Underwater survival in the dog tick Dermacentor variabilis (Acari:Ixodidae)

Ticks are blood-feeding arthropods known for their long survivability off the host. Although ticks are terrestrial, they can survive extended periods of time submerged underwater. A plastron is an alternative respiration system that can absorb oxygen from water via a thin layer of air trapped by hydrophobic hairs or other cuticular projections. The complex spiracular plate of ticks has been postulated to serve as a plastron but that function has not been verified. This study provides evidence of plastron respiration in the American dog tick, Dermacentor variabilis, and for the first time confirmed the existence of plastron respiration in Ixodidae. Longer survival rates in oxygenated water indicate that underwater respiration requires oxygen. Wetting the spiracular plate with alcohol debilitates any potential plastron function and lowers the survival rate. Survival underwater may also be enhanced by metabolic depression and possibly anaerobic respiration. This study describes the first example of plastron respiration in the Ixodidae.     

Plastron respiration in adult beetles of the suborder Myxophaga

The kind of physical gill known as a plastron is a gas film of constant volume and an extensive water-air interface. Such films are held by hydrofuge structures, and they resist wetting at the hydrostatic pressures to which they are normally subjected in nature. In well-aerated water a plastron enables the insect to be immersed indefinitely. A plastron is here recorded for the first time in adult beetles of the suborder Myxophaga. In both known genera of the family Torridincolidae a plastron is present on the abdomen. In Ptyopteryx of Brazil the plastron forms a diffraction grating that is responsible for the iridescence oi the abdomen. The structure of the plastron of Torridincola of Rhodesia is like that of some other beetles, and it is not iridescent. Among adult beetles plastron respiration has been independently evolved in at least eight quite different groups, and within some of these groups it has been evolved on more than one occasion.     

The structure and probable function of the peritreme in intertidal Gamasina (Acarina: Mesostigmata)

The peritremes of the British intertidal Gamasina are structurally similar to those described for species occurring in soil and leaf litter, the highly elaborate peritreme plastrons present in species occurring in low oxygen tension environments are not typical of the fauna. It is shown that elaborate peritremes can form effective gas gills if an airfilm is supported by the peritrematic slit; however, if the airfilm is supported by the micropapillae on the floor of the peritreme, then the respiratory efficiency of the gas gilhbecomes comparable with that of elaborate peritreme plastrons. Evidence is presented reinforcing the theory that the plastron is supported by the micropapillae. The absence of elaborate respiratory peritreme plastrons among the intertidal Gamasina suggests that these mites are preadapted to respiration during tidal inundation. Other preadaptations include the distal parabolic shape of the micropapillae, however in the plastron forming species the micropapillae are shorter and basally thicker to prevent bending under pressure Developmental trends within the peritremes, with respect to plastron formation are discussed.     

Submersion respiration in small diving beetles (Dytiscidae)

Some small diving beetles can survive submerged through weeks and months, because they can extract oxygen, dissolved in the water, through respiratory pores in their integument. An air flux from the outside to the inside through the respiratory pores has been demonstrated. All diving beetles capable of such pore respiration are small, but not all small diving beetles have pore respiration. With increasing size, more and more of the surface must be covered by respiratory pores to meet the increasing demand of oxygen. In running water species the pore-respiration mode is regarded as an adaptation to life in current exposed substrates, thus they avoid the risk of being swept away during frequent surface visits. In stagnant water species the pore respiration mode reduces the risk of falling victim to pelagic predators. The submersion tolerant species can switch to surface respiration, e.g. during low oxygen content. The pore respiratory mechanism is believed to be a specialised plastron. The oxygen flux through the scattered, small respiratory pore area may be enhanced by a functional thinning of the boundary layer.     

Fine structure of the eggs of blowflies Aldrichina grahami and Chrysomya pacifica (Diptera: Calliphoridae)

We report here the fine structure of the eggs of blowflies Aldrichina grahami (Aldrich) and Chrysomya pacifica Kurahashi. For A. grahami, the plastron is wide and extends to almost the entire length of the eggs. The plastron near the micropyle is truncated. The polygonal patterns of chorionic sculpture bear a distinct swollen boundary. Regarding C. pacifica, the plastron is narrow and extends to almost the entire length of the eggs. The plastron near the micropyle bifurcates to a Y-shape, but the arms of the 'Y' are short. Information presented herein allows some distinctive features to differentiate among eggs of blowfly species.     

The water impermeability of discontinuous animal integuments]

Discontinuous integuments formed by macroscopic discrete elements are characters mainly of land animals, both vertebrates and invertebrates. Waterproof properties of these integuments manifest themselves in capture and preservation of some air in their layers during submergence. The functions of integument air layer are various in different groups of animals, viz. participation in respiration (invertebrates with integuments in the form of plastron), insulation (especially birds and mammals), regulation of pelage state in the water (mammals). The principal condition of watertightness is high density of integument elements forming a system of capillaries with variable cross-sections. The general physical principle underlying the watertightness of integuments consist in a effect of surface tension arresting the water penetration deep into the coats soon as meniscus of a liquid has attend the widening section of intraintegumental capillary. For the watertightness of plastron formed by a single hair layer considerable stiffness of the whole system and high values of capillary pressure are important. Multilayered pliable mammalian pelage is characterized by a rather low capillary pressure because its effect is supplemented by the pressure that arise in the pelage air layer as a result of its compression during submergence. In birds with their structurally complicated integuments and diverse locomotory adaptations a wide spectrum of variability of plumage watertightness is observed.     

A juvenile eurysternid turtle (Testudines: Eurysternidae) from the upper Kimmeridgian (Upper Jurassic) of Nusplingen (SW Germany)

A juvenile turtle from the upper Kimmeridgian (Upper Jurassic) of Nusplingen is identified as an eurysternid turtle. It differs in plastral morphology from a juvenile eurysternid turtle from the latest Kimmerdigian of Kelheim described in the 19th century, which represents a comparably early developmental stage. Both juveniles have primordial ribs not yet transformed into costals and lack all other carapacial elements whereas the plastral elements are well developed. The new specimen from Nusplingen has a more robust plastron type when compared to the very gracile, bow- or arc-shaped plastron type of the formerly described juvenile. Both plastron types are also represented by yet undescribed additional juvenile, medium-sized and/or larger eurysternid specimens. The juvenile specimens thus likely document the presence of two morphologically very similar eurysternid taxa in the Upper Jurassic of southern Germany. Both plastron types are different from those described for Idiochelys and Solnhofia but may resemble plastron morphology of Eurysternum, which is, however, only incompletely known.     

Structural characterization of the micropatterned egg plastron in the mosquito,Aedes albopictus

Eggs of the Asian tiger mosquito Aedes albopictus are fully covered with an air‐covering plastron network that enables them to stay below the surface of the water. Scanning electron microscopy revealed that the chorionic surface was covered with clusters of globular tubercles of different sizes. Histologically, the eggshell provides a typical water‐repellent microarchitecture consisting of an outer exochorion, a membranous endochorion and an intermediate pillar layer. The eggshell has a distinct chorionic tubercle that increases the surface area for gas exchange to enhance respiration capacity. In particular, the chorionic wall gives rise to a hexagonal pattern with smooth and elevated boundaries. In A. albopictus, a set of micropatterns for each hexagon was specified, and each central tubercle was surrounded by 20 small peripheral tubercles. Our fine structural analysis revealed that the partitioning of the surface into numerous hexagonal chorionic sculptures is associated with the Voronoi partition (tessellation), based on the distance to a point in a specific area of the plane. Therefore, the micropatterned surface of the mosquito eggshell appears to not only resist wetting by hydrostatic pressure but also provide resistance to lysate deposition by the biofouling process.     

Plastron respiration in bugs and beetles

In the naucorid bug, Alphelocheirus, the plastron hairs are twice as thick and nearly twice as dense (ca. 4 × 10⁶/mm²) as they had been thought to be by previous workers. From experiments and calculations it seems clear that when the plastron is subjected to excess pressures it is wetted long before there is any question of the collapse of the hair pile itself. The plastron of Aphelocheirus is thus like the plastrons of other insects in that it is wetted long before the structures supporting the air film collapse.A plastron has been independently evolved in at least five subfamilies of the Naucoridae. A plastron is here recorded for the first time in bugs of the family Helotrephidae.It has been claimed that the plastron-bearing elmid beetles are unable to fly. Many, if not most, of these beetles fly after they emerge from their pupal cells. However, once they have begun to live under water they cease to be able to fly: the flight muscles degenerate, and this degeneration seems to be irreversible.The structure of the plastron scales of several kinds of weevils is described. The resistance of the plastron of the rice water weevil, Lissorhoptrus, to wetting at excess pressures is examined. An explanation is advanced to account for the fact that weevils and other plastron-bearing beetles that live in still waters can often swim whereas those, like elmids and dryopids, that live in running waters cannot swim.     

Physical gills in Elmidae (Coleoptera: Byrrhoidea): Structure and evolutionary pattern of plastron in Stenelmis and related genera

Many species within Elmidae (Coleoptera: Byrrhoidea) have plastrons composed of flattened setae. However, some genera display fine plastrons on the epicuticle, called plastron hairs. In Japanese elmids, members of the genera Stenelmis, Ordobrevia, Nomuraelmis and Leptelmis bear ventral plastron hairs. Based on a maximum likelihood tree including most Japanese genera within Elmidae, we found that these genera are monophyletic and that plastron hairs are a derived character in Elmidae. We also found that the genus Graphelmis bears jigsaw puzzle‐like plastron scales with plastron hair‐like projections, and is sister to the group with plastron hairs.     

Comparative Morphology of Arthropod Exterior Surfaces with the Capability of Binding a Film of Air Underwater

The air-binding surfaces of aquatic and semi-aquatic insects and spiders were examined under a scanning electron microscope. The bristles that are instrumental in binding the film of air show distinctive characteristics within each phylogenetic group. Very small species can often maintain themselves in a bubble of air underwater although they lack the dense bristle coat characteristic of larger plastron breathers. Relatively large insects generally require a morphologically adapted chamber in which to carry their air supplies. The ecological advantages of an underwater respiration employing portable air are discussed.     

Chorionic sculpturing in some sandfly eggs (Diptera,Psychodidae)1

The outer chorionic sculpturing of the eggs of thirteen species of neotropical sandflies was examined by scanning electron microscope. Sculpture patterns are species specific and include polygons, parallel ridges and mountain or volcano-like structures. Two closely related and undescribed species of the intermedia group were distinguished by this means. The role of sculpturing in plastron respiration is discussed.     

A diffusion equation for tapered plastrons

An equation for the diffusion of oxygen along a plastron of uniform thickness was provided by Thorpe and Crisp in their classical work on plastrons. Since then it has been discovered that some plastrons are tapered, e.g. those of many spiracular gills, in which the thickness of the plastron becomes less as the distance from the spiracle increases. Here a differential equation is provided for calculating the efficiency of a uniformly tapered plastron. A series of curves is also given that show the efficiency of the tapered plastron as a function of the distance from the spiracle, thickness, and degree of taper. Using these curves the efficiency of the plastron can be estimated immediately without the necessity of solving the differential equations.     

Plastron respiration in marine intertidal oribatid mites (Acari,Fortuyniidae and Selenoribatidae)

Plastron respiration was investigated in the fortuyniid Alismobates inexpectatus, Fortuynia atlantica and the selenoribatid Carinozetes bermudensis. All these taxa inhabit intertidal zones of subtropical and tropical coasts and are exposed to tidal flooding. The utilization of plastron mechanisms enables these species to respire under water. Cerotegumental structures consisting of micropapillae and pillars bearing an outer sheet provide extensive areas where air is retained supplying the tracheal system with oxygen. A. inexpectatus and F. atlantica possess a dorsal and ventral plastron connected laterally by cuticular channels of the van der Hammen’s organ, whereas the specific configuration of these channels varies between the genera. The plastron of Carinozetes species spans the whole body except for all movable parts as legs and genital and anal valves. Plastron structures in juveniles of the families Fortuyniidae and Selenoribatidae were investigated for the first time in detail. Air-retaining cerotegument is also present in immatures of these taxa but is concentrated along lateral and ventral folds where series of pores lead into supposed tracheal organs. In juveniles of A. inexpectatus and F. atlantica, these organs are tubes with a length of approximately 3–15 μm, and in Carinozetes immatures, these organs are short saccules (0.5–1 μm).     

Evidence that a late-emerging population of trunk neural crest cells forms the plastron bones in the turtle Trachemys scripta

The origin of the turtle plastron is not known, but these nine bones have been homologized to the exoskeletal components of the clavicles, the interclavicular bone, and gastralia. Earlier evidence from our laboratory showed that the bone-forming cells of the plastron were positive for HNK-1 and PDGFRalpha, two markers of the skeletogenic neural crest. This study looks at the embryonic origin of these plastron-forming cells. We show that the HNK-1+ cells are also positive for p75 and FoxD3, confirming their neural crest identity, and that they originate from the dorsal neural tube of stage 17 turtle embryos, several days after the original wave of neural crest cells have migrated and differentiated. DiI studies show that these are migratory cells, and they can be observed in the lateral regions of the embryo and can be seen forming intramembranous bone in the ventral (plastron) regions. Before migrating ventrally, these late-emerging neural crest cells reside for over a week in a carapacial staging area above the neural tube and vertebrae. It is speculated that this staging area is where they lose the inability to form skeletal cells.     

ADAPTIVE EVOLUTION OF PLASTRON SHAPE IN EMYDINE TURTLES

Morphology reflects ecological pressures, phylogeny, and genetic and biophysical constraints. Disentangling their influence is fundamental to understanding selection and trait evolution. Here, we assess the contributions of function, phylogeny, and habitat to patterns of plastron (ventral shell) shape variation in emydine turtles. We quantify shape variation using geometric morphometrics, and determine the influence of several variables on shape using path analysis. Factors influencing plastron shape variation are similar between emydine turtles and the more inclusive Testudinoidea. We evaluate the fit of various evolutionary models to the shape data to investigate the selective landscape responsible for the observed morphological patterns. The presence of a hinge on the plastron accounts for most morphological variance, but phylogeny and habitat also correlate with shape. The distribution of shape variance across emydine phylogeny is most consistent with an evolutionary model containing two adaptive zones—one for turtles with kinetic plastra, and one for turtles with rigid plastra. Models with more complex adaptive landscapes often fit the data only as well as the null model (purely stochastic evolution). The adaptive landscape of plastron shape in Emydinae may be relatively simple because plastral kinesis imposes overriding mechanical constraints on the evolution of form.     

Ecomorphological variation in shell shape of the freshwater turtle Pseudemys concinna inhabiting different aquatic flow regimes

Populations of species that inhabit a range of environments frequently display divergent morphologies that correlate with differences in ecological parameters. The velocity of water flow (i.e., flow velocity) is a critical feature of aquatic environments that has been shown to influence morphology in a broad range of taxa. The focus of this study was to evaluate the relationship between flow velocity and shell morphology for males and females of the semi-aquatic freshwater turtle Pseudemys concinna. For both sexes, the carapace and plastron show significant morphological differences between habitats characterized by slow-flowing (i.e., lentic) and fast-flowing (i.e., lotic) water. In general, the most prominent pattern for both sexes is that the shells of individuals from lotic habitats are more streamlined (small height-to-length ratio) than the shells of individuals from lentic habitats. Of the two shell components (carapace and plastron), the carapace shows greater divergence between habitats, particularly for males. These results are consistent with adaptations to flow velocity, and suggest that variation in shape may be more constrained in females. I also provide empirical evidence for an adaptive benefit of the observed shape change (i.e., drag reduction) and a brief comment on the relative roles of genetic divergence and phenotypic plasticity in generating shape differences observed in this species.     

Evidence for the neural crest origin of turtle plastron bones

Summary: The migrating cranial neural crest cells of birds, fish, and mammals have been shown to form the membranous bones of the cranium and face. These findings have been extrapolated to suggest that all the dermal bones of the vertebrate exoskeleton are derived from the neural crest ectomesenchyme. However, only one group of extant animals, the Chelonians, has an extensive bony exoskeleton in the trunk. We have previously shown that the autapomorphic carapacial and plastron bones of the turtle shell arise from dermal intramembranous ossification. Here, we show that the bones of the plastron stain positively for HNK‐1 and PDGFRα and are therefore most likely of neural crest origin. This extends the hypothesis of the neural crest origin of the exoskeleton to include the turtle plastron. genesis 31:111–117, 2001. © 2001 Wiley‐Liss, Inc.     

Behaviour and physiology: the thermal strategy of leatherback turtles

Background

Adult leatherback turtles (Dermochelys coriacea) exhibit thermal gradients between their bodies and the environment of ≥8°C in sub-polar waters and ≤4°C in the tropics. There has been no direct evidence for thermoregulation in leatherbacks although modelling and morphological studies have given an indication of how thermoregulation may be achieved.

Methodology/Principal Findings

We show for the first time that leatherbacks are indeed capable of thermoregulation from studies on juvenile leatherbacks of 16 and 37 kg. In cold water (< 25°C), flipper stroke frequency increased, heat loss through the plastron, carapace and flippers was minimized, and a positive thermal gradient of up to 2.3°C was maintained between body and environment. In warm water (25 – 31°C), turtles were inactive and heat loss through their plastron, carapace and flippers increased. The thermal gradient was minimized (0.5°C). Using a scaling model, we estimate that a 300 kg adult leatherback is able to maintain a maximum thermal gradient of 18.2°C in cold sub-polar waters.

Conclusions/Significance

In juvenile leatherbacks, heat gain is controlled behaviourally by increasing activity while heat flux is regulated physiologically, presumably by regulation of blood flow distribution. Hence, harnessing physiology and behaviour allows leatherbacks to keep warm while foraging in cold sub-polar waters and to prevent overheating in a tropical environment.