Emphasis> and the tobacco budworm <Emphasis Type="Italic">H. assulta</Emphasis>

The evolution of reproductive isolation is a prerequisite in the formation of new species. Although there are numerous studies on ejaculates in lepidopteran insects, ejaculate comparisons among sibling species have not been adequately addressed to understand possible reproductive barriers to hybridization. Here, we examined the interspecific and intraspecific variations of ejaculates in the sibling noctuid moths Helicoverpa armigera and Helicoverpa assulta. We found that there were considerable variations in the number of apyrene and eupyrene sperm and the length of eupyrene sperm. Male pupal mass explained not only a significant proportion of the variation in apyrene sperm number in both H. armigera and H. assulta, but also a significant proportion of the variation in eupyrene sperm number in H. assulta. There was a significant positive relationship between the number of eupyrene sperm and the number of apyrene sperm in both species. No difference in the length of eupyrene sperm was found between them; however, ejaculates of H. armigera had many more eupyrene sperm than H. assulta had. In H. armigera, large males generally mated with large females. The evolutionary consequences of these differences are discussed in this paper.     

Thermal biology of flight in a butterfly: genotype,flight metabolism,and environmental conditions

Knowledge of the effects of thermal conditions on animal movement and dispersal is necessary for a mechanistic understanding of the consequences of climate change and habitat fragmentation. In particular, the flight of ectothermic insects such as small butterflies is greatly influenced by ambient temperature. Here, variation in body temperature during flight is investigated in an ecological model species, the Glanville fritillary butterfly (Melitaea cinxia). Attention is paid on the effects of flight metabolism, genotypes at candidate loci, and environmental conditions. Measurements were made under a natural range of conditions using infrared thermal imaging. Heating of flight muscles by flight metabolism has been presumed to be negligible in small butterflies. However, the results demonstrate that Glanville fritillary males with high flight metabolic rate maintain elevated body temperature better during flight than males with a low rate of flight metabolism. This effect is likely to have a significant influence on the dispersal performance and fitness of butterflies and demonstrates the possible importance of intraspecific physiological variation on dispersal in other similar ectothermic insects. The results also suggest that individuals having an advantage in low ambient temperatures can be susceptible to overheating at high temperatures. Further, tolerance of high temperatures may be important for flight performance, as indicated by an association of heat‐shock protein (Hsp70) genotype with flight metabolic rate and body temperature at takeoff. The dynamics of body temperature at flight and factors affecting it also differed significantly between female and male butterflies, indicating that thermal dynamics are governed by different mechanisms in the two sexes. This study contributes to knowledge about factors affecting intraspecific variation in dispersal‐related thermal performance in butterflies and other insects. Such information is needed for predictive models of the evolution of dispersal in the face of habitat fragmentation and climate change.     

The influence of diet on the duration and thermal sensitivity of development in the linden bug Pyrrhocoris apterus L. (Heteroptera: Pyrrhocoridae)

Temperature and nutrition are crucial environmental variables that determine rates of growth and development in insects. However, the simultaneous effect of these factors on life‐history traits is rarely addressed. In the present study, the influence of two diets (linden fruit and sunflower seeds) on the duration of immature stages and thermal reaction norms for development is tested in the bug Pyrrhocoris apterus L. (Heteroptera: Pyrrhocoridae). Eggs and larvae are reared at five constant temperatures (20, 22, 24, 26 and 28 °C) under an LD 20 : 4 h photocycle. Development rates deviate from linearity in the studied thermal range, especially in larvae; therefore, a nonlinear (power‐law) approximation is also attempted. Parental diet causes no change in thermal reaction norms for egg development. However, the progeny of sunflower‐fed bugs are more variable in terms of their development time, suggesting a transgenerational effect. Larval mortality rates increase in cooler conditions and are always higher on sunflower seeds. This is accompanied by more variable, less temperature‐dependent and generally slower larval development. A review of previously published case studies on temperature–diet interactions in the control of insect development leads to two general conclusions. First, there are two approaches for assessing the temperature‐dependent development in insects: one based on the concept of the sum of degree‐days and the other based on the concept of reaction norm. Despite an obvious non‐exclusiveness, the two approaches appear to have developed in isolation from each other. Second, three principal patterns of temperature–diet interactions can be recognized. The pattern found in P. apterus (the direct effects of diet are stronger at higher temperatures and much weaker or absent at lower temperatures) appears to be the most widespread.     

VARIATION IN THERMAL PERFORMANCE AND REACTION NORMS AMONG POPULATIONS OF DROSOPHILA MELANOGASTER

The major goal of evolutionary thermal biology is to understand how variation in temperature shapes phenotypic evolution. Comparing thermal reaction norms among populations from different thermal environments allows us to gain insights into the evolutionary mechanisms underlying thermal adaptation. Here, we have examined thermal adaptation in six wild populations of the fruit fly (Drosophila melanogaster) from markedly different natural environments by analyzing thermal reaction norms for fecundity, thorax length, wing area, and ovariole number under ecologically realistic fluctuating temperature regimes in the laboratory. Contrary to expectation, we found only minor differences in the thermal optima for fecundity among populations. Differentiation among populations was mainly due to differences in absolute (and partly also relative) thermal fecundity performance. Despite significant variation among populations in the absolute values of morphological traits, we observed only minor differentiation in their reaction norms. Overall, the thermal reaction norms for all traits examined were remarkably similar among different populations. Our results therefore suggest that thermal adaptation in D. melanogaster predominantly involves evolutionary changes in absolute trait values rather than in aspects of thermal reaction norms.     

Clonal variation in foliar chemistry of aspen: effects on gypsy moths and forest tent caterpillars

Quaking aspen (Populus tremuloides) exhibits striking intraspecific variation in concentrations of phenolic glycosides, compounds that play important roles in mediating interactions with herbivorous insects. This research was conducted to assess the contribution of genetic variation to overall phenotypic variation in aspen chemistry and interactions with gypsy moths (Lymantria dispar) and forest tent caterpillars (Malacosoma disstria). Thirteen aspen clones were propagated from field-collected root material. Insect performance assays, measuring survival, development, growth, and food utilization indices, were conducted with second and/or fourth instars. Leaf samples were assayed for water, nitrogen, total nonstructural carbohydrates, condensed tannins, and phenolic glycosides. Results showed substantial among-clone variation in the performance of both insect species. Chemical analyses revealed significant among-clone variation in all foliar constituents and that variation in allelochemical contents differed more than variation in primary metabolites. Regression analyses indicated that phenolic glycosides were the dominant factor responsible for among-clone variation in insect performance. We also found significant genetic trade-offs between growth and defense among aspen clones. Our results suggest that genetic factors are likely responsible for much of the tremendous phenotypic variation in secondary chemistry exhibited by aspen, and that the genetic structure of aspen populations may play important roles in the evolution of interactions with phytophagous insects. Received: 14 May 1996 / Accepted: 29 January 1997     

Stable and variable life‐history responses to temperature and photoperiod in the beet webworm,Loxostege sticticalis

The beet webworm, Loxostege sticticalis L. (Lepidoptera: Crambidae), unlike many temperate insects and despite its wide distribution range, has a geographically stable value of the critical photoperiod for diapause induction. It has thus been hypothesized that the species could adjust its life cycle to different climates in an alternative way, which should be reflected in geographical variation and/or environmental plasticity of some other ecophysiological trait. Three remote populations of the beet webworm were studied. The insects were reared from egg to adult at several combinations of temperature and photoperiod in order to measure development times of all the immature stages and pre‐pupal body weight, and to characterize the sensitivity of these life‐history traits to the two ecological factors. The thermal reaction norms for immature development appeared to be significantly different in the three populations. There was also a significant effect of photoperiod on development time as well as on the thermal sensitivity and lower temperature threshold for larval development. Pre‐pupae from the northernmost population were heavier and their body mass was more strongly affected by photoperiod than in the other two, but attainment of a greater weight under short‐day conditions, especially combined with higher temperatures, was common for all the three populations. Nevertheless, all the discovered geographical and environmentally induced differences in life‐history traits were very small and their adaptive significance remains problematic.     

Control of preimaginal development by photoperiod and temperature in the dock leaf beetle <Emphasis Type="Italic">Gastrophysa viridula</Emphasis> (De Geer) (Coleoptera,Chrysomelidae)

Development times of preimaginal stages and immature adult body mass of the dock leaf beetle Gastrophysa viridula from Bryansk (53°N, 34°E) were determined at six constant temperatures ranging from 16 to 26°C and at two photoperiods, 12L: 12D or 22L: 2D. The lower developmental threshold (LDT) and sum of degreedays (SDD) were calculated for eggs, larvae, and pupae; the values for the total preimaginal development were 7.9°C and 274.8 °C×d, respectively. The thermal reaction norms for preimaginal development were not affected by photoperiod, although the adult body mass was on average greater under short-day conditions. Our results confirm the previously published data on G. viridula concerning diapause induction by shorter daylengths, greater body mass under lower temperatures, and narrowing of the thermal optimum range during preimaginal development. The thermal reaction norms for larval development were shown to be the same in individuals that died during the pupal stage, and in those that successfully emerged as adults. Our results are more precise than the previous data on this species due to a larger sample size, careful control of rearing conditions, and the use of an alternative method for calculating LDT and SDD. In particular, a more accurate estimation of LDTs allowed us to demonstrate that rate isomorphy was violated in this species, contrary to what had been stated by Honěk and co-authors (2003).     

Morphological variation versus genetic divergence: a taxonomic implication for Mogannia species (Cicadidae: Cicadinae)

Species showing intraspecific morphological variation tend to be very difficult to identify using morphological characters. One such example is the cicada genus Mogannia where some species show considerable intraspecific variation mainly exhibited by wing pattern and body colouration. Thirty-one variants covering different putative species of Mogannia were recognized and illustrated in the present paper. Molecular data of mitochondrial COI and Cytb sequences were employed to test the level of variation and phylogeny of them. The existence of a ‘barcoding gap’ between intraspecific and interspecific genetic divergences and the reciprocally monophyletic clades indicate that all the closely related variants represent a single species, and that all these variants correspond to six species, respectively. However, the evolutionary relationships of intraspecific variants are not resolved possibly due to insufficient genetic variation among them. Our results indicated that some morphological characters, especially the wing pattern and body colouration, and even the number of apical processes of the aedeagus in a couple of related species, must be used with great caution in delimiting Mogannia species and their relatives. The factors responsible for intraspecific morphological variation and phylogeny of Mogannia spp. are preliminarily discussed.     

A procedure for calculating the thermal constants associated with the flight period of Cerambyx welensii Küster and Prinobius myardi Mulsant from field observations

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Use of intraspecific variation in thermal responses for estimating an elevational cline in the timing of cold hardening in a sub‐boreal conifer

To avoid winter frost damage, evergreen coniferous species develop cold hardiness with suitable phenology for the local climate regime. Along the elevational gradient, a genetic cline in autumn phenology is often recognised among coniferous populations, but further quantification of evolutionary adaptation related to the local environment and its responsible signals generating the phenological variation are poorly understood. We evaluated the timing of cold hardening among populations of Abies sachalinensis, based on time series freezing tests using trees derived from four seed source populations × three planting sites. Furthermore, we constructed a model to estimate the development of hardening from field temperatures and the intraspecific variations occurring during this process. An elevational cline was detected such that high‐elevation populations developed cold hardiness earlier than low‐elevation populations, representing significant genetic control. Because development occurred earlier at high‐elevation planting sites, the genetic trend across elevation overlapped with the environmental trend. Based on the trade‐off between later hardening to lengthen the active growth period and earlier hardening to avoid frost damage, this genetic cline would be adaptive to the local climate. Our modelling approach estimated intraspecific variation in two model components: the threshold temperature, which was the criterion for determining whether the trees accumulated the thermal value, and the chilling requirement for trees to achieve adequate cold hardiness. A higher threshold temperature and a lower chilling requirement could be responsible for the earlier phenology of the high‐elevation population. These thermal responses may be one of the important factors driving the elevation‐dependent adaptation of A. sachalinensis.     

Sexual size dimorphism within species increases with body size in insects

Studies examining interspecific differences in sexual size dimorphism (SSD) typically assume that the degree of sexual differences in body size is invariable within species. This work was conducted to assess validity of this assumption. As a result of a systematic literature survey, datasets for 158 insect species were retrieved. Each dataset contained adult or pupal weights of males and females for two or more different subsets, typically originating from different conditions during immature development. For each species, an analysis was conducted to examine dependence of SSD on body size, the latter variable being used as a proxy of environmental quality. A considerable variation in SSD was revealed at the intraspecific level in insects. The results suggest that environmental conditions may strongly affect the degree, though not the direction of SSD within species. In most species, female size appeared to be more sensitive to environmental conditions than male size: with conditions improving, there was a larger relative increase in female than male size. As a consequence, sexual differences in size were shown to increase with increasing body size in species with female-biased SSD (females were the larger sex in more than 80% of the species examined). The results were consistent across different insect orders and ecological subdivisions. Mechanisms leading to intraspecific variation in SSD are discussed. This study underlines the need to consider intraspecific variation in SSD in comparative studies.     

A three‐dimensional geometric morphometric study of the development of sulcus versus shell outline in Permian neospiriferine brachiopods

The morphological variation of the sulcal development and shell outline in large Permian neospiriferine brachiopods including Fasciculatia Waterhouse, 2004 is investigated using geometric morphometrics. The sulcal tongues of spiriferide brachiopods can be, in a qualitative sense, categorized into three types according to the degree of their development: short sulcal tongue, long sulcal tongue and geniculated sulcal tongue. All three types have been noted within Fasciculatia striatoparadoxa, regardless of the nature of the substrate which they originally inhabited. To quantify its morphological variation both in sulcal development and shell outline, 51 brachiopod shells were scanned with a three‐dimensional (3‐D) surface imaging device, and their 3‐D models were reconstructed. Using two landmarks and 58 semilandmarks designated on the surface of the reconstructed 3‐D models, a landmark‐based morphometric analysis was performed. Our result demonstrates a significant intraspecific variation of sulcal development in F. striatoparadoxa and its relatives. Local environmental factors, especially the intensity of ambient water flow, are invoked as the most likely cause for this intraspecific variation. Additionally, this study also shows that there are considerable interspecific distinctions in shell outline among Fasciculatia species, independent of the high variation in the sulcal development. The strong stability of overall shell outline at species level implies a decoupled morphological development between sulcal tongue and whole shell outline. The 3‐D morphometric approach applied here demonstrates its great utility as a tool for quantifying and analysing the morphological variation of highly convex brachiopod shells.