Turtle Ants

The intricate eusocial lifestyle of ants is as fascinating in its uniqueness, as it is in its likeness to human societies. From fungus farming, to aphid ranching, to complex raiding tactics, parallels between ants and humans are inescapable. Although individual ants have next to no large-scale impact, working together a cohesive force capable of shaping entire ecosystems is formed: the colony. The success of ant colonies lies in the incredible selfless cooperation of individual ants for the good of the whole.

Polymorphism of castes within Leaf Cutter ants.

Like humans increase overall productivity by exchange of goods and services, ants increase colonial efficiency by dividing tasks among polymorphic castes. Each caste is responsible for a specific task, so body morphology evolved to best suit the caste function. While foraging worker ants are smaller and numerous, soldier ants are usually large and have powerful mandibles to defend the nest in case of an attack. Different environments have different selection pressures, and so ant caste morphology sees a great diversity of shapes and functions across the globe. One of the most robust defensive mechanisms within ants is the hardened head plate of Turtle ants.

Head disc of a turtle ant.

Head First 

Turtle ants are tree-dwellers and inhabit the canopy in equatorial tropics of North and South America. While most ants diligently construct or dig out their nests, some turtle ants have found a clever way to avoid the arduous building process. When searching for an appropriate nesting site, turtle ants look for branches that have been killed by wood boring beetles. These beetles excavate tunnels in the wood with their strong mandibles, and the turtle ants take over the hollowed out tunnels themselves. This not only saves time, but the wooden walls of their home are too tough for most insect predators to break through. Other turtle ants have been described to chew out the tunnels in the wood themselves with short convex mandibles.

Turtle ant defending the entrance.

But not every tunnel will fit the job. The defense tactic of turtle ants relies on burrows with an entrance diameter that precisely matches the size of the soldier ant caste’s head. The head of a turtle ant soldier is a highly sclerotized circular disc that acts as a door-like barricade during an attack. As the soldier ant backs into the tunnel, the disc snugly fits the opening, effectively excluding hopeful predators. Experimental work has shown that a colony will not settle a branch unless the entrance properly fits the soldier’s head, demonstrating the reliance of turtle ants on this defense. However, if an entrance is made larger after the ants have settled, two soldier ants may align themselves back to back so that both head discs cover the opening.

several turtle ants defending the nest entrance.

The tactic of defending a burrow with ones own body is called phragmosis, and is not exclusive to turtle ants. The Black Rugose Trapdoor spider, for example, has a hardened abdomen that it uses in an analogous fashion, giving it its name.

The sclerotized abdomen of a trapdoor spider.

 

Back to the Roots

Living in a 30m high arboreal environment, accidental or predator induced falling is inevitable and could leave the ant unable to find the way back to the nest. Being stranded in the dangerous insect world below is almost certain demise for a lone ant, so a safe way home is important for colonial survival. While turtle ants do not possess wings, they have evolved an alternative way to navigate the skies in the event of a fall.

Using their legs and enlarged heads turtle ants (along with some other arboreal ants) are able to control their fall with remarkable precision, essentially gliding back to the roots or trunk of their tree. Experiments have shown that the aim of gliding ants incredibly precise, with 85% of ants making a successful landing on the trunk of their nest tree. The ants navigate using visual cues, and are able to reposition themselves mid-flight to direct the glide path towards the tree.

Fitting in Pays Off

 Among the bustling metropolis of a Crematogaster ampla (not turtle ant) nest, certain members have a concealed identity and an ulterior motive. These intruders are so good at masking their true nature that even the native inhabitants see them as their own. This is a species of mimic turtle ant (Cephalotes specularis) that has adapted to look and behave like the ant Crematogastra ampla, and live along side it as a thieving social parasite. Social parasitism (when one animal exploits the lifestyle of another) is common in animals, but not often to this degree of perfection.

Mimic ant on the right, host ant on the left.

The mimic turtle ant nests along side the host, and follows the chemical pheromone trail laid by foraging ants to sources of food. The job of the mimic is very risky, as the host species is aggressively territorial, and upon finding non-mimic turtle ants encroaching on resources, C. ampla will often dismember the intruder.

To avoid dismembering, the mimic ants have to blend in visually and behaviorally. As opposed to the rough and hairy surface of most turtle ants, mimic ants have a black shiny abdomen to match the host. The low-lying body structure of turtle ants would also be conspicuous, so the mimics raise their abdomen when they are in the vicinity of their host. Despite these adaptations to fit in, the mimic ants have one undefended flank. They don’t have the same chemical scent as the host ant, making them vulnerable to tactile detection. If the host ant physically contacts the mimic, they will become aggravated and chase the intruder away. To avoid this, mimic ants often stay on the edges of the ant trail, minimizing chances of being smelled out.

The lesson is clear: laziness can get you by, but getting caught can be dismembering.

Sources:

Head First: Creighton, W. S. and R. E. Gregg, 1954. Studies on the Habits and Distribution of Cryptocerus Texanus Santschi (Hymenpotera: Formicidae), Psyche, 61:2 41-57

Back to the Roots: Yanoviak S. P., Dudley R., and M. Kaspari, 2005. Directed Arial Descent in Canopy Ants. Letters to Nature, 433: 624-626

Fitting in Pays Off: Powell, S., Del-Claro, K., Feitosa, R. M., Brandão, C. R., 2014. Mimicry and Eavesdropping Enable a New Form of Social Parasitism in Ants. American Society of Naturalists, 184, 4: 500-509

Please visit http://www.alexanderwild.com for a whole whack of cool turtle ant photos.

 

Sea Slug

What better creature to show off the incredible diversity and elaborate adaptations of invertebrates than the multifaceted and undoubtedly adorable sea slug? Looking at the ornate colour display and alien morphology of the sea slug, it is difficult to imagine that it bears any relation to its modestly dressed land-lugger counterpart.

Sea slugs are found in nearly all coastal marine environments, but their abundance and polymorphism flourishes in tropical waters. The vulnerable soft-bodied nature of the sea slug has resulted in creative adaptations of the outer layer (mantle) to deter predators. Whereas some become almost invisibly camouflaged into the background, others flamboyantly display their distastefulness to predators with bright aposematic coloration. As well as being popular with divers, sea slugs are eaten by humans in parts of Russia and China, and have been instrumental to neurobiological studies because of their unusually large axons.

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Smelling the Way  All sea slugs possess two chemosensory tentacles on their heads called rhinophores. As the tongue does in mammals, rhinophores  are able to detect molecules dissolved in water, ultimately functioning as the smell and taste organs. Because sea slugs have poor (or no) vision, it is very important to have sensitive rhinophores in order to find food or detect pheromones of potential mates.

Rhinophores have a variety of shapes aimed to maximize the surface area so that more sensory cells to be exposed to water. Having two rhinophores enables the sea slugs to detect the relative difference in chemical concentration between the left and right rhinophore, thereby determining the direction of the smelly source. Hammer head sharks similarly navigate with smell by having nostrils on opposite sides of the “hammer”, and moths have enlarged and very elaborate antennae for the same reason. Since the rhinophores are so important to the sea slug lifestyle, many have adapted to retract them into the body when under attack.

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Stolen Weapons

A nudibranch everting its mouth to eat an anemone.

All slugs are molluscan gastropods that have lost or greatly reduced their shell, and sea slugs are no different. While the loss of the shell increases the mobility, it obviously leaves slugs more vulnerable to predation compared to their home-owning snail relatives.  The seemingly counterintuitive loss of protection has freed the mantle of the sea slug to be subject to vigorous natural selection, resulting in terrific alternatives in self defence.

Many sea slugs are toxic or otherwise unpleasant to predators, but there’s a twist: most sea slugs are unable to produce deterrents themselves. So where do they get them from? The answer is simple and brilliant: they steal them from their prey.

Aeolid nudibranch simple anatomy.

Although as a group sea slugs have a wide range of prey, individual species are restricted to a single food source.  This means that defensive strategies vary greatly among sea slugs, and are specific to the dietary restriction each type of slug. Sponge eating sea slugs for example concentrate toxic sponge biomolecules in their outer layer, thus becoming distasteful to predators.

Sea slugs that feed on anemones and hydroids have adapted to take up the cnidocytes (stinging cells of anemones and jelly fish) undigested. From the digestive tract they transfer the cnidocytes into bundles on their extremities called cnidosacs where hopeful predators would attack first. Nudibranch (naked gilled) aeolid sea slugs have beautiful horn-like projections called cerata, at the tips of which the cnidosacs are located. In case the cnidocytes fail to deter , the cerata readily fall off to distract the predator, and can later regrow leaving the slug essentially undamaged. This behaviour is called autotomy, and is the same tactic a salamander uses when it loses its tail.

Sea hares have a unique talent among sea slugs: they are able to produce ink isolated from the algae they eat. Unlike the speedy octopus, which vanishes like a magician in the cloud  of ink, the sea hare would not get very far by the time the ink dissipated in the water. Why and how sea hares use ink was therefore a mystery to biologists until quite recently. Interestingly, the ink itself has few deterring properties on its own, but it mixes with a viscous substance released from separate glands called opaline. The ink/opaline mixture then coats the antennae of predators such as lobsters, dulling their senses and effectively killing their appetite. Other research has even suggested that the ink mixture is so appealing to some predators, that it distracts them from the sea hare itself, diverting all attention to the ink.

Sea hare excreting red ink.

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Solar Powered Sea Slug?

Elysia chloritica full of tasty chloroplasts.

Like other slugs steal the defences from their prey, the algivorous sea slug Elysia chloritica has developed a curious tactic, earning it the nickname “Crawling Leaf” . While the majority of the consumed algae is digested by the slug, the chloroplast is left undamaged and transported to the mantle giving the slug a leaf-like appearance. The sea slug mantle even provides an environment for the chloroplasts to continue functioning and photosynthesizing for up to three months (though how it does that is still a mystery). For some time it was believed that E. chloritica was actually harnessing the sugars produced by the chlorophyll as an energy supply, essentially making the slug a living solar panel. Unfortunately for fun fact enthusiasts, more recent studies involving photosynthesis inhibiting drugs refute this hypothesis.  Current research suggests that the slug may be sequestering the chloroplasts in order to blend in with the surrounding green environment.

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Mating Like Sea Hares

Sea Hare mating chain. Sexy!

Sea hares are a type of sea slug with rounded bunny-ear rhinophores, and exhibit an interesting and sexy mating behaviour. Sea hares are hermaphroditic, like all slugs, but uniquely get together in groups of three to six individuals for reproductive “orgies”. The sea hare aggregation forms what is called a mating chain, whereby the front individual acts as a male, the back individual as a female, and those in-between act as both males and females. Pheromone release by sexually mature sea hares triggers this get-together, and sometimes attracts members other species suggesting low pheromone specificity.

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More Fun With Sea Slugs! 

Sea Slug recipe translation from 1913, a highly amusing read: http://www.hallman.org/indian/slug.html

Very informative sea slug site (albeit a bit outdated): http://www.seaslugforum.net

Sea Hares (and other molluscs): http://www.molluscs.at/gastropoda/index.html?/gastropoda/sea/aplysiomorpha.html

Awesome documentary that features sea slugs and other incredible animals of the Lembeh strait in indonesia: http://www.youtube.com/watch?v=nJMZ6reOB0E