Shell color polymorphism in marine gastropods

Recognition of Conus species using a combined approach of supervised learning and deep learning-based feature extraction

Cone snails are venomous marine gastropods comprising more than 950 species widely distributed across different habitats. Their conical shells are remarkably similar to those of other invertebrates in terms of color, pattern, and size. For these reasons, assigning taxonomic signatures to cone snail shells is a challenging task. In this report, we propose an ensemble learning strategy based on the combination of Random Forest (RF) and XGBoost (XGB) methods. We used 47,600 cone shell images of uniform size (224 x 224 pixels), which were split into an 80:20 train-test ratio. Prior to performing subsequent operations, these images were subjected to pre-processing and transformation. After applying a deep learning approach (Visual Geometry Group with a 16-layer deep model architecture) for feature extraction, model specificity was further assessed by including multiple related and unrelated seashell images. Both classifiers demonstrated comparable recognition ability on random test samples. The evaluation results suggested that RF outperformed XGB due to its high accuracy in recognizing Conus species, with an average precision of 95.78%. The area under the receiver operating characteristic curve was 0.99, indicating the model's optimal performance. The learning and validation curves also demonstrated a robust fit, with the training score reaching 1 and the validation score gradually increasing to 95 as more data was provided. These values indicate a well-trained model that generalizes effectively to validation data without significant overfitting. The gradual improvement in the validation score curve is crucial for ensuring model reliability and minimizing the risk of overfitting. Our findings revealed an interactive visualization. The performance of our proposed model suggests its potential for use with datasets of other mollusks, and optimal results may be achieved for their categorization and taxonomical characterization.

A sex chromosome polymorphism maintains divergent plumage phenotypes between extensively hybridizing yellowhammers (Emberiza citrinella) and pine buntings (E. leucocephalos)

Under allopatric speciation, populations of a species become isolated by a geographic barrier and develop reproductive isolation through genetic differentiation. When populations meet in secondary contact, the strength of evolved reproductive barriers determines the extent of hybridization and whether the populations will continue to diverge or merge together. The yellowhammer (Emberiza citrinella) and pine bunting (E. leucocephalos) are avian sister species that diverged in allopatry on either side of Eurasia during the Pleistocene glaciations. Though they differ greatly in plumage and form distinct genetic clusters in allopatry, these taxa show negligible mitochondrial DNA differentiation and hybridize extensively where they overlap in central Siberia, lending uncertainty to the state of reproductive isolation in the system. To assess the strength of reproductive barriers between taxa, we examined genomic differentiation across the system. We found that extensive admixture has occurred in sympatry, indicating that reproductive barriers between taxa are weak. We also identified a putative Z chromosome inversion region that underlies plumage variation in the system, with the 'pine bunting' haplotype showing dominance over the 'yellowhammer' haplotype. Our results suggest that yellowhammers and pine buntings are currently at a crossroads and that evolutionary forces may push this system towards either continued differentiation or population merging. However, even if these taxa merge, recombination suppression between putative chromosome Z inversion haplotypes may maintain divergent plumage phenotypes within the system. In this way, our findings highlight the important role hybridization plays in increasing the genetic and phenotypic variation as well as the evolvability of a system.

Temporal stability and directional change in a color cline of a marine snail from NW Spain

The evolution and maintenance of color clines is a classic topic of research in evolutionary ecology. However, studies analyzing the temporal dynamics of such clines are much less frequent, due to the difficulty of obtaining reliable data about past color distributions along environmental gradients. In this article, we describe a case of decades-long temporal stability and directional change in a color cline of the marine snail Littorina saxatilis along the coastal inlet of the Ría de Vigo (NW Spain). L. saxatilis from this area shows a clear color cline with 3 distinct areas from the innermost to the more wave-exposed localities of the Ría: the inner, protected localities show an abundance of fawn-like individuals; the intermediate localities show a high diversity of colors; and the outer, wave-exposed localities show populations with a high frequency of a black and lineated morph. We compare data from the 1970s and 2022 in the same localities, showing that the cline has kept relatively stable for at least over half a century, except for some directional change and local variability in the frequency of certain morphs. Multiple regression analyses and biodiversity measures are presented to provide clues into the selective pressures that might be involved in the maintenance of this color cline. Future research avenues to properly test the explanatory power of these selective agents as well as the possible origins of the cline are discussed.

Dominant gingers - Discovery and inheritance of a new shell polymorphism in the great pond snail Lymnaea stagnalis

Color polymorphism is a classic study system for evolutionary genetics. One of the most color-polymorphic animal taxa is mollusks, but the investigation of the genetic basis of color determination is often hindered by their life history and the limited availability of genetic resources. Here, we report on the discovery of shell color polymorphism in a much-used model species, the great pond snail Lymnaea stagnalis. While their shell is usually beige, some individuals from a Greek population show a distinct red shell color, which we nicknamed Ginger. Moreover, we found that the inheritance fits simple, single-locus Mendelian inheritance with dominance of the Ginger allele. We also compared crucial life-history traits between Ginger and wild-type individuals, and found no differences between morphs. We conclude that the relative simplicity of this polymorphism will provide new opportunities for a deeper understanding of the genetic basis of shell color polymorphism and its evolutionary origin.

Effects of heating rate and shell colour on the cardiac thermal performance in a polymorphic gastropod Batillaria attramentaria

Heating rate has gained extensive attention in mechanistic understanding of physiological responses to changing thermal conditions in the context of climate change. In polymorphic gastropods, differences in the absorption of solar energy between dark- and light-coloured individuals lead to supposable differences in their heating rates and body temperatures in sunshine. In the present study, we examined the effect of heating rate on heart rate (HR) in a polymorphic gastropod Batillaria attramentaria. By using biomimetic models, we found that daily maximum temperature of snails with a dark unbanded shell (D-type morph) was higher than snails with a white line on the upper side of each whorl (UL-type morph) by 0.6 °C when exposed to sunlight, but there was no apparent difference in heating rates between D- and UL-type models. We measured HR of snails at various heating rates from 3.0 to 9.0 °C h^-1. Faster heating rates significantly increased maximum thermal tolerance in both D- and UL-type snails, highlighting the importance to have thorough knowledge on the heating rate in the field to obtain accurate maximum thermal limit of gastropods. Critical temperature at which HR precipitously declines was higher in D-type snails than UL-type snails. Our results suggested that the impacts of heating rate as well as the shell colour should be considered to gain a mechanistic understanding of the population dynamics of polymorphic gastropods.

A decade of progress in marine evolutionary biology

This article summarizes the Evolutionary Applications Special Issue, "A decade of progress in Marine Evolutionary Biology." The globally connected ocean, from its pelagic depths to its highly varied coastlines, inspired Charles Darwin to develop the theory of evolution during the voyage of the Beagle. As technology has developed, there has been a dramatic increase in our knowledge about life on our blue planet. This Special Issue, composed of 19 original papers and seven reviews, represents a small contribution to the larger picture of recent research in evolutionary biology, and how such advancements come about through the connection of researchers, their fields, and their knowledge. The first European network for marine evolutionary biology, the Linnaeus Centre for Marine Evolutionary Biology (CeMEB), was developed to study evolutionary processes in the marine environment under global change. Though hosted by the University of Gothenburg in Sweden, the network quickly grew to encompass researchers throughout Europe and beyond. Today, more than a decade after its foundation, CeMEB's focus on the evolutionary consequences of global change is more relevant than ever, and knowledge gained from marine evolution research is urgently needed in management and conservation. This Special Issue, organized and developed through the CeMEB network, contains contributions from all over the world and provides a snapshot of the current state of the field, thus forming an important basis for future research directions.