In a body of water as large as the Pacific Ocean, you might think that salinity levels would be fairly constant; and they are in the ocean depths. But head closer in to shore, and it can be a very different matter. At stormy times of year, coastal freshwater run-off can dramatically reduce the water's salinity, and the situation is often exacerbated at low tide. So how do coastal sea creatures cope with this environmental variability? Some have gone for the metabolically costly option of maintaining high tissue salt levels,even when the sea is relatively dilute, while others go with the flow,allowing their salt levels to rise and fall. The medium-sized graceful crab(Cancer gracilis), found along the Pacific coast of North America, is one coastal resident that has opted for the low-cost alternative, allowing its salt levels to fluctuate, even when the salinity becomes perilously low. Iain McGaw from the University of Nevada at Las Vegas was intrigued to find out how the animals cope during a bout of low salinity with an additional metabolic burden; digestion. Knowing that most animals ramp up their metabolism during digestion, McGaw was curious to find out how the crabs balance the potentially conflicting demands of low salinity survival with the demands of digesting dinner (p. 3766).
Fortunately, McGaw didn't have to worry about keeping the crabs in the Mojave Desert. He had long-term contacts with the Bamfield Marine Sciences Centre on the west coast of Vancouver island, so he packed up his Nevada lab and drove almost 1500 miles north to the remote field station to investigate the crabs' habits. Having set up home for a 15-month sabbatical, McGaw collected crabs from the ocean, ready to measure their physiological responses to digestion in low salinity.
But first he needed to know how the crabs responded to low salinity alone. McGaw measured the unfed animals' blood flow and oxygen uptake rates as he simulated coastal dilution during low tide by dropping the salinity from 100%seawater to 65% for a 6-hour period. The crustaceans lowered their heart rates and blood flow, as well as reducing their oxygen uptake by clamping their gills shut and becoming inactive. However, their metabolic rates and cardiac activity rapidly returned to normal when the crabs were returned to 100%seawater. Next he measured the crabs' physiological responses to a fish dinner in 100% seawater; their blood flow and heart rates increased slightly, but their oxygen demand rocketed 2-3 fold and remained high for several days as the animals digested their meal and incorporated it into their own tissues.
So what happened to the crabs' cardiac activity and metabolic rate when McGaw simulated a drop in salinity as the sea went out after dining? He fed the crabs in 100% seawater, before dropping the salinity levels to 65% 3 hours later. Their heart rates rose a little as the salinity dropped and their oxygen uptake fell slightly too, but soon recovered as if the crabs were feeding in the comfort of full-strength seawater. The crustaceans' metabolism was reacting as if they were digesting their meal in high salinity, despite the uncomfortably low salt levels; they appeared to prioritize digestion over their metabolic responses to low salinity.
McGaw admits that he was surprised, he had thought the crustaceans would prioritize their low-salinity survival strategy in the harsh conditions, but it seems that digesting dinner takes precedence. Which made McGaw wonder exactly what was going on in the crustacean's guts(p. 3777), but first he had to figure out how to film the crabs' internal workings.
McGaw hit on the idea of filming the animals with a portable low-power X-ray machine, much like an airport baggage X-ray system. Knowing that the crabs' digestive systems would easily sort and discard lead-glass tracking beads from a hearty meal, a colleague suggested that electrolytic iron powder might pass through the animals' systems unhindered. Mixing the powder with fish, the crabs tucked in and McGaw was ready to feed the crabs in 100%seawater before X-ray filming their digestive tracts over a range of salinities.
Tracking a meal through the crustacean's digestive tract, McGaw realised that the animals were able to slow the digestive process as the salinity dropped, so crabs fed at 100% seawater digested their meals faster than animals fed in 80% seawater. However, the crabs in the lowest salinity (60%)conditions hardly ever completed digesting their meals. Having ground up the fish slowly in their foregut, the crabs regurgitated the majority of the meal about 6 hours later, possibly saving themselves the burden of completing digestion and the metabolically costly process of incorporating the meal's components into their own tissues.
McGaw admits that he isn't sure whether the crabs make an active decision to conserve energy for other body systems by discarding the meal before cellular uptake starts, or they simply lack the reserves needed to fuel the entire digestion process. However, he's keen to compare the graceful crab's digestive behaviour with species that regulate their ion levels to find out whether osmoregulators are able to balance or prioritize the conflicting demands of ionic regulation and digestion.