[5] A study of population samples from New Zealand and from the Tierra del Fuego region indicate they are the same species, despite the large distance and deep ocean between these locations.

[6] Its specific epithet (gregaria) derives from its behaviour in the immature phase to form very large shoals or swarms of many tens of thousands of individuals in shallow coastal waters.

They have been found on the sea floor in the outer Marlborough Sounds and along the coastline from Cook Strait south to Campbell Island, where they have been observed at depths of 1,000 m (3,300 ft).

[8] In New Zealand waters, the larvae of M. gregaria undergo 5 stages of larval development from mid-winter until the postlarval metamorphosis in spring, when they form large swarms on the surface and heaped up on beaches.

[10] The post-larval stage typically lasts until February when the animals begin the benthic phase of their life-cycle and settle on the sea floor.