Eye-Size Variability in Deep-Sea Lanternfishes (Myctophidae): An Ecological and Phylogenetic Study

One of the most common visual adaptations seen in the mesopelagic zone (200–1000 m), where the amount of light diminishes exponentially with depth and where bioluminescent organisms predominate, is the enlargement of the eye and pupil area. However, it remains unclear how eye size is influenced by depth, other environmental conditions and phylogeny. In this study, we determine the factors influencing variability in eye size and assess whether this variability is explained by ecological differences in habitat and lifestyle within a family of mesopelagic fishes characterized by broad intra- and interspecific variance in depth range and luminous patterns. We focus our study on the lanternfish family (Myctophidae) and hypothesise that lanternfishes with a deeper distribution and/or a reduction of bioluminescent emissions have smaller eyes and that ecological factors rather than phylogenetic relationships will drive the evolution of the visual system. Eye diameter and standard length were measured in 237 individuals from 61 species of lanternfishes representing all the recognised tribes within the family in addition to compiling an ecological dataset including depth distribution during night and day and the location and sexual dimorphism of luminous organs. Hypotheses were tested by investigating the relationship between the relative size of the eye (corrected for body size) and variations in depth and/or patterns of luminous-organs using phylogenetic comparative analyses. Results show a great variability in relative eye size within the Myctophidae at all taxonomic levels (from subfamily to genus), suggesting that this character may have evolved several times. However, variability in eye size within the family could not be explained by any of our ecological variables (bioluminescence and depth patterns), and appears to be driven solely by phylogenetic relationships.     

Molecular phylogenetics of the Pectinidae (Mollusca: Bivalvia) and effect of increased taxon sampling and outgroup selection on tree topology

Evolutionary relationships of the Pectinidae were examined using two mitochondrial genes (12S rRNA, 16S rRNA) and one nuclear gene (Histone H3) for 46 species. Outgroup taxa from Propeamussidae, Spondylidae and Limidae were also sequenced to examine the impact of outgroup choice on pectinid topologies. Our phylogenetic analyses resolved the Pectinidae as monophyletic, but many of the subfamilies and tribes within the family do not form monophyletic clades. The paraphyletic Aequipectinini group is the most basal member of the Pectinidae, with the Chlamydinae and Palliolinae representing the most recently derived pectinid groups. These results are in contrast with the current morphological hypothesis of Pectinidae evolution, which suggests the Chlamydinae and Pallioline are basal groups within the Pectinidae. Ingroup topology was found to be sensitive to outgroup choice and increasing taxon sampling within the Pectinidae resulted in more robust phylogenies.     

Endosymbiosis in statu nascendi: close phylogenetic relationship between obligately endosymbiotic and obligately free-living Polynucleobacter strains (Betaproteobacteria)

Bacterial strains affiliated to the phylogenetically shallow subcluster C (PnecC) of the Polynucleobacter cluster, which is characterized by a minimal 16S rRNA gene sequence similarity of approximately 98.5%, have been reported to occur as obligate endosymbionts of ciliates (Euplotes spp.), as well as to occur as free-living cells in the pelagic zone of freshwater habitats. We investigated if these two groups of closely related bacteria represent strains fundamentally differing in lifestyle, or if they simply represent different stages of a facultative endosymbiotic lifestyle. The phylogenetic analysis of 16S rRNA gene and 16S-23S ITS sequences of five endosymbiont strains from two different Euplotes species and 40 pure culture strains demonstrated host-species-specific clustering of the endosymbiont sequences within the PnecC subcluster. The sequences of the endosymbionts showed characteristics indicating an obligate endosymbiotic lifestyle. Cultivation experiments revealed fundamental differences in physiological adaptations, and determination of the genome sizes indicated a slight size reduction in endosymbiotic strains. We conclude that the two groups of PnecC bacteria represent obligately free-living and obligately endosymbiotic strains, respectively, and do not represent different stages of the same complex life cycle. These closely related strains occupy completely separated ecological niches. To our best knowledge, this is the closest phylogenetic relationship between obligate endosymbionts and obligately free-living bacteria ever revealed.     

Phylogeny and spatio-temporal distribution of European Pectinidae (Mollusca: Bivalvia)

Phylogenetic relationships within Pectinidae (Bivalvia, Pteriomorphia) have been investigated primarily for Pacific and Western Atlantic or commercially valuable taxa. Most molecular phylogenetic studies have revealed monophyly of pectinid bivalves but interrelationships of the different clades are still inconsistent. However, non-commercial European Pectinidae has mostly been neglected in earlier investigations and therefore the evolution and radiation of the European Pectinidae is poorly understood. Since the fossil record of this group is well investigated, the evolutionary age of phylogenetic diversification and radiation events within this group can be dated. Thus, the connection of geological and climatic changes to radiation events within this group can be assessed. We investigated the phylogenetic relationships within European Pectinidae using mitochondrial (12S and 16S) and nuclear (18S, 28S and H3) gene markers and performed relaxed molecular clock approaches to gain information on the evolutionary age and the connection between Cenozoic climatic changes and diversification within this group. The results show concordance of radiation events with the Middle Miocene cooling event and the following climatic period with slowly decreasing temperatures. However, geological changes such as the uplift of the Gomphotherium Landbridge or the closure and re-opening of the Strait of Gibraltar also had great impact on diversification and distribution patterns within European Pectinidae.     

The first scallop

Scallops (Pectinidae) are a highly diverse bivalve family with a long evolutionary history, but insufficient knowledge on the internal shell characters of initial taxa has hampered clarification of their phylogenetic roots. Here, morphological details of the shell interior of the basal pectinid Pleuronectites laevigatus from the Middle Triassic are documented for the first time. It is shown that ligament morphology, lack of internal buttresses and hinge articulation, presence of a right anterior auricular scroll, procrescent discs, and differential valve convexity of Pleuronectites link Pectinidae with Early Triassic aviculopectinoids rather than with entoliids, as recently proposed. The key adaptations of Pectinidae, i.e. the ctenolium and the alivincular-alate ligament system, probably evolved in conjunction with a marked size increase that required improvements in the shell attachment and in the system for opening the valves. Although Pleuronectites laevigatus is recognized as the earliest known member of the Pectinidae, a diphyletic origin of this family from different stocks within the Aviculopectinoidea cannot be ruled out.     

Moringua edwardsi (Moringuidae: Anguilliformes): Cranial specialization for head‐first burrowing?

The order Anguilliformes forms a natural group of eel-like species. Moringua edwardsi (Moringuidae) is of special interest because of its peculiar fossorial lifestyle: this species burrows head-first. Externally pronounced morphological specializations for a fossorial lifestyle include: reduced eyes, lack of color, low or absent paired vertical fins, elongated, cylindrical body, reduced head pores of the lateral line system, etc. Many fossorial amphibians, reptiles, and even mammals have evolved similar external specializations related to burrowing. The present study focuses on osteological and myological features of M. edwardsi in order to evaluate the structural modifications that may have evolved as adaptations to burrowing. Convergent evolutionary structures and possible relations with head-first burrowing, miniaturization, feeding habits, etc., were investigated. Body elongation, reduction of the eyes, modified cranial lateral line system, and modified skull shape (pointed though firm) can be considered specializations for head-first burrowing. Hyperossification can probably be regarded more as a specialization to both head-first burrowing and feeding, even though an impact of miniaturization cannot be excluded. Hypertrophied adductor mandibulae muscles and the enlarged coronoid process can be associated with both feeding requirements (it enhances bite forces necessary for their predatory behavior) and with a burrowing lifestyle, as well as miniaturization.     

Chromosome rearrangements in Pectinidae (Bivalvia: Pteriomorphia) implied based on chromosomal localization of histone H3 gene in four scallops

Chromosomal structural rearrangement in four scallops, Chlamys farreri (n = 19), Patinopecten yessoensis (n = 19), Chlamys nobilis (n = 16) and Argopecten irradians (n = 16), was studied by fluorescence in situ hybridization using histone H3 gene probes. The results show that histone H3 gene sites differ strikingly with regard to number, location, and intensity among, or even within these species. For example, two histone H3 gene loci were detected on the metaphase chromosomes of P. yessoensis, while one locus was found in the others. In P. yessoensis, differing intensities of hybridization signals were detected between homologues 5 and 11, and within homologue 11. These data suggest that the histone H3 gene is a qualified chromosome marker for the preliminary understanding of the historical chromosomal reconstructing of the Pectinidae family. The variable distribution patterns of the histone H3 gene suggest that gene duplication/diminution as well as chromosome rearrangements by inversion and translocation may have played important roles in the genomic evolution of Pectinidae. We also compiled our present results with former published data regarding the chromosome mapping of rDNAs in species of the Pectinidae family. Such comparative chromosomal mapping should improve our understanding of historical chromosomal reconstructions of modern-day scallops.     

Repeated eye reduction events reveal multiple pathways to degeneration in a family of marine snails

Eye reduction occurs in many troglobitic, fossorial, and deep‐sea animals but there is no clear consensus on its evolutionary mechanism. Given the highly conserved and pleiotropic nature of many genes instrumental to eye development, degeneration might be expected to follow consistent evolutionary trajectories in closely related animals. We tested this in a comparative study of ocular anatomy in solariellid snails from deep and shallow marine habitats using morphological, histological, and tomographic techniques, contextualized phylogenetically. Of 67 species studied, 15 lack retinal pigmentation and at least seven have eyes enveloped by surrounding epithelium. Independent instances of reduction follow numerous different morphological trajectories. We estimate eye loss has evolved at least seven times within Solariellidae, in at least three different ways: characters such as pigmentation loss, obstruction of eye aperture, and “lens” degeneration can occur in any order. In one instance, two morphologically distinct reduction pathways appear within a single genus, Bathymophila. Even amongst closely related animals living at similar depths and presumably with similar selective pressures, the processes leading to eye loss have more evolutionary plasticity than previously realized. Although there is selective pressure driving eye reduction, it is clearly not morphologically or developmentally constrained as has been suggested by previous studies.     

Four new species of the myrmecophile Diplocotes Westwood (Coleoptera: Ptinidae) from Queensland and South Australia

Abstract  Four new species of the Australian ptinid genus Diplocotes are described. Two of these species are from the dry tropical and subtropical areas of northern and central Queensland, while the other two are from the arid areas of South Australia. While the four new species described here have not been observed with ants in the field, the species of this genus are known to be myrmecophilous, and similar interactions may occur between the new species and their host ants. Additionally, many characters are shared with other unrelated myrmecophilous species, and may be convergent adaptations to the ant-associated lifestyle.     

Molecular biogeography of cave life: a study using mitochondrial DNA from bathysciine beetles

This study focuses on phylogenetic relationships in two distinct species assemblages of cave-dwelling beetles with similar disjunct distributions in the Pyrenees and Sardinia. One assemblage contains six species in the genera Ovobathysciola (four species) and Patriziella (two species) on Sardinia and one species of Anillochlamys in the Pyrenees. Species within the two Sardinian genera co-occur in the same karst area. Although, they are believed to be each others closest relative, they have very different body types (globular body with short appendages in Ovobathysciola; elongated body with long appendages in Patriziella), which are believed to reflect different degrees of adaptation to cave life. The other assemblage of Bathysciine beetles includes three species in the genus Speonomus in the Pyrenees and one on Sardinia. All the species are rare and many are endangered. One issue of particular interest was whether Ovobathysciola and Patriziella are reciprocally monophyletic or whether each of the Patriziella species evolved independently from the co-occurring Ovobathysciola species, as the similar morphology of the Patriziella species might be due to convergence rather than common descent. Based on DNA sequences of the cytochrome oxidase I (COI) region of the mtDNA, neither scenario was supported. Rather, the two Patriziella species are sister taxa embedded within the Ovobathysciola radiation. In addition, the well-dated geological history of this region allowed us to calibrate absolute rates of COI evolution, the first such estimates for any insect. Finally this study suggests that the evolutionary acquisition of typical cave adaptations (e.g., elongated body and appendages) may occur at about the same rate as loss of traits (e.g., eyes and pigmentation) associated with cave life.     

Getting a grip on the evolution of grasping in musteloid carnivorans: a three‐dimensional analysis of forelimb shape

The ability to grasp and manipulate is often considered a hallmark of hominins and associated with the evolution of their bipedal locomotion and tool use. Yet, many other mammals use their forelimbs to grasp and manipulate objects. Previous investigations have suggested that grasping may be derived from digging behaviour, arboreal locomotion or hunting behaviour. Here, we test the arboreal origin of grasping and investigate whether an arboreal lifestyle could confer a greater grasping ability in musteloid carnivorans. Moreover, we investigate the morphological adaptations related to grasping and the differences between arboreal species with different grasping abilities. We predict that if grasping is derived from an arboreal lifestyle, then the anatomical specializations of the forelimb for arboreality must be similar to those involved in grasping. We further predict that arboreal species with a well‐developed manipulation ability will have articulations that facilitate radio‐ulnar rotation. We use ancestral character state reconstructions of lifestyle and grasping ability to understand the evolution of both traits. Finally, we use a surface sliding semi‐landmark approach capable of quantifying the articulations in their full complexity. Our results largely confirm our predictions, demonstrating that musteloids with greater grasping skills differ markedly from others in the shape of their forelimb bones. These analyses further suggest that the evolution of an arboreal lifestyle likely preceded the development of enhanced grasping ability.     

Dominant structural and functional adaptations of distant chemosensory systems in phylogenesis of Gastropoda

The invasion of land by gastropods independent and repeated in the course of their evolution, was shown to be accompanied by appearance of organisationally similar olfactory tentacular organs and special integrative centres. The majority of primary and secondary water gastropods in different phylogenetic groups had a different, more primitive organisation of the tentacular sensory system as compared to the terrestrial species. Regularities of the phylogenetic adaptations of mollusks to the habitat media and lifestyle are discussed.     

Comparative character of the deep-water and inshore cottoid fishes endemic to Lake Baikal

Lake Baikal's 29 endemic species of cottoid fishes form three groups: shallow-water species in depths to 350 m; eurybathic species from 50 to 1300 m; and abyssal species from 400 to 1600 m. These groups differ in their abilities to withstand high hydrostatic pressure. As in marine deep-water fishes, abyssal cottoids in Baikal have few or no cones in the retina, and some have tubular eyes. Their seismosensory systems predominate, based chiefly on free neuromasts. The proportion of species with canal systems decreases with depth. Diversity of the predominantly gammarid foods also decreases from 45 species in shallow water to five species in deep water, and the lateral line system plays the dominant role in food detection at all depths. Two abyssal cottoids have become secondary pelagic, achieving close to neutral buoyancy through high lipid levels and reduced skeletal mineralization. These forms take advantage of the abundant pelagic planktonic amphipod populations. The adaptations of abyssal forms parallel those seen in deep-water marine fishes.     

THE POTENTIAL FOR COEVOLUTION IN A HOST-PARASITOID SYSTEM. II. GENETIC VARIATION WITHIN A POPULATION OF WASPS IN THE ABILITY TO PARASITIZE AN APHID HOST

Much of the study of coevolution has focused on the adaptations that have resulted from interactions between species. For reciprocal evolution to occur, there must be genetic variation in each species for traits that directly affect their interaction. Here I report evidence of significant additive genetic variance within a population of parasitic wasps in the ability to successfully parasitize an aphid host. These data, combined with companion work documenting clonal variation in a population of aphids from the same site, provide evidence that within the same population both a host and its parasitoid have the potential for specific and reciprocal genetic interactions.     

An evolutionary perspective on caching by corvids

A principal finding in the food-caching literature is that species differences in hoarding propensity are positively correlated with species differences in degree of adaptations to caching behaviour, such as performance on spatial memory tasks and hippocampal volume. However, there are examples that do not fit this pattern. We argue that these examples can be better understood by considering the phylogenetic relatedness between species. We reconstruct the ancestral state for caching behaviour in corvids and assess when transitions in caching behaviour occurred within the corvid phylogeny. Our analysis shows that the common ancestor of all corvids was a moderate cacher. This result suggests that corvids followed a bi-directional evolutionary trajectory in which caching was secondarily lost twice and there were at least two independent transitions from moderate to specialized caching. The independent evolution of specialized cachers in the two groups must, therefore, be a case of convergent evolution. This is exemplified by the fact that specialized cachers show structurally different adaptations serving the same function to intense caching, such as different pouches to transport food. Finally, we argue that convergent evolution may have led to adaptations in memory and hippocampus that serve the same function but differ in design, and that these different adaptations may explain the examples that do not fit the pattern predicted by the adaptive specialization hypothesis.     

Testing an Adaptive Hypothesis Through Context-Dependence: Effects of Water Depth on Foraging Behaviour in Three Species of Garter Snakes

Adaptive hypotheses based on interspecific comparisons can be tested by evaluating the context‐dependence of the behaviour of individual organisms. Drummond (Behaviour, 86, 1983, 1) categorized garter snake species (Thamnophis) as terrestrial–aquatic generalists or aquatic specialists based on diet and aquatic foraging behaviour. He hypothesized that the characteristic foraging behaviours of aquatic specialists – including frequent crawling on the underwater substrate and a high rate of underwater predatory strikes – are adaptations for feeding on relatively widely dispersed aquatic prey. Drummond's hypothesis based on interspecific comparisons suggests that individual snakes might change their foraging in the direction of aquatic specialist behaviour with an increase in water depth (which increases prey dispersion). I tested this prediction through laboratory observations of Mexican Pacific lowlands garter snakes (T. validus) feeding on minnows in shallow (2 cm) and deep (3–7 cm) water. Members of this species are appropriate subjects because they are ecologically intermediate between the generalists and aquatic specialists studied by Drummond, and thus might be expected to show more variation in aquatic foraging behaviour than those species. T. validus showed significantly higher frequencies of crawling on the underwater substrate and of underwater strikes in the deep water than in the shallow water; i.e. increased water depth shifted the behaviour of these snakes toward that of aquatic specialists, thus supporting Drummond's hypothesis. Individuals of an aquatic specialist species, the narrow‐headed garter snake (T. rufipunctatus), showed less pronounced changes in behaviour with increased water depth. Western ribbon snakes (T. proximus), which feed primarily at the land–water interface (and are expected to act like terrestrial–aquatic generalists), typically refused to feed in deep water. Interspecific differences in underwater visual acuity may underly the behavioural differences among the three species by determining whether changes in foraging behaviour with water depth are advantageous. Information on phylogenetic relationships suggests that the facultative behaviour of T. validus may represent an intermediate stage in the evolution of aquatic specialization.     

Differences in microhabitat,abundance, biomass and body size between oxybiotic and thiobiotic free-living marine nematodes

Summary Analysis of samples from a 15 m deep sandy bottom in the northern part of Øresund, Denmark shows that the nematode fauna comprises 2.9 million individuals per m² down to 20 cm depth which averages of 0.40 g organic carbon. Highest densities are found in the uppermost cm. Biomass attains its highest value at 1–2 cm depth. Three results are discussed: 1) nematodes in the uppermost cm are, on average, three times smaller than those between 1–8 cm depth (60 ng org. C vs. 200 ng org. C ind. weight); 2) a distinct abundance trough is found at 8 cm depth, which correlates with the physical space occupied by the gallery of the lugworm Arenicola marina as well as with its possibly unselective feeding on the interstitial meiofauna at that depth; 3) the thiobiotic (deeper-living) species are significantly more slender than the oxybiotic (surface-dwelling) species, suggesting surface-dependent adaptations to life in oxygen poor and sulphide rich sediments where large amounts of dissolved organic matter occur. This is in contrast to previous assumptions that the body length or body shape reflects adaptations to the physical constraints of interstitial space.