Komodo Dragons and Invasive Crayfish

Nature-Nurture-Noise (6)

I have been fascinated with Komodo Dragons since I first learned of them as a kid. A lizard that hunts pigs and deer seemed otherworldly. In 2017, I finally had an opportunity to see some in the wild at Komodo National Park, which consists of three small islands: Komodo, Rincon and Padar, soon to be their last refuge.  The individual pictured above was one of the larger and more curious of the ten or so we encountered. The long snake like forked tongue—a dragonish feature—is their primary sensory organ, constantly flicking, tasting the air with sensitive receptors, by means of which they detect prey, which consist mainly of Timor deer (Rusa timorensis). But they are not at all averse to the much larger water buffalo. Komodo dragons are the apex predators on the islands, a role facilitated by their size (males can approach 200 pounds after a large meal), by far the largest of any lizard.

Humans have fallen prey to these beasts, which is why all visitors are accompanied by rangers. But they consume human flesh mostly in the form of cadavers; In the past, graveyards were a favored source of carrion. Now, villagers go to great lengths to ensure the spirits of their dead remain undisturbed. For Komodo dragons are as much scavenger as hunters. Their diet is purely carnivorous in either case, to the extent that they avoid the gastric contents of the herbivores they consume, sometimes regurgitating it. Zero tolerance for plant matter.

As can be seen in these photos, Komodo dragons are constantly drooling, which does nothing to lessen their eerie countenance. Their saliva is thought to be mildly venomous, which may speed death in prey that survive an initial attack; escape is only temporary, as dragons assiduously track their injured victims. Just before disembarking, I saw an adult Timor deer buck, kneeling in chest high sea water, a strange tableau. Upon closer inspection with my binoculars, I could see nasty wounds on its head and flank. On the shore, a dragon waited patiently.

Komodo dragons can be seen in a few zoos, but they are not particularly charismatic in that environment. When when well fed, they mostly sleep. But it is in zoos that another dimension of these uncanny creatures was revealed: virgin birth. Parthenogenesis was first documented at the London Zoo in 2005, a female last exposed to males two year prior layed a clutch of eggs, some of which survived. Then, in 2006, a female at the Chester Zoo in England, who had never been exposed to a male dragon, gave birth to seven viable offspring, all male. And, in 2008, parthenogenesis was again documented, this time at the Wichita zoo.

There is potential here for a study along the lines of that conducted on armadillos (post 2 in this series). It would be fascinating to look for evidence of random individual differences, through comparisons among sibling clones. That’s not a priority, though, for those who care for them; and there are clearly logistical issues in making these measurements on such formidable creatures.

This type of virgin birth is called sporadic (sometimes facultative) parthenogenesis, and it seems to be a latent capacity in many animals. Sporadic parthenogenesis is best documented in captive situations. Other examples in captivity include several shark and ray species, and a number of snake species. Among birds, sporadic parthenogenesis has been documented in captive California condors, chickens, turkeys, ducks and pigeons. Mammals, seem to have epigenetic constraints (genomic imprinting) that preclude parthenogenesis.

Though more common in captive environments, sporadic parthenogenesis is probably much more common in nature than previously believed 10.1098/rsbl.2012.0666 . Virgin births in nature have been demonstrated for a several lizards and snakes, and one crocodile species. And the list is growing. Individual night lizards (Lepidophyma smithii) can produce both parthenogenic and sexual offspring in the same clutch https://doi.org/10.1111/mec.15617. This species should be considered as a candidate model organism for random developmental variation. But there is a much better option at hand.

Crayfish Invaders

It is called the marbled crayfish (Procambarus virginalis). The scientific species name says it all. As a species, marbled crayfish are of recent vintage, born around 1995. At least that is when this species was first recognized as such in the German aquarium trade. It began with a single incidence of sporadic parthenogenesis in a female slough crayfish, Procambrus fallax, native to rivers of the southeastern U.S., The slough crayfish has long been popular in the aquarium trade. The creation of the marbled crayfish from the slough crayfish was a singular event, the result of defective meiosis in the mother of this species. Usually this would end in a quick termination of defective embryos. This time, though, the lucky mother drew the equivalent of an Inside Straight and Royal Flush combined. She produced healthy female offspring, genetically identical to her and each other.  Her direct descendants, in turn, generated more clonal offspring until a new species was born, the females—and they were all females—of which would not interbreed with males of the slough crayfish, the parent species. Keeping the line pure.

Marbled crayfish soon spread throughout the Aquarium trade in Europe and America. Unfortunately, for many other crayfish species, some marbled crayfish either escaped or more probably were deliberately released, first in Europe. The problem for other crayfish was that these marbled crayfish outcompeted them, in part because of their superior fighting ability.

Marbled crayfish have now spread throughout Europe, aided in no small part by their asexual mode of reproduction. A single marbled crayfish can be the source of a whole new population, which is especially problematic for an invasive species. One was released in Madagascar, probably around the year 2001; now they number in the millions and are rapidly displacing native crayfish species there.

On the plus side, marbled crayfish are ideal for investigating random sources of individual differences. Gunther Vogt was the first to recognize this. In his lab at the University of Heidelberg he reared marbled crayfish in highly standardized conditions to minimize environmental variation; he then monitored a number of physical and behavioral traits https://doi.org/10.1111/j.1469-7998.2008.00473.x.

Let’s begin with coloration. The marbling pattern in marbled crayfish results from the interactions of two types of pigment cells: one type produces red pigment, the other white. The interactions of these two pigments over the entire carapace produces patterns as variable as human fingerprints. Every genetically identical individual crayfish has her own unique pattern, which she retains throughout her life despite repeated molts during which the old carapace is shed and replaced by a new one. These patterns can be used much like fingerprints for individual identification. Also, like human fingerprints, the marbling pattern is subject to fluctuating asymmetry: The left and right halves are not identical. For identification purposes, each individual must be photographed on both sides.

Perhaps it’s not surprising to find developmental randomness in color patterns given that the number of possible configurations is vast. But what about size variation? Shouldn’t all marbled crayfish clones reared in isolation, under near identical conditions, be the same size at a given age? Quite unexpectedly, they aren’t. At day 152 their length and weight were measured revealing massive size variation. Even more surprising, to me at least, was the variation in life-history traits, for example, age at first reproduction. One began spawning as early as day 157 (about 5 months), another at day 531 (about 18 months), quite a spread. Lifespan had an even broader range (437-910 days). We will return to the random component in lifespan for diverse creatures, including humans, in a later post.

There were interesting behavioral differences within these clones as well. From the time they could first swim there were noticeable differences in how they moved and how much. Morover, as juveniles and adults, Some preferred to be alone, others aggregated. Some adults from the same batch preferred to sit while resting in their shelters, others lay on their backs, consistently. When it came time to molt, some preferred to do it in the morning, some at night. The same was true for egg laying.

There was also variation in aggressive behavior and social dominance. Even when given equal access to food the dominant individuals grew faster, further stabilizing the dominance hierarchy, reinforcing offensive and defensive behavior. The same relationship between dominance and faster growth is found in sexual crayfish as well, but usually explained as genetic differences in the efficiency of food conversion and aggression. In marbled crayfish, it seems, randomly generated behavioral and physiological differences, however slight at the outset, can produce a wide range of behavioral individuality. When it comes to behavior, of course, we look to the brain, where the potential for random processes is immense. Marbled crayfish have not been investigated in that regard. For that we must turn to another animal, the most widely and thoroughly studied in all of biology. That will be the subject of the next post in this series.