Fighting Tooth and Claw

Velvet Swimming Crab, Necora puber

The Velvet Swimming Crab (Necora puber) is unique in a couple of senses. Firstly, it is the only member of the genus Necora, having been classified as such following reexamination of type material in 1987 (1). Secondly it, unusually for a swimming (or portunid) crab, occurs predominantly on hard substrates (2). It is also the largest swimming crab to be found in the waters surrounding Ireland and Britain, its final pair of pereiopods being flattened, fin-like to aid swimming. However its most striking features are its red eyes (lending it the alternative common name, “Devil Crab) and its velvety appearance. This is due to tiny hairs covering the carapace and appendages which allow a layer of algae to cover the crab. An important commercial crab (3), N. puber is usually found in shallow water to a depth of 20 m where it feeds on a variety of organisms including other crustaceans, molluscs and brown algae.

Velvet Swimming Crab, Necora puber, with swimming pereiopod on right

Interaction between individual crabs are more often than not antagonistic, with fights occurring between all sizes of crabs. They seem to prefer a solitary lifestyle and it is assumed that this is because large numbers of crabs increase the presence of predators and competition for food (4). In laboratory conditions (4), N. puber individuals were seen surprisingly not to engage in an escalation of violence, but rather in sporadic bursts. Fights between individuals were won in the majority of cases by the larger crab, yet smaller crabs never backed down from confrontation. This might seem a bit foolhardy on their parts, having little chance of winning, but under the laboratory conditions all crabs were in intermoult stage and had a hard outer shell. In vivo, fights may be one by smaller crabs against larger but softer ones.

Velvet Swimming Crab, Necora puber

References:

1. Holthuis, 1987. Zoologische Mededelingen 61 pp. 1-14
2. Norman and Jones, 1992. Estuarine, Coastal and Shelf Science 34 pp. 71-83
3. Robson et al., 2007. Food Microbiology 24 pp. 419-424
4. Thorpe et al., 1994. Behavioural Processes 32 pp. 235-246

The Threat to a Seemingly Widespread Species: Small Copper Butterfly

When it comes to species under threat, its the specialists that seem to be in the most trouble. Relying on, say, a single rare plant species for food or existing only in a specific microhabitat spells trouble in the extinction stakes. Yet the news is seemingly equally as grim for some widespread species.

Small Copper Butterfly, Lycaena phlaeas

The Small Copper (Lycaena phlaeas) is a common butterfly of hedgerows and woodland verges in Ireland, although it is also frequently spotted in fields and even gardens. It is considered common throughout Europe, Asia, North America and even north Africa, feeding on Common Sorrel (Rumex acetosa) and other Rumex spp. (1). However a detailed study of numbers in north Wales showed a population level decline of 89%, a level comparable to that of species considered rare and threatened (2). Habitat loss and fragmentation are the causes of the reduction in numbers. This illustrates the difficulty in foreseeing what consequences our further misuse of natural resources will have on a range of species.

References:

1. Endo et al., 1985. Journal of Insect Physiology 31 pp. 525-532
2. León-Cortés et al., 2000. Ecological Entomology 25 pp. 285–294

Lavender in the Salt

Sea Lavender, Limonium humile

With its sprigs of purple flowers on stalked shoots, Sea Lavender (Limonium humile) does indeed resemble its namesake Lavender (Lavandula spp.). Its there though that the similarity ends, as the resemblance is superficial with no relationship existing. L. humile can be found growing, sometimes with local abundance, in salt marshes where its flower bearing stems can rise to 20 cm above a basal rosette of rounded leaves. Salt marshes pose the dual problem of high salinity and low oxygen levels. Both of these pose there own particular problems for L. humile, but the lack of oxygen to the root systems is possibly the greater of the two evils. It, along with other Limonium spp., overcome this problem by switching root respiration to a very high level of lactate fermentation, a protracted level much greater than the transient lactate glycolysis seen in most plants at the transition from normoxic to anoxic conditions (1). The lactate subsequently produced in the Limonium spp. root cells is of a degree that would lethally acidify them and is transported out of the cells into the environment, thus maintaining cell homeostasis.

Sea Lavender, Limonium humile

References:

1. Rivoal and Hanson, 1993. Plant Physiology 101 pp. 553–560

Two Sun Planet

As recently mentioned on this blog, its been a miserable summer sun-wise. Not so for a planet recently discovered by the Kepler spacecraft. Launched in March 2009, Kepler's aim is to find and analyse terrestrial and larger planets in or near the habitable zone of a wide variety of stars (1). And its come up with quite the find: a planet in orbit around two suns (2). Known as Kepler-16b, the Saturn sized planet describes a 229-day orbit around its two stars, which in turn eclipse each other. The stars are 20% and 69% the size of our own star, the sun, and have a 41 day orbit.

Kepler-16b (in blue) orbiting its two stars (orange and yellow).
Note that distances are not to scale

References:

1. http://kepler.nasa.gov/
2. Doyle et al., 2011. Science 333 pp. 1602-1606

Sun Bathing Butterfly

Silver-washed Fritillary, Argynnis paphia

The sun has been in short supply in Ireland this year, and with September in full swing it looks (and feels) like we've had our, some may say inadequate, share for the year. These some would not include the Silver-washed Fritillary, Argynnis paphia. A large and attractive species native to Ireland that is common in woodlands, it has a yellow-orange upperwing marked with black that are similar to other Fritillary species, but is distinguished by the silvery sheen of the underwings (1). The adults are often found on their favourite food of brambles, basking in the sun. Here, they use their wings to absorb solar radiation and maintain their temperature above the ambient. By adjusting the angle of its wings at rest, it can absorb as much or as little sun as possible. Under artificial conditions, A. paphia was seen to adjust its wing position to maintain a thoracic temperature of 34±1.5 °C (3).

References:

1. Sterry, 2004. Collins Complete Guide to Irish Wildlife p. 106
2. Kammer and Brachi, 1973. Comparative Biochemistry and Physiology 45 pp. 1057-1063
3. Vielmetter, 1958. Journal of Insect Physiology 2 pp. 13-16

Cormorant Washing Line

Cormorant, Phalacrocorax carbo

The Cormorant (Phalacrocorax carbo) is one of the largest seabirds in Ireland, and also one of the most familiar. It can often be seen feeding in estuaries and perched in trees, where they sometimes breed (1). With its dark body and long, hooked bill it could be mistaken for its relative the Shag (P. aristotelis). However, the Shag is a smaller bird by about 20 cm and is rarely seen inland. Adult Cormorants have an attractive blue-green sheen to their plumage, a yellow and white bare patch of skin on its face and a white thigh patch that disappears after the breeding season (1).

Cormorant, Phalacrocorax carbo

Recently, I observed a Cormorant perched on an old mooring post in a river estuary. It held its wings out from its body in a drooping fashion that is known as a spread-wing posture. Many birds engage in this type of behaviour. Cormorant feathers retain moisture when diving for food, which decreases buoyancy and aids underwater pursuit of prey. Only the outer layer of feathers are wettable however, and a a layer of insulating air is maintained next the skin when diving (2). Yet the outer feathers remain wet upon emergence and therefore need to be dried. Hence the spread-wing posture (as seen in the photographs) of the Cormorant, a living washing line.

References:

1. Sterry, 2004. Collins Complete Guide to Irish Wildlife p. 34.
2. Elowson, 1984. The Auk 101 pp. 371-383

Spooky Dragonflies

Male Common Darter, Sympetrum striolatum

The Common Darter (Sympetrum striolatum) is, as its name suggests, a common and widespread dragonfly in Ireland, indeed in Europe as a whole. Flying between June and October, it can often seen resting on paths or stones in sunny, open areas. S. striolatum is a very aggressive, territorial species and will “dart” at other, often larger dragonfly species. Males and females differ in their colouring, with mature males having orange-red, unwaisted abdomens in contrast to the the dull, yellow-brown females.

Female Common Darter, Sympetrum striolatum

Individuals are most often seen near favoured breeding sites of shallow ponds and small lakes, so it was therefore a surprise when in Hungry S. striolatum (along with five other species of Sympetrum)  individuals were observed in large numbers in a cemetery that was nowhere near any water source (1). Were they just being spooky or was there a more rational answer?

Common Darter, Sympetrum striolatum

The dragonflies were seemingly attracted to polished black gravestones, where their behaviour was the same as if by water. This involved perching near the gravestones and defending their perch, flying individuals repeatedly touching the gravestone surfaces with their ventral sides and pairs in tandem position frequently circling over the gravestones. It was found that the gravestones reflect highly and horizontally polarised light in a manner almost identical to smooth water. S. striolatum, along with many other dragonfly species, find their aquatic habitats by means of polarotaxis, the use of reflected polarised light to discriminate between surfaces (2). This explains the attraction to the gravestones, an attraction that is so strong that females are sometimes lead to oviposit on the gravestones.

References:

1. Horváth et al., 2007. Freshwater Biology 52 pp. 1700–1709
2. Bernáth et al., 2002. Freshwater Biology 47 pp. 1707–1719