Science often answers questions that I never would have thought to ask. For example, what happens to a pig carcass when you leave it on the ocean bottom?
This isn’t science trivia. While scientists know quite a bit about how bodies decompose on land, forensic researchers Gail Anderson and Lynne Bell point out at the start of their new PLoS One study, most of what we know about undersea carcasses comes from studies of whales, porpoises, and sharks. Very little is known about the waterlogged afterlives of mammals that are more like humans in size and anatomy.
To change that, Anderson looked to pigs – often used as proxies for humans in such studies because of their comparable size and skin. In a previous experiment with N. Hobischak, Anderson documented what happened to a trio of freshly-killed pigs that the researchers anchored in the shallows of British Columbia’s Howe Sound. That study outlined what sort of critters came to dine on the pigs, the effects of depth on decomposition, and other factors of breakdown, but there was a problem. The researchers had to dive to gather their data, meaning that they were missing some of the changes in the porcine breakdown. They lacked a continuous picture of what was happening to the pigs. Picking up where the previous study left off, the new research by Anderson and Bell tells a more detailed tale of what happens to such submerged bodies.
At the rate of one pig each year between 2006 and 2008, the researchers lowered three pig carcasses into British Columbia’s Saanish Inlet at a depth of about 313 feet. The choice of location was critical to what eventually unfolded. For most of the year, Anderson and Bell point out, the inlet water is relatively low in oxygen, only replenished in the fall. And yet, despite these hypoxic conditions, the inlet hosts a rich assemblage of marine critters that would eventually play a part in the breakdown. And as a scientific benefit, the inlet is the base site for the Victoria Underwater Network Under Sea (VENUS), equipped with instruments that allowed Anderson and Bell to gain a constant stream of data on temperature, pressure, dissolved oxygen, and other measurements from the study sites.
Of course, “carcass deployment” and study presented some unique challenges. Each pig had to be weighted down so that they’d stay within range of the underwater cameras needed to photograph them. (Although this didn’t prevent the first carcass from being dragged out of range by crabs after 23 days.) And the researchers had to think carefully about turning on the camera lights. Some of the little scavengers would scatter when the lights flicked on, although the typically returned very quickly. Even so, to prevent the lights from greatly affecting the habits of the dinner guests, the researchers turned on the camera lights sparingly.
I’ll let Anderson and Bell describe what happened once the first two pigs were in position:
Within minutes of placement, large numbers of Munida quadrispina Benedict (squat lobsters, Family Galatheidae) arrived at Carcass 1 and 2 and began to pick at the skin, attracted to the entire carcass, with some preference for the orifices. Scanning the camera around the area showed that very large numbers of M. quadrispina were actively moving towards the carcasses from all areas.
Three-spot shrimp and Dungeness crabs soon appeared, as well, and the scavengers were so quick with their work that, as the researchers report, “On Day 2, a substantial portion of the rump area of Carcass 1 was removed, and a large flap of skin and flesh from the abdominal area was opened.” Crustaceans did most of the disassembly, although a passing blunt-nose sixgill shark also took a sizable chunk from the pig.
The rest of the paper is full of macabre details delivered in a scientific deadpan that could almost pass as gallows humor – “[O]n Day 4 M. magister was seen pulling the tongue out of the mouth and consuming it” – but the scientific details of the breakdown gave Anderson and Bell a serious look at what happens to bodies under shifting sea conditions.
Pig carcasses 1 and 2 followed a similar pattern. Both dropped at times of relatively low dissolved oxygen, the pigs immediately attracted a horde of shrimp, squat lobsters, and crabs that rapidly dismantled the bodies. Pig carcass 3, however, was lowered at a time of even lower dissolved oxygen. Some squat lobsters showed up at the beginning, but their claws weren’t powerful enough to do anything more than graze the surface of the carcass. Without larger crabs with more powerful snipping apparatus, the squat lobsters left and a furry mat of bacteria grew over the body. The pig lay there almost undisturbed until the oxygen in the water was refreshed. Only then did both invertebrate and vertebrate scavengers make short work of the carcass.
The pigs didn’t just rot away. They were consumed down to the bone. While the first carcass was dragged out of view before the end of the experiment, total skeletonization took 38 days for carcass 2, and 135 days for carcass 3 owing to the long period of stasis due to low oxygen.
The overall pattern was quite different from what Anderson observed in the previous study. In the Howe Sound experiment, the pig carcasses went through a familiar pattern of decomposition that included initial floating, sinking, and “bloat and float” – which is exactly what it sounds like – before finally settling on the bottom. In the new experiment, the pigs didn’t fill up with decompositional gases. The water pressure at their depth prevented them from doing the dead pig’s float. The pigs stayed put unless dragged out of place by the scavengers.
One of the lessons from all this, Anderson and Bell found, is that the arthropod community surrounding a cadaver found at sea isn’t a reliable indicator of when that body entered the water. Anderson and Bell didn’t observe any rigid succession of scavengers that could be used to estimate time of death. Instead, varying oxygen levels dictated the degree of scavenging – skeletonization of carcass 3 took three times as long because of low oxygen conditions at the time the pig entered the water. Translating this to search and rescue efforts, a body that came to rest at a depth more than 200 feet in very low-oxygen conditions will be more likely to be in place and intact because the conditions are inhospitable to scavengers.
And at a much more minute level, studying the damage done by the scavengers can be critical to investigating human bodies that wind up in the ocean. “[I]t is not uncommon for disarticulated human appendages to be recovered washed up on beaches,” Anderson and Bell write, “leading to media speculation of dismemberment and foul play.” But, as the researchers found, scavengers can easily dismember and disarticulate a body, leaving tell-tale damage behind. Understanding how scavengers feed, and how quickly they can alter a body, can be critical to accurately reconstructing marine crime scenes. When authorities think they have a maritime murder on their hands, crabs can be critical in court.
Anderson, G., Bell, L. 2014. Deep coastal marine taphonomy: Investigation into carcass decomposition in the Saanich Inlet, British Columbia using a baited camera. PLoS One. 9 (10): e110710. doi:10.1371/journal.pone.0110710