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Examples of animals with a predator-prey relation

Martes foina (stone marten) preying on Glis glis (edible dormouse)
Bakaloudis et al. (2012)

Tyto alba (barn owl) preying on Mus musculus (house mouse)
Bontzorlos et al. (2005)

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Two parasitic organisms

Viscum album (mistletoe) & various deciduous trees
Noetzli et al. (2003)

Haplosporidium pinnae & Pinna nobilis (pen shell )
Catanese et al. (2018)

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[Yiannis Issaris/AP]

With rapid efficiency, a mysterious parasite is seeking out and killing a giant species of clam found only in the Mediterranean Sea. Unless scientists can find a way of stopping it soon, they say the mollusk could go extinct.

For thousands of years, the emblematic noble pen shell has been intrinsically connected to human civilization.

The largest bivalve in the Mediterranean can grow to more than a meter (three feet) long and has provided food and one of the world's rarest materials: sea silk spun from fibers it uses to secure itself to the seabed. The mollusk also contributes to clear water by filtering out organic particulates.

The pen shell, Pinna nobilis, has been on the European Union's protected species list for decades because of overfishing, pollution and the destruction of its natural habitat, meaning any fishing is banned.

But the ban is often poorly enforced, with the animal harvested for food or for its shell, which is used for decorative purposes.

The pen shells, which have a life span of several decades and take years to reach reproductive age, were already dying faster than they could be replaced. So the spread of the microscopic parasite, which first appeared in the western Mediterranean in late 2016 and was identified just this year as a new species, has alarmed experts.

Exactly how the parasite kills isn't completely clear, although scientists have found it attacks the pen shell's digestive system. The infected animal is also unable to close its shell, incapacitating its defense against predators. Once infected, death is almost certain.

"In less than a year it wiped out [the pen shell population of] the Spanish coast," said Maria del Mar Otero of the International Union for Conservation of Nature.

Soon parts of France, Malta, Tunisia and Italy were affected. In recent weeks, tests confirmed the same parasite, Haplosporidium pinnae, is responsible for pen shell die-offs in parts of Greece, and researchers have reported mass mortality as far east as Turkey and Cyprus.

Scientists are now racing to understand how the parasite spreads and its life cycle - essential information for a successful rescue program. One theory is that it could be spreading through phytoplankton, the bivalve's food source, but nobody knows for certain.

"We cannot be sure of anything at this point," said Pantelis Katharios, senior researcher at the Institute of Marine Biology, Biotechnology and Aquaculture of the Hellenic Center for Marine Research, or HCMR.

"What we know now is that the Pinnas are dying, that the cause is this parasite, and we know that it's spreading very, very rapidly," he said. "And that is going to be a huge problem (for) the ecology and the balance of the ecosystem in the Mediterranean."

Yiannis Issaris, marine ecologist at the HCMR's Institute of Oceanography, initially noticed widespread pen shell death off the coast of Anavyssos, southeast of Athens, in mid-summer. He suspected the culprit could be the same one causing mortality in Spain - and testing proved him right.

Now the question is how to tackle the outbreak. "This is very fresh for the scientific community," Issaris said. "We're still at the stage of recording where it has spread to."

Some parts of Greece still have healthy populations of the mollusks, he noted, whereas in other areas they have been wiped out.

Protecting the pen shell in its natural habitat - sandy seabeds or seagrass meadows - appears "difficult to impossible," Issaris said, particularly without knowing how the parasite spreads. In Spain, some healthy individuals were moved to aquariums.

Although tests are pending in some Mediterranean countries witnessing mass die-offs, experts have little doubt what the results will show.

"It's very, very, very likely" to be the same parasite, Otero said. "There's almost no question."

One thing's clear: the parasite is very particular in its choice of victim. A smaller, related species, the Pinna rudis, which also exists outside the Mediterranean, is unaffected.

"We don't know how it has appeared in the Mediterranean... We only know that it causes mortality only in the Pinna nobilis," Issaris said.

In the clear, shallow waters south of the Greek capital, dozens of dead pen shells lie scattered in a seagrass meadow, testament to the devastation.

Peering through a microscope at the tissue of an infected individual in his office in Crete, Katharios points out the culprit: small, oval-shaped parasites spread throughout the sample.

Scientists are puzzling over why an organism would be so lethal to the very species it depends on for its own survival.

"Normally parasites in nature do not have any benefit from harming the host, because they depend on the host," Katharios explained. "But once in a while we may come across incidents like this, where we have massive mortalities."

This could just be a natural phenomenon in which the parasite will eventually be wiped out along with its host, he said. Another possibility is that it originated in a different species and for some reason jumped to the pen shell.

A third is that the pen shell's immune system has been compromised by factors such as pollution, climate change or water temperature fluctuations.

"It's extremely, extremely difficult to find the truth at this stage," Katharios said. [AP]

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New parasite decimates giant clam species in Mediterranean, ELENA BECATOROS | Kathimerini

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Yiannis Issaris/AP] With rapid efficiency, a mysterious parasite is seeking out and killing a giant species of clam found only in the Mediterranean Sea. Unless scientists can find a way of stopping it soon, they say the mollusk could go extinct. For thousands of years, the emblematic noble pen shell has been intrinsically connected to human civilization.

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The ecological niche of two characteristic animals

Sciurus anomalus (Caucasian squirrel)
Diurnal species that stay active all year except in winter. Main activity during early morning and before sunset. Territorial animals that marks their territories with urine and feces. Probably solitary during non-mating seasons.

Erinaceus concolor (Hedgehog)
Animals with relatively constant home ranges, that may overlap between individuals. However, aggressive interactions between hedgehogs are rare, and mutual avoidance appears to be normal.

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Established in 1964, the IUCN Red List of Threatened Species has evolved to become the world's most comprehensive information source on the global conservation status of animal, fungi and plant species.

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Sharing the same food - Niche partitioning

The process by which natural selection drives competing species into different patterns of resource use or different niches. Coexistence is obtained through the differentiation of their realized ecological niches.

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The sun dips low over the bay, its fading rays gilding the avocets as they swish their heads through the water. The egrets eye their own reflections as if in profound self-contemplation. A willet flashes past, its black-and-white wings an exclamation in the dusk.

Faced with such beauty, two words come irrepressibly to mind: niche partitioning.

Okay, it's not a particularly lovely phrase in itself. But niche partitioning is why we can have so many different kinds of long-legged, long-billed shorebirds stalking through the same water, apparently hunting the same food, and yet all coexisting.

Natural selection favors the fittest. This should lead you to expect that one shorebird species would be the fittest-the best at hunting and surviving and reproducing in this shallow-water-and-mud habitat-and, out-competing all other shorebirds, eventually become the only shorebird around. Why doesn't this happen? Niche partitioning.

A niche is a set of biological conditions: temperature, habitat, types of food, etc. A species' niche is the set of conditions in which it thrives. Some species have broad niches, being able to do well under a large range of conditions; think of a cockroach, or a pigeon, or the common house mouse. My little buddies the juncos have a fairly broad niche, which is why I was able to study them under a variety of different environmental conditions. Still, there are many things that are outside their niche: juncos could not survive in a punishing desert, or Antarctica, or by trying to live off of nectar from flowers. Other species have narrow niches, specializing in very specific circumstances: cave salamanders, or tubeworms found only next to deep-sea hydrothermal vents.

The niche of shorebirds is, well, the shore. They feed in the same areas and with the same basic strategy: stick your bill in the water (or mud), pull out food, eat, repeat.

Except that this isn't the whole story. The reason that they can all coexist is that they don't occupy the same niche; they partition that broad shore niche into narrower, more specific niches, and each species specializes on its own smaller niche. You can see evidence of this in the shapes of their bodies. The shorter-billed Willet won't be able to reach deep enough into the mud to catch the prey that the Long-billed Curlew can.

The upcurved bill of the avocet is well-suited to its characteristic scything behavior, where it swishes the bill through the water just above the surface of the mud, capturing anything the bill touches as it sweeps. The straight-billed stilt is instead adapted for pecking and grabbing at organisms it sees.

Some of these variations are quite subtle. The bill of the Greater Yellowlegs looks very similar to that of the Willet, but look closely and you'll see a slight upward curve to the yellowlegs' bill that the willet's bill lacks.

It's not always about the bill. Great and Snowy Egrets have basically identical bills, body plans, and hunting strategies; the difference between them is simply that of size.

Size matters. Here is our snowy, stalking along in the water, looking for prey and then stabbing down to seize it. What kind of prey is she getting?

Worms. Yum! I watched this Snowy Egret catch half a dozen worms in a few minutes, barely taking more than a few steps between each capture. On the same evening, in the same place, I watch a Great Egret stand very still for a long while, barely twitching, then plunge down to make his kill.

It isn't that the Snowy Egret couldn't have caught that fish, or that the Great Egret couldn't catch worms. There are some large prey that the Great Egret can take which are out of the snowy's repertoire-I have seen a Great Egret eat a vole, which I don't think a snowy could subdue or fit down its throat-but that little fish is fair game for both of them. The difference here isn't one of ability but of costs and benefits. For the smaller snowy, the worms are big enough to be a good snack, worth the work of catching them. For the larger Great Egret, the worms are so small relative to his body size that it's better, instead, to ignore them and wait for something larger. The end result is that these two near-identical species can forage side-by-side, eating different things.

This is niche partitioning: a bay-ful of prey animals divided among predators large and small, visual and tactile hunters, long- and short-billed.

The divisions among the niches aren't perfect. The Snowy and Great Egrets would both happily eat a small fish, as would pretty much any other bird mentioned in this post. Animals can adjust their behavior, too: stilts usually forage visually, spotting prey and then pecking at it; but at night, or when the wind is so strong that it disturbs the surface of the water and makes it hard to see, stilts switch to scything through the water like avocets. Still, even slight differences in niche allow all of these species to pursue their own specialties most of the time-and let us humans enjoy diverse shores.

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The sun dips low over the bay, its fading rays gilding the avocets as they swish their heads through the water. The egrets eye their own reflections as if in profound self-contemplation. A willet flashes past, its black-and-white wings an exclamation in the dusk. Faced with such beauty, two words come irrepressibly to mind: niche partitioning.

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Trophic pyramid