Jellyfish that bear an unusual resemblance to breast implants have arrived on Nelson shores.

The black-lined marine animal "the size of a teacup and maybe the size of a saucer" is apparently harmless, if the behaviour of one swimmer at Little Kaiteriteri is anything to go by. Fellow swimmer Sara-Lee Dabinette was in the area this week and noticed a man weighing up the jellyfish with one in each hand.

Curious, she went closer and discovered that the jellyfish did indeed "feel like mammaries", and resembled silicon breast implants, except for the black lines.

Stephens Bay bach owner John Midgley, who has taken annual holidays in the area for more than a decade, noticed swarms of what he described as black-rimmed jellyfish in the Astrolabe Roadstead this week. He said it was the first time in all his years in the area he had seen such jellyfish.

"There were swarms of them. They're almost clear with a black ring around them."

Mr Midgley said they ranged in size from that of a teacup to maybe a saucer, and did not appear to have any stingers. He had noticed them from the roadstead (near Adele Island) and north to Tonga Island.

"They look like a standard jellyfish but with an odd colour. They've very definitely got a black rim, and there are hundreds in each swarm," Mr Midgley said.

The Encyclopedia of New Zealand says the best-known jellyfish species around New Zealand include the stinging lion's-mane jellyfish (Cyanea species) and the moon jelly (Aurelia species).

Niwa Nelson regional manager Ken Grange said that without seeing one it was a bit difficult to say what it might be but swarms are normal for this time of year.

"In late summer they're going to be everywhere," Dr Grange said.

Dennis Gordon, Niwa principal scientist and jellyfish expert in Wellington, said that without seeing the jellyfish himself it was difficult to say exactly what the species might be floating on to Nelson shores, but the description, including that the jellyfish have dark radials extending out from its centre, sounded like they could be hydroid medusa, formally known as Aequorea forskalea.

Dr Gordon said they might be described by some as "breast implants which had been badly handled".

The species is described as being up to 175mm wide, saucer-shaped, thick in the centre, gradually thinning towards the margin.

Dr Gordon said there is an actual species called "black jellyfish" (Chrysaora achlyos) which is not found in New Zealand.

He said Aequorea forskalea appear in warm summers, often associated with La Nina, but New Zealand was currently not in this or an El Nino phase.

"We had them in March 2010, for example, when the sea conditions were warm owing to La Nina."

Dr Gordon said jellyfish were useful in determining the health of the marine environment.

They are sometimes harbingers of pollution that kills other marine organisms.

- © Fairfax NZ News