Close-up, this slater-like crustacean looks like just another critter you'd find scuttling along the sand at Auckland's Piha Beach.

But the tiny creature has just yielded new evidence that animals can have biological clocks influenced by the tide, as well as the more familiar circadian clock which follows the day/night cycle and regulates our own behaviour.

While the molecular mechanism of the circadian clock in humans is well known, including its location in the human brain and the genes involved, the mechanisms of other biological clocks are not.

Many animals are known to have "extra" biological clocks that regulate feeding or reproduction according to the tide or lunar cycle but scientists have been unsure of how they work, particularly over longer periods.

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University of Auckland researchers James Cheeseman and Professor Mike Walker carried out a study of Scyphax ornatus, a nocturnal sand-burrowing isopod that feeds on the plant and animal detritus that is moved up the beach by the incoming tide.

Leaving their burrows only at night, the animals need to maximise the amount of time for feeding before the tide comes in.

In the wild, they appear able to follow a semilunar or approximately fortnightly feeding cycle, meaning something other than the circadian clock must be regulating their behaviour.

Taking the animals from Piha into the laboratory, the study used artificially manipulated light and tidal cycles to test several hypotheses for the mechanism of the semilunar clock that controls their behaviour.

Their study, just published in the journal Scientific Reports, found the animals followed internal biological clocks even when deprived of external stimuli.

"What we have found is that, in the laboratory, with light and tide cycles artificially manipulated, these animals follow the same rules of behaviour as they would in the wild," Cheeseman said.

"So we can very accurately change the semilunar rhythm by changing the perceived length of the day and tidal cycles.

"That tells us their semilunar or fortnightly behaviour continues to be regulated by the interaction of circatidal and circadian clocks even where there is either no external stimuli or they are in an environment with artificial light cycles or tidal cycles."

Walker said circalunar and circatidal behaviour in animals was well known by early Maori who followed a fishing and planting calendar over the circalunar cycle.

Rain, wind, maybe snow for shortest day of the year

This year's winter solstice may start mild, but by the end of the shortest day of the year on Wednesday there will be rain, wind and even some snow.

NIWA meteorologist Ben Noll says a storm is brewing that will direct subtropical winds towards New Zealand in the second half of the week.

But first up is the June solstice at 4.24pm on Wednesday.

This is the time at which the sun's zenith reaches its northernmost point and the North Pole tilts directly toward the sun, giving 24 hours of daylight or "Midnight Sun."

Conversely, there is no sunlight at all south of the Antarctic Circle.

"In New Zealand, the length of daylight on the shortest day of the year varies: Auckland has 9 hours and 37 minutes, Wellington 9 hours and 11 minutes, but Invercargill 8 hours and 35 minutes (or about 7 hours less daylight than on the longest day of the year, December 22nd)," Mr Noll said.

Many Northern Hemisphere countries consider the June Solstice the official start of summer.

But that doesn't mean that it marks the official start of winter in the Southern Hemisphere.

In New Zealand, Australia and South Africa, winter begins on June 1 and ends on August 31, otherwise known as "meteorological winter" combining the months with the most similar weather together, which makes record-keeping easier.

Just one month from the solstice, Invercargill will have about 30 minutes more daylight, Wellington 25 minutes, and Auckland 21 minutes.