The creatures form precise lines in the ocean, sometimes stretching in length to more than 5km.

Walking her dog along Nelson's Tahunanui Beach on Friday afternoon, Daphna Koppenjan was surprised to see clumps of small orangey-red crustaceans spread along the sand.

The critters were mostly in small creeks running down to the sea, or in spots of water had been left by the outgoing tide.

"It started off on the beach with the occasional one. As we walked further toward the end, it became more and more on the beach, then we had these swarms," Koppenjan said.

DAPHNA KOPPENJAN A swarm of gregarious squat lobster on Nelson's Tahunanui Beach on Friday.

She has been visiting the dog walking area of the beach regularly for four years and it's the first time she's seen the small creatures washed up on the shore.

READ MORE:

* Lobster krill swarm

* Southern seas red with krill

* Calls to restrict krill fishing industry

Koppenjan isn't the first to notice the sudden arrival around Tasman Bay of the critters, which have the appealing sounding common name of gregarious squat lobster.

TIM O'CONNELL/STUFF A large number of lobster krill have washed up on the shores around Nelson in recent days or caused the sea to take on a reddish appearance.

Earlier this week, an influx of squat lobster had been spotted around Tasman Bay region, turning the beaches red.



The gregarious squat lobster is commonly found in shallow coastal waters along the eastern coast of the South Island. They are also widely distributed around the southern oceans, which is unique among the group of crustaceans to which they belong.

The species was originally described from Patagonia in 1793 and has attracted attention from many explorers and naturalists. For example, pirate, explorer and natural historian William Dampier wrote of seeing great shoals of them in the Falkland Islands, "which coloured the sea in red spots for a mile in compass".

They have also been seen in such places as Chile and Tasmania, and appear to defy a well-recognised barrier for marine species, known as the East Pacific Barrier: thousands of kilometres of uninhabitable deep ocean from one side to the other that typically prevents dispersal.

A study of populations from New Zealand and the southern tip of South America indicate they are the same species, not two similar species that look similar.

Munida gregaria play an important role in the ocean food web. After their migrating stage - which is when they form dense swarms - they live on the seafloor, scavenging on detritus and occasionally doing a bit of active predation.

In turn, the gregarious squat lobster is eaten by a long list of species, including many commercially important fish, squid, octopus, king crabs, birds and likely whales.