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The Greatest Show on Earth, happening now

May14
The best time to see Atlantic horseshoe crabs (Limulus polyphemus) is on the nights of the full and new moon in May and June.

The best time to see Atlantic horseshoe crabs (Limulus polyphemus) is on the nights of the full and new moon in May and June.

I am still in Mozambique, and will be here for a few more weeks, but I simply must take a quick break from describing African nature to highlight a spectacular phenomenon that is taking place right now along the eastern coast of North America – the mass spawning of the Atlantic horseshoe crabs (Limulus polyphemus). Watching these magnificent animals is to me one of the most beautiful natural events that one can witness, and I encourage everybody living on the East Coast to take a trip to the beach this and next month (this year the best time to see them are nights of May 24th, and June 9th and 23rd.) What follows is a short excerpt from my book “Relics” (Chicago University Press 2011), describing my experience of watching horseshoe crabs on the beaches of the Delaware Bay.

“As hundreds of biting flies did their best to drain us of every drop of blood, my friend and fellow photographer Joe Warfel and I stood on the beach, waiting for the spectacle to begin. The sun grew dim, and the high tide was nearing its peak. There were a few people on the beach when we first arrived, but by now they had all disappeared, and we were the only witnesses to what was about to unfold. I started to tell Joe how strange it was that nobody else stayed to watch, but swallowed a fly and decided to quietly enjoy the rest of the evening. First came the big females. Nearly all had males in tow. In the dimming light we could see spiky tails of hundreds more as they tumbled in the waves, trying to get to the dry land. By the time the sun fully set, the beach was covered with hundreds of glistening, enormous animals. Females dug in the sand, making holes to deposit their eggs, nearly 4,000 in a single nest, while the males fought for the privilege of fathering the embryos. Fertilization in horseshoe crabs is external, and often multiple males share the fatherhood of a single clutch. Equipped with a pair of big, compound eyes (plus eight smaller ones), capable of seeing the ultraviolet range of the light spectrum, male horseshoe crabs are very good at locating females even in the melee of waves, sand, and hundreds of other males.

Delaware Bay is the best place in the world to see these magnificent animals. On a good night one could easily see 100,000 horseshoe crabs.

Delaware Bay is the best place in the world to see these magnificent animals. On a good night one could easily see 100,000 horseshoe crabs.

Horseshoe crabs have been around longer than most groups of organisms that surround us now. A recent discovery in the fossil deposits of Manitoba, an interesting little creature named Lunataspis aurora, proves that horseshoe crabs quite similar to modern forms were already present in the Ordovician, 445 million years ago. By the time the first dinosaurs started terrorizing the land in the Triassic (about 245 million years ago), horseshoe crabs were already relics of a long-gone era. And yet they persisted. Dinosaurs came and went, the Earth changed its polarity and climate many times over, but horseshoe crabs slowly plowed forward. Yet during this time they changed surprisingly little. Species from the Jurassic were so similar to modern forms that I doubt I would notice anything unusual if one crawled in front of me on the beach in Delaware. Somehow horseshoe crabs had stumbled upon a lifestyle and morphology so successful that they were able to weather changes to our planet that wiped out thousands of seemingly more imposing lineages (dinosaurs and trilobites immediately come to mind.) But despite claims to the contrary by creationists and other lunatics, they kept evolving. Modern horseshoe crabs, limited to three species in Southeast Asia and one in eastern North America, differ in many details from their fossil relatives. We know, for example, that many, if not most of fossil horseshoe crabs lived in freshwater, often in shallow swamps overgrown with dense vegetation, and some might have even been almost entirely terrestrial. Currently only the mangrove horseshoe crab Carcinoscorpius rotundicauda from the Malayan Peninsula routinely enters rivers, and is the only species to lay eggs in fresh or brackish water.

Even Sir David Attenborough, the man who probably witnessed more natural spectacles than any other human being, is fascinated by the spawning of horseshoe crabs. Here he demonstrates the improper way of holding a horseshoe crab (never hold them by their telson) while on the beach in Delaware during the filming of the BBC series "Life in the Undergrowth".

Even Sir David Attenborough, a man who probably witnessed more natural spectacles than any other human being, is fascinated by the spawning of horseshoe crabs. Here he demonstrates the improper way of holding a horseshoe crab (never hold them by their telson) while on the beach in Delaware during the filming of the BBC series “Life in the Undergrowth”.

The following morning Joe and I found the beach covered with horseshoe crab eggs. Well-rested and ready to start a bright new day the flesh-piercing flies attacked us with a renewed enthusiasm. Flailing our arms and swatting dozens at a time we went about flipping crabs stuck on their backs in the sand, and started to look for particularly big clutches of eggs. Although females burry the eggs in the sand, the returning tide washes out many of them. Freshly laid eggs are small, not larger then half a grain of rice. Surprisingly, the eggs grow as they develop, eventually becoming more than twice as large. This, of course, is impossible. The “growth” is an illusion, the result of the production of an external, thin membrane by the developing embryo. A fully developed egg, which at this stage has spent two weeks in the sand, resembles a tiny glass aquarium, with a petite horseshoe crab twirling inside, impatient to break the walls of its miniature prison. Once free, the larva (or at least the lucky ones) catches a wave back into the ocean and will spend about a week floating freely, before settling on the bottom of the shallow shore waters to begin life akin to that of its parents.[…]“

Tiny horseshoe crab larvae, known as the trilobite larvae, twirling in their aquarium-like egg shells. Soon they will break free to begin a short pelagic period, after which they settle on the bottom of the ocean to begin a lifestyle similar to that of their parents.

Tiny horseshoe crab larvae, known as the trilobite larvae, twirling in their aquarium-like egg shells. Soon they will break free to begin a short pelagic period, after which they settle on the bottom of the ocean to begin a lifestyle similar to that of their parents.

Just like their distant relatives, scorpions, horseshoe crabs display green fluorescence under the ultraviolet light.

Just like their distant relatives, scorpions, horseshoe crabs display green fluorescence under the ultraviolet light.

Limulus5

Atlantic horseshoe crabs on the Prime Hook Beach near Milford, Delaware.

Mozambique Diary: I have fallen and I can’t get up

May10
Sunrise over Nhagutua Gorge in Gorongosa National Park

Sunrise over Nhagutua Gorge in Gorongosa National Park

The best part of traveling with a group of biologists in a place like the Cheringoma Plateau is the impossibility of ever being bored. Not only can you witness hilarious and exotic injuries (where else do people get bitten by bats?), but every day brings new discoveries of things you never thought existed. At our first camp, which was located on the rim of a deep, stunningly beautiful limestone gorge called Nhagutua, our team’s myrmecologist Leeanne Alonso found a colony of what is arguably the most amazing ant species on the planet. Leeanne was collecting ants running on the trunk of a large Knobthorn (Acacia nigrescens), and on a whim pulled off a piece of bark from the tree. To her surprise she found underneath a nest of tiny, yellow ants, which she immediately recognized as members of the genus Melissotarsus.

Leeanne Alonso collecting Melissotarsus ants from an acacia tree

Leeanne Alonso collecting Melissotarsus ants from an acacia tree

A few things make Melissotarsus stand out among other ants. Unlike other species, their adult workers retain the ability to spin silk (a characteristic typical of many ant larvae), which is produced from a gland on the underside of the head and spun with special brushes on the tarsus. The brushes pull the viscous fibers from the silk spigots, stretch, shear it, and line the wood corridors with it. The silk lining presumably helps prevent the invasion of the colony by predators, and may help keep the corridors free from fungi and other pathogens.

Melissotarsus ants are incapable of walking or even standing on flat, smooth surfaces, and immediately fall on their back, unable to right themselves up.

Melissotarsus ants are incapable of walking or even standing on flat, smooth surfaces, and immediately fall on their back, unable to right themselves up.

Melissotarsus workers never leave the nest to forage outside, for two important reasons. For one, they have a strongly reduced sting, which makes them highly vulnerable to attacks by other ants. In fact, minutes after Leeanne had opened their nest it was invaded by Crematogaster and Pheidole ants, which quickly wiped out all workers and larvae present in the exposed corridors. But even more importantly the ants could not forage outside their narrow corridors even if the world outside was safe and friendly, for the simple fact that they cannot walk. Yes, these six-legged insects are incapable of walking or even standing on flat surfaces outside the narrow confines of their nest. Inside the narrow wood corridors they move in a way reminiscent of a rock climber squeezing between two vertical walls by pressing the back and knees against the wall (a move known as stemming). Their first and third pair of legs points down, like in all other insects, but the second pair of legs is usually held up, with the feet pressing against the ceiling. I placed a few individuals on the flat surface of my portable photo studio, and they immediately fell on their back and flailed their legs helplessly.

Opened corridors of the Melissotarsus (probably M. emeryi) colony within the wood of Knobthorn (Acacia nigrescens). The yellow objects are diaspidid scale insects, which the ants raise for their meat.

Opened corridors of the Melissotarsus (probably M. emeryi) colony within the wood of Knobthorn (Acacia nigrescens). The yellow objects are diaspidid scale insects, which the ants raise for their meat.

Since these insects cannot leave their nest inside the tree, they are forced to obtain all their food there. They do so by raising scale insects, mostly of the family Diaspididae. Symbiotic relationships between ants and scale insects are not uncommon – many ant species tend and protect them in exchange for honeydew produced by these floem-feeding insects. Melissotarsus also collect honeydew from some species of scale insects and groom them constantly, removing wax from the insects’ bodies and preventing the formation of the protective shield that scale insects are typically covered with. But the bulk of their food comes not from the sugary water exuded by the scales (in fact, most of the species they raise don’t produce honeydew), but rather from slaughtering the scale insects for meat. This, as far as I know, is the only example outside of human societies of an organism raising another species for its flesh, and adds animal husbandry to the already impressive list of seemingly human skills (e.g., architecture, farming, solar navigation) that ants had evolved long before our species first appeared on Earth.

(You can see photos of another species of Melissotarsus on Alex Wild’s blog.)

A Melissotarsus worker walking through a narrow passages of her nest; notice the second pair of legs, which is pressed against the ceiling of the corridor.

A Melissotarsus worker walking through a narrow passages of her nest; notice the second pair of legs, which is pressed against the ceiling of the corridor.

A winged, reproductive male of Melissotarsus. These are the only members of the colony capable of walking on flat surfaces.

A winged, reproductive male of Melissotarsus. These are the only members of the colony capable of walking on flat surfaces.

Mozambique Diary: Sylvan katydids of Gorongosa

Apr8
Male Elegant sylvan katydid (Acauloplax exigua) in his typical resting position.

Male Elegant sylvan katydid (Acauloplax exigua) in his typical resting position.

A few nights ago, as I was walking towards my cabin along the edge of the Chitengo Camp, I heard a call of a cricket that I did not quite recognize. Cricket calls are unmistakable for their clean, almost melodious quality, very different from the call of a cicada or a katydid, which tend to be more “noisy.” But his call was very pure, almost bird-like, and it was coming from high in a tree. I expected to find a black, wide-winged Homoeogryllus cricket among the leaves, but to my delight the mystery caller turned out to be a gorgeous Blue-legged sylvan katydid (Zabalius ophthalmicus). This and a few related species produce some of the most pure tone, almost whistle-like calls, a rarity among katydids.

Sylvan katydids (Pseudophyllinae) are uncommon in this part of the continent. The greatest diversity of these insects is found in the rainforests of Central and West Africa, where most live high in the canopy. In Mozambique I expected to find only four species, and indeed found them all in Gorongosa (which of course is not to say that more are not to be discovered.) Virtually all sylvan katydids, true to their common name, are associated with forests and other woody habitats, and few are found in the open savanna.

Southern African sylvan katydids display two very distinct types of mimesis (camouflage). Species found in the canopy of broad-leaved trees (e.g., Ficus) are superb mimics of foliage, complete with leaf-like venation of their wings and fake fungal spots or other “damage.” During the day they rest with their wings spread flat against the lower surface of the leaf, and are absolutely impossible to find. (Interestingly, this type of behavior does not occur in any katydid species found in the New World – all North and South American katydids hold their wings in a vertical position, and during the day rest on twigs with their wings facing up in an imitation of a small leaf.)

Greater bark katydid (Cymatomerella spilophora)

Greater bark katydid (Cymatomerella spilophora) in the woodland of Gorongosa

The second group of species of Mozambican sylvan katydids are bark mimics. These species are associated with more open habitats, mostly miombo or mopane woodland, and spend the day resting on trunks of small-leaved trees, such as Acacia or Brachystegia. Their wings are held similarly flat against the bark, and their coloration is mottled, resembling the surface of the trunk. In addition, their legs are strongly flattened and covered with dense hairs, which helps them eliminate the shadow cast by their bodies.

One of the Mozambican species, The Common bark katydid (Cymatomera denticollis), is unusual among katydids in its ability to produce chemical defenses. Production of repellant chemicals has been documented in a few Neotropical species, but this is the first example of such a behavior in an African species. These insects, when threatened by a predator, fan their wings and reveal a brightly colored, red, orange, and black abdomen. At the same time a gland on their abdomen sprays a strongly smelling liquid. I have had no chance to look into the chemical composition of this substance, but its smell is very much reminiscent of that produced by their (unrelated) South American counterparts. In those katydids the repellant substances were identified as methylpyrazines, and I would not be surprised if the African species produced related compounds.

A song of the Blue-legged sylvan katydid (Zabalius ophthalmicus). Click here to listen to the recording: first played at the normal speed, followed by a fragment slowed down by a factor of 10 (for the katydid-challenged listeners, i.e., most males over 35).

A song of the Blue-legged sylvan katydid (Zabalius ophthalmicus). Click here to listen to the recording: first played at the normal speed, followed by a fragment slowed down by a factor of 10 (for the katydid-challenged listeners, i.e., most males over 35).

Female Elegant sylvan katydid, showing fake leaf damage and leaf-like venation on her wings.

Female Elegant sylvan katydid, showing fake leaf damage and leaf-like venation on her wings.

Sylvan katydids of Gorongosa in their typical daily resting poses: Blue-legged sylvan katydid (Zabalius ophthalmicus), Elegant sylvan katydid (Acauloplax exigua), Common bark katydid (Cymatomera denticollis) and Greater bark katydid (Cymatomerella spilophora).

Sylvan katydids of Gorongosa in their typical daily resting poses: Blue-legged sylvan katydid (Zabalius ophthalmicus), Elegant sylvan katydid (Acauloplax exigua), Common bark katydid (Cymatomera denticollis) and Greater bark katydid (Cymatomerella spilophora).