Your local rock pool is much more of an extreme environment than you think. Photo / File
Your local rock pool is much more of an extreme environment than you think. Photo / File
In the third of a five-part series looking at Auckland University's research in the region's blue backyard, science reporter Jamie Morton looks at some of the hardy little creatures that live in our rock pools.
Rock pools might appear safe, placid refuges for the marine life that dwell in them - but work by two Auckland researchers shows how these favourite playgrounds of lazy childhood summers are some of the most extreme environments in nature.
And the work by University of Auckland's Dr Anthony Hickey and Dr Neill Herbert on New Zealand fish - particularly some of our more intriguing species - could have ramifications that stretch well beyond our coastlines - and even into medical laboratories.
One charismatic group of small, endemic blenny-like fish, called triplefins, are found in various niches, from abyssal depths to rock pools and estuaries, and most of the 27 species appear to have evolved in New Zealand waters between 12 and 25 million years ago.
More recently, Hickey, Herbert and colleague Professor Nigel Birch have received a Marsden Fund grant to study triplefin physiology.
"The inter-tidal environment is a very harsh place to live," Hickey explained.
"High-energy crashing waves mean that a fish must be hardy and have muscles, fins and armour adapted to survive repeated bursts to fight surge and cling to rock and hide in seaweed."
Importantly, rock pools could become isolated at high tide, posing perilous issues for small pool-dwelling fish.
Life constantly changes with the rise and fall of the tide, and day and night, and fish must respond to two "clocks", dictated by sun and moon.
"If it rains, salts in isolated pools are diluted, and if it is hot, water evaporates concentrating salts," Hickey said.
"Therefore, a fish's gills, gut and kidneys must be adept at pumping salts and water."
Adding to the unpleasantness was that, as all life in rock pools produce waste, resident animals must deal with each other's excreta until the tide returned with fresh salt water.
Much of their research focused on the extraordinary changes in temperature and oxygen in rock pools.
"Temperature has enormous effects on metabolism, the hotter it gets the faster reactions occur, and for a fish this means metabolism accelerates on hot days and oxygen and metabolic fuels are depleted fast," Hickey said.
Children play in rock pools at Takapuna Beach. Photo / File
"Rock pool temperatures soar or plummet in summer and winter, and also fluctuate wildly during the day."
Their work had revealed how metabolism in rock pool triplefin species was much more robust across temperatures.
"There are clinical settings where hyperpyrexia - or getting too hot - is an issue.
"Malignant hyperthermia is one setting, where a patient has a genetic predisposition to common anaesthetics and they respond by overheating and have a heart attack."
Like humans die in heat waves, generally from heart failure, the same effect occurred in other animals, including fish.
"The circulatory system seems to be the weak link with rising temperatures, and energy releasing mitochondria appear to be at a nexus in heart failure," Hickey said.
"We have found that, in a warming world, heart mitochondria provide insights as to which fish will live or die as temperature increases."
The triplefin fish Bellapiscus medius. Photo / Paul Caiger
Oxygen was also linked to temperature to some extent, as less oxygen was dissolved in warm water.
In rock pools oxygen could "super concentrate" and bubble from algae as it photosynthesised.
On warm nights, however, oxygen could become incredibly low as algae and animals respired in the dark.
"Almost all animals are dependent on oxygen and the adult human brain has about five minutes before hypoxic damage sets in," Hickey said.
"Lack of oxygen occurs in numerous clinical situations, such as stroke, heart attack, surgery, birth, organ transplant and cancers."
Research by PhD students Jules Devaux and Tristan Mcarley had found that a rock pool triplefin fish, Bellapiscus medius, had brains and brain mitochondria that were robust to low oxygen, as they only responded to oxygen when levels reached about 1.5 per cent of that which humans are exposed to in the atmosphere.
The triplefin fish Bellapiscus medius. Photo / Paul Caiger
"These fish clearly provide a useful model to study hypoxic brain function, as they have evolved mechanisms to avoid hypoxic brain damage," he said.
"Our current and ongoing work points again to mitochondria, as the rock pool species' brains have mitochondria that tolerate acidification, a condition that initiates seizures in us, and in the other subtidal triplefin species initiates mitochondrial failure."
The students were continuing to explore whole animal responses to temperature, oxygen, and more recently environmental toxicants.
"So when you visit the beach this summer, have some respect for the hardy little fish and other critters within rock pools and the rock pools themselves," he said.
"Contemplate how these animals routinely tolerate extreme swings in temperature, or two to three hours of little to no oxygen."
Five intriguing facts about rock pool fish
1. Rock pool tiddlers can function at oxygen levels of about 1.5 per cent that of what we get from the atmosphere. We pass out at 10 per cent.
2. The rock pool triplefin species Bellapiscus medius and Forsterygion lapillum have 30 per cent larger brain to body mass ratios than deeper subtidal species. Scientists don't know why - but perhaps it's to remember where their rock pools are.
3. Blennies, a group within which triplefins sit, can remember their rock pool for up to a month and this requires spatial mapping and memory.
4. The triplefin Forstrygion varium, measuring about 8cm long, is able to find its way home among marauding snapper after being displaced by up to 700m. That is the same as a grown male navigating 16km through town amid troublesome goths, punks, bogans and emos.
5. When corrected for temperature, the triplefin brain can respire at about the same rate per gram as a mammalian brain.
