Visual perception and camouflage response to 3D backgrounds and cast shadows in the European cuttlefish, Sepia officinalis.

To conceal themselves on the seafloor, European cuttlefish, Sepia officinalis, express a large repertoire of body patterns. Scenes with 3D relief are especially challenging because it is not possible either to directly recover visual depth from the 2D retinal image or for the cuttlefish to alter its body shape to resemble nearby objects. Here, we characterised cuttlefish camouflage responses to 3D relief, and to cast shadows, which are complementary depth cues. Animals were recorded in the presence of cylindrical objects of fixed (15 mm) diameter, but varying in height, greyscale and strength of cast shadows, and to corresponding 2D pictorial images. With the cylinders, the cuttlefish expressed a '3D' body pattern, which is distinct from previously described Uniform, Mottle and Disruptive camouflage patterns. This pattern was insensitive to variation in object height, contrast and cast shadow, except when shadows were most pronounced, in which case the body patterns resembled those used on the 2D backgrounds. This suggests that stationary cast shadows are not used as visual depth cues by cuttlefish, and that rather than directly matching the 2D retinal image, the camouflage response is a two-stage process whereby the animal first classifies the physical environment and then selects an appropriate pattern. Each type of pattern is triggered by specific cues that may compete, allowing the animal to select the most suitable camouflage, so the camouflage response is categorical rather than continuously variable. These findings give unique insight into how an invertebrate senses its visual environment to generate the body pattern response.

Parasites contribute to ecologically dependent postmating isolation in the adaptive radiation of three-spined stickleback.

Spatial variation in parasitic infections is common, and has the potential to drive population divergence and the reproductive isolation of hosts. However, despite support from theory and model laboratory systems, little strong evidence has been forthcoming from the wild. Here, we show that parasites are likely to cause reproductive isolation in the adaptive radiation of three-spined stickleback. Adjacent wild populations on the Scottish island of North Uist differ greatly and consistently in the occurrence of different parasites that have substantial effects on fitness. Laboratory-reared fish are more resistant to experimental infection by parasite species from their own population. Furthermore, hybrid backcrosses between the host populations are more resistant to parasites from the parental population to which they are more closely related. These patterns provide strong evidence that parasites can cause ecological speciation, by contributing to selection against migrants and ecologically dependent postmating isolation.

The evolutionary ecology of dwarfism in three-spined sticklebacks.

1. Body size is a defining phenotypic trait, but the ecological causes of its evolution are poorly understood. Most studies have considered only a single putative causal agent and have failed to recognise that different environmental agents are often correlated. 2. Darwin suggested that although trait variation across populations is often associated with abiotic variation, evolution is more likely to be driven by biotic factors correlated with the abiotic variation. This hypothesis has received little explicit attention. 3. We use structural equation modelling to quantify the relative importance of abiotic (pH, metal concentrations) and biotic (competition, predation) factors in the evolution of body size in three-spined sticklebacks Gasterosteus aculeatus on the island of North Uist, Scotland. We combine phenotypic data from multiple isolated populations, detailed characterisation of their environment and a common garden experiment that establishes the genetic basis of size differences. 4. Three-spined sticklebacks on North Uist show almost unprecedented intraspecific evolution of body size that has taken place rapidly (<16,000 years). The smallest fish mature at only 7% of the mass of ancestral, anadromous fish. Dwarfism is associated with reduced abundance of a smaller competitor species, the nine-spined stickleback Pungitius pungitius, and with low pH indicative of poor resource conditions. Dwarfism also tends to occur where an important predator, the brown trout Salmo trutta, is also small. The abundance of P. pungitius and the size of S. trutta are themselves related to underlying abiotic environmental variation. 5. Despite the close association between abiotic and biotic factors across populations, our results support Darwin's hypothesis that biotic factors, associated with variation in the abiotic environment, are more important in explaining evolution than is abiotic variation per se. This study demonstrates the importance of considering the relationships between environmental variables before conclusions can be drawn about the causes of (body size) evolution on islands.

Molecular markers reveal spatially segregated cryptic species in a critically endangered fish, the common skate (Dipturus batis).

Many sharks and skates are particularly vulnerable to overfishing because of their large size, slow growth, late maturity and low fecundity. In Europe dramatic population declines have taken place in common skate (Dipturus batis L.), one of the largest demersal fish in regional shelf seas, leading to extirpations from substantial parts of its former range. Here we report the discovery of cryptic species in common skate collected from the northeast Atlantic continental shelf. Data from nuclear microsatellite markers indicated two clearly distinct clades and phylogenetic analysis of mitochondrial DNA sequences demonstrated monophyly of each one of them. Capture locations showed evidence of strong spatial segregation, with one taxon occurring mainly in waters off the southern British Isles and around Rockall, while the other was restricted to more northerly shelf waters. These apparently cryptic species showed overlapping substrate and depth preferences, but distributional limits were closely related to temperature gradients, potentially indicating thermal limits to their distributions. This discovery of hidden diversity within a large, critically endangered marine vertebrate demonstrates how marine biodiversity can be underestimated, even in such a relatively well-studied and heavily exploited region.

Deformed wing virus implicated in overwintering honeybee colony losses.

The worldwide decline in honeybee colonies during the past 50 years has often been linked to the spread of the parasitic mite Varroa destructor and its interaction with certain honeybee viruses. Recently in the United States, dramatic honeybee losses (colony collapse disorder) have been reported; however, there remains no clear explanation for these colony losses, with parasitic mites, viruses, bacteria, and fungal diseases all being proposed as possible candidates. Common characteristics that most failing colonies share is a lack of overt disease symptoms and the disappearance of workers from what appears to be normally functioning colonies. In this study, we used quantitative PCR to monitor the presence of three honeybee viruses, deformed wing virus (DWV), acute bee paralysis virus (ABPV), and black queen cell virus (BQCV), during a 1-year period in 15 asymptomatic, varroa mite-positive honeybee colonies in Southern England, and 3 asymptomatic colonies confirmed to be varroa mite free. All colonies with varroa mites underwent control treatments to ensure that mite populations remained low throughout the study. Despite this, multiple virus infections were detected, yet a significant correlation was observed only between DWV viral load and overwintering colony losses. The long-held view has been that DWV is relatively harmless to the overall health status of honeybee colonies unless it is in association with severe varroa mite infestations. Our findings suggest that DWV can potentially act independently of varroa mites to bring about colony losses. Therefore, DWV may be a major factor in overwintering colony losses.