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Posts Tagged ‘research news’

Parasites are organisms (living things) that require a host organism to survive and reproduce, usually at the host’s expense. Examples are many and include those that live on the outside of their host (ectoparasites e.g. fleas, ticks), and those that live inside their hosts (endoparasites e.g. tapeworms, liver flukes), ranging in size from microscopic viruses that hide inside our cells to large multicellular animals such as parasitic wasps and botflies (if you like gory horror stories, read about the latter here – science is often weirder than fiction!).  

Researchers at Liverpool and Glasgow Universities (1) investigated the nematode Heterorhabditis bacteriophora’s infection of greater waxmoth larvae (nematodes form the Nematoda phylum and are essentially tube-shaped worms, hence their alternative name “roundworms”). Heterorhabditis is an obligate parasite – meaning it cannot survive without a host. As a larval worm, it lives in the soil until it finds an insect larva host to enter. Once inside the insect, Heterorhabditis releases a bacteria species, Photorhabdus luminescens, that kills the insect host and digests it to form a nutrient-rich soup that Heterorhabditis eats. Living off the pre-digested insect, Heterorhabditis matures and reproduces hermaphroditically (i.e. the nematode is both male and female) and the new nematode larvae mature within the dead insect host. Eventually the insect host is devoured and it splits, releasing thousands of new Heterorhabditis larvae into the environment ready to infect another insect host and restart the life-cycle.

While this macabre scene is taking place inside the dead waxmoth larvae, the waxmoth remains potentially attractive to predators, such as birds, because, unlike after a normal death, the larva doesn’t dry out and shrivel. While birds are not affected by Heterorhabditis,  being eaten by a bird would be a big problem for the nematode as it will be killed by the bird’s digestive system.

Robin (thanks to Smudge9000)Heterorhabditis has an ingenious way to avoid this early demise. A few days after Heterorhabditis infects the waxmoth larva, the larva changes colour – it becomes bioluminescent (glows) for a short while, but it also permanently changes to bright pink in colour. Birds have good colour vision, and the research team demonstrated that European robins, Erithacus rubecula, were significantly more likely to choose to eat uninfected waxmoth larvae over infected ones. The team also noticed that if birds did peck at or eat a pink, infected larvae they would later be more likely to choose uninfected larvae, leading the team to suspect that the nematode also makes the waxmoth taste unpleasant. By changing its hosts colour, and reinforcing this colour warning with a foul taste, Heterorhabditis persuades potential avian predators not to eat infected larvae, allowing the parasite to continue its lifecycle in the waxmoth without interference.

References

1. Fenton A et al. 2011. Parasite-induced warning coloration: a novel form of host manipulation. Animal Behaviour 81: 417-422

Further Information

Parasites 
Daily Parasite
Wikipedia
The Life Tree
Aberystwyth University
Nematodes
University of Nebraska-Lincoln
Earthlife
Berkeley University
Wikipedia
Bioluminescence
San Diego Natural History Museum
– National Geographic: why deep-sea creatures glow

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Killer whales (Orcinus orca), or ‘orcas’, are the largest member of the dolphin family. Like all whales and dolphins, orcas are mammals – they breathe air, and mothers produce milk for their live-born young.

Killer whale mother and calf (photo by Sam)Killer whales were originally thought to solely eat fish but, increasingly, research is demonstrating that killer whales in different populations specialise in different prey – and that these populations may even be distinct enough to be separate species.  For example, in the northeast Pacific, off the coast of the USA and Canada, three distinct populations have been observed: the resident population lives close to the shore year-round, feeding only on fish (usually salmon). A transient population moves in and out of the area – these feed on marine mammals such as seal lions and other whales. The third, offshore, population are less well-known but researchers have recently found evidence that they may be specialist shark eaters, preying on Pacific sleeper sharks. These populations act differently to each other, make different vocalisations, and do not interbreed. Similarly in Antarctica, populations specialise on fish, or seals, or minke whales, with some observed hunting penguins.

Killer whales have different strategies for each prey, probably learned from other members of their group (called a ‘pod’). Those hunting seals among the ice of Antarctica will coordinate their swimming into a rush towards an ice floe, creating a bow-wave that can knock a seal off the floe and into the water where it can be caught. 

Other seal hunting pods risk stranding themselves as they launch onto a seal colony’s beach in an attempt to grab an unwary seal close to the ocean edge.

Those populations that hunt other whales, however, will stop using echolocation, which could alert their prey to their presence, while they stalk their prey, and (once caught up to the prey) will harry mother and calf pairs to exhaust the calf, before separating the calf from the help of its mother and then swimming on top of the calf to drown it. 

Killer whales are extremely intelligent predators and their behavioural repertoire is intriguing; I look forward to marine biologists unveiling more of their lives.

References

Ford JKB et al. 2011. Shark predation and tooth wear in a population of northeastern Pacific killer whales. Aquatic Biology 11: 213-224

Morell V. 2011. Killer whales earn their name. Science 331: 274-276

Morin PA et al. 2010. Complete mitochondrial genome phylogeographic analysis of killer whales (Orcinus orca) indicates multiple species. Genome Research 20: 908-916

Further Information

MarineBio

Smithsonian Ocean Portal

Convention on Migratory Species

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Most predators rely on their camouflage and behaviour to get close enough to their prey to strike before the prey notices them and can take avoidance actions. A few predators, however, use an alternative plan – they don’t hide their presence but instead use deceptive means to stop the prey recognising that they are in fact a predator. These “aggressive mimics” use lures that represent a different species to trick their prey into coming closer; for example the anglerfish uses a dangling lure that imitates a worm or bug to encourage smaller fish to approach this ‘food’ before the small fish instead becomes the meal. Bolas spiders, by contrast, produce imitation female moth pheromones that lure male moths close enough to the spider to be caught.

An Australian research team has shown that assassin bugs of the species Stenolemus bituberus use two separate techniques to catch their spider prey. When stalking, the bug will slowly creep towards the spider until it is in striking distance. Its second technique is very different – the assassin bug will lure the spider to approach within striking distance by plucking the spider’s web threads, pretending to be prey caught in the spider’s web.

By comparing examples of web vibrations made by the assassin bugs to those made by actual struggling prey, falling leaves (which elicited no response from the spiders), and courting male spiders (which prompted the females to take up a mating position), the study demonstrated that the bugs specifically imitate the vibrations made by struggling prey. Spiders can be a dangerous prey that can counter-attack and eat the assassin bug instead, and high-frequency vibrations can lead to a fast, aggressive approach by the spider that could endanger the assassin bug. By mimicking weakly struggling prey using low-frequency vibrations instead, the assassin bug lures the spider into a slower, less dangerous (to the bug) approach by the spider that enables the bug to effectively draw the spider within attacking range for an easy arachnid meal.

Reference

Wignall AE and Taylor PW. 2010. Assassin bug uses aggressive mimicry to lure spider prey. Proceedings of the Royal Society B doi: 10.1098/rspb.2010.2060

Further Info

Angler fish
National Geographic
Sea and Sky
Arkive

Bolas spiders
University of Kentucky
Video of the spider lassoing prey (not for arachnophobes!)

Assassin bugs

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Researchers working in Papua New Guinea have analysed rainforest plant-herbivore interactions and found that an average of 251 herbivore (plant eating) species are associated with each rainforest tree species, 48 of which feed exclusively on that single tree species. With around 200 tree species in the lowland rainforest studied, this means the trees interact with somewhere in the region of 9,600 herbivorous insect species, which is staggering.

 Rainforest (thanks to tauntingpanda)Although these results used a fair amount of extrapolation of collected data from a limited research site and number of tree species, the research highlights the complexity of food-webs and inter-species relationships, particularly in ecosystems as species-rich as rainforests, and illustrates the huge number of species affected by the loss of each felled tree.

References

Novotny V et al 2010. Guild-specific patterns of species richness and host specialization in plant-herbivore food webs from a tropical forest. Journal of Animal Ecology 79: 1193-1203

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The IUCN (International Union for Conservation of Nature) compiles a ‘Red List’ where each species is described and their status classified – from Critically Endangered (50%+ probability of extinction within 10 years or 3 generations (whichever is longer)) through Endangered (20% probability of extinction within 20yrs or 5 generations) and Vulnerable (probability of extinction of 10% within 100yrs) to Near Threatened and Least Concern (Fig 1).  A paper by Hoffmann et al (2010) published in Science today has analysed this data on 25,780 species of the world’s vertebrates (all mammals, birds, amphibians, and cartilaginous fish (sharks and rays), and most reptiles and bony fish) and found that one-fifth of Earth’s vertebrates are Threatened and that this figure is increasing yearly.

Figure 1. Red List categories (from IUCN website)

This is sad, but not wholly unexpected news – after all, although extinctions have always happened in our planet’s history, the current levels are 100-1000 times above pre-human rates (Pimm et al 1995) and it’s well known that a vast number of species are in trouble. Fortunately it’s not all bad news as Hoffman’s study also demonstrates that conservation does work and, although current levels of conservation are not enough to overcome the significant threats to animals, there have been successes with species being downgraded in recent time due to the efforts to conserve them. Hoffmann mentions that the Humpback Whale Megaptera novaeangliae has moved from Vulnerable to Least Concern since the commercial whaling ban in 1955, and Butchart et al (2006) calculated that between 1994 and 2004 at least 16 bird species were saved from extinction by the conservation programmes implemented for them; so a turn-around of the extinction trend is possible with concerted effort, political will, and public support.

With Hoffmann concluding that, on average, 52 species move one category closer to extinction each year, the time for us all to take action is now.

References and Further Info

Butchart SHM et al 2006. How many bird extinctions have we prevented? Oryx 40: 266-278

Hoffmann M et al 2010. The impact of conservation on the status of the World’s vertebrates. Science – published online 26 Oct 2010 (doi 10.1126/science.1194442)

IUCN Red Data category specifications

Pimm SL et al 1995. The future of biodiversity. Science 269: 347-350

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The estuarine, or saltwater, crocodile (Crocodylus porosus) is the largest reptile in the world at around 4.5m in length, and with individuals over 7m long spotted.  They are found in freshwater and estuarine habitats such as mangroves, rivers and estuaries, from India through SE Asia and along the island chains to Australia and the South-east Pacific. This wide distribution of one species, with considerable tracts of open ocean in-between populations that should be un-navigable to the crocodiles, set Australian scientists wondering why this saltwater barrier has not lead to the separated populations becoming different species (‘speciation’). Their recent work has demonstrated that estuarine crocodiles can travel vast distances across the oceans by using water currents to speed up their journeys, enabling the seemingly unconnected populations to intermingle and breed. Yes, surfing crocodiles exist!

The team captured and radio-tagged 27 estuarine crocodiles in Australia then followed their satellite-tracked movements and compared their travels to the water speed and direction in that section of the river or ocean at that time. Their data showed that the crocodiles had two distinct travel modes:

(1) a short-range movement of around 1-3km per day in one direction; this was their typical daily pattern and is likely to have been day-to-day travel within their home range, and

(2) a less frequent long-range movement of more than 25km per day in a constant direction (although for the analysis all journeys of over 10km per day were included as long-range).

Intriguingly, while short-distance travel did not follow a set tidal pattern, long journeys were always started within one hour of the tide changing direction, giving the crocodile 6-8 hours travelling with a ‘tailwind’ current helping them along (less than 4% of long-distance travel was against the current and this dramatically reduced their travel speed). When the tide turned against them again, the crocodiles would stop their journey and climb out or dive to the bottom of the river, resting while the tide was not in their favour.

Some of the crocodiles travelled incredible distances – one taking an ocean voyage that coincided with a strong sea current that helped him travel 590km in 25 days, entering a different river along the Australian coastline. Another moved more than 411km in 19 days, later returning all the way back to the exact location within the river where it was originally captured – so they are excellent navigators as well.

The researchers suggest that using the ocean and river currents for migratory travel allows the crocodiles to travel far further than they would be able to do under their own power as crocodiles, despite being aquatic, are not really well-built for swimming – either in speed or efficiency. Both males and females made these long-distance journeys but the purpose of the travel is not yet known although it may be to take advantage of fish migrations. These travels do mean, however, that throughout their 10,000sq km range the ocean does not act as a barrier to gene flow between the various populations of estuarine crocodiles, maintaining the population as one species despite the ocean barriers in between.

Reference

Campbell HA, Watts ME, Sullivan S, Read MA, Choukroun S, Irwin SR & Franklin CE. 2010. Estuarine crocodiles ride surface currents to facilitate long-distance travel. Journal of Animal Ecology 79: 955-964

Further info

Ecology Asia

Unique Australian animals

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Today the BBC’s ‘Horizon’ programme focuses on the recently-finished census of our planet’s seas and oceans. Ten years of research by over 2,700 scientists collaborating from 80 countries around the globe, have lead to discoveries of more than 1,200 new species and the rediscovery of others thought to have gone extinct years ago – 50 million years ago in the case of one shrimp. New marine habitats and ecosystems were found, as well as a greater insight into the state of the world’s oceans and the effects on marine life today. I’ll certainly be tuning in to BBC2 at 9pm tonight – after all, you can’t go wrong when David Attenborough’s narrating!!

Further Info:

BBC Horizon Programme Info
Census of Marine Life: A decade of discovery – official website 
BBC News: “Marine census publication marks ‘decade of discovery’ “

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