OSTRACODS' OXYGEN PUZZLE

Abstract

According to palaeontological records, two billion years ago atmospheric oxygen levels were a tenth of the levels they are today. Prehistoric water-breathing crustaceans presumably adapted to these low levels, but when oxygen levels rocketed, some creatures took advantage of the metabolic advantages offered and allowed their blood oxygen levels to rise. However,crustaceans seem to have stuck to their old-fashioned low-oxygen habits; they have remarkably low blood oxygenation levels even to this day. Oddly enough,despite the large evolutionary gap between crustaceans and warm-blooded animals, modern mammalian brains share the low oxygenation levels of crab and crayfish circulation. Jean-Charles Massabuau and Laure Corbari at Bordeaux University wondered if this striking similarity was evidence that the crustaceans' prehistoric oxygen regulation strategy was preserved over evolutionary time (p. 4415).Massabuau reasoned that if he could show how primitive water-breathers were able to regulate their tissue oxygenation levels in prehistoric times, this would reveal an early adaptation strategy that could have maintained low tissue oxygenation levels in animals throughout the course of evolution. Massabuau needed a primitive animal, so when Pierre Carbonel told him about tiny creatures called ostracods that hadn't changed much in the past 500 million years, Massabuau realised he had found an `open window into the past'to study ancient oxygen regulation strategies.The team decided to focus on the ostracods' breathing apparatus. Ostracods are minute crustaceans, complete with little appendages that beat rhythmically to waft water into the animal's breathing cavity, where gas exchange occurs by diffusion. Wondering whether the diminutive creatures were able to adjust their breathing rate at different oxygen levels, the team designed miniature aquaria, allowing them to change the oxygenation levels in the mini-habitats. The team videotaped the crustaceans to see whether the animals would beat their breathing appendages faster as oxygen levels dropped. The team were surprised when they saw that the ostracods didn't change the beating frequency of their breathing apparatus at different oxygen levels. Clearly, the ostracods couldn't regulate their ventilation in response to changes in water oxygenation. So how had the animals coped for millions of years?The team suspected that ostracods might regulate their oxygen by choosing to live at particular oxygen levels. To test this, they needed to study the distribution of the little sediment-dwellers in their natural habitat. They didn't have to travel far to collect sediment core samples: the Bay of Arcachon in France, where ostracods live, is right outside their lab. The researchers then determined the ostracods' preferred oxygen levels by measuring the oxygen profile of each sediment sample, freezing and slicing up the samples and counting the number of animals in each slice. The team found that the crustaceans were escaping both oxygen-rich and oxygen-depleted regions and migrating to sediments where the oxygen levels were slightly higher than those required by their tissues. The ostracods were regulating their body oxygenation simply by crawling to their comfort zone.So 500 million years ago, these tiny crustaceans were already managing their tissue oxygenation levels using a behavioural rather than a physiological strategy, which could have been conserved over evolutionary time. But what is the evolutionary advantage to maintaining such low body tissue oxygenation? `One explanation is that metabolism produces oxygen free radicals, which damage cells', says Massabuau, `so maintaining low oxygen levels may protect an animal's tissues from free radical attack'. It seems ostracods may have a very good reason for being stuck in the past.