Forget The Bachelor - it's a tiny crab that takes the cake for the ultimate act of courtship.

Kiwi scientists have observed how the parasitic pea crab, the unwanted surprises we sometimes find inside edible shellfish, find love by tickling their way into the female's home.

Many things about the crustaceans have long baffled scientists - but perhaps the biggest mystery was how they reached their mates.

Using night vision cameras, University of Auckland researcher Oliver Trottier tracked male pea crabs as they left the safety of their homes - in this case green-lipped mussels - to search for a mate.

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His research suggests they honed in on a pheromone emitted by female crabs and pumped through the part of the mussel where it exhaled.

It typically took the crab 40 minutes to be sure his mate was inside, but actually breaking in without getting crushed was the real challenge.

Over hours, and usually at night when there were fewer predators about, the male crab used its claws to constantly tickle and scratch the mussel until it became docile enough for the intruder to quickly sneak in.

Once the male shared its sperm, it then left again, "probably just so he's not robbing the female of dinner".

Mr Trottier said he was startled to observe the process, which he later unsuccessfully tried to mimic by scratching mussels with tweezers.

Better understanding the parasites had potential benefits for the New Zealand shellfish industry which has had export shipments returned when just one crab was found inside a container of 100 mussels, he said.