Richard Robinson / Dorid nudibranch (Archidoris wellingtonensis) in Dusky Sound, Fiordland National Park, New Zealand.

Richard Robinson / Nudibranch (Archidoris wellingtonensis) in Dusky Sound, Fiordland National Park, New Zealand. April 2014.

Nick Upton / Sea lemon (Archidoris pseudoargus) sea slug among Common Barnacles (Semibalanus balanoides) attached to rock exposed on a low spring tide, Crail, Scotland, UK, July

Nick Upton / Sea lemon (Archidoris pseudoargus) sea slug among Common Barnacles (Semibalanus balanoides) attached to rock exposed on a low spring tide, Crail, Scotland, UK, July.

Nick Upton / Sea lemon (Archidoris pseudoargus) sea slug among Common Barnacles (Semibalanus balanoides) attached to rock exposed on a low spring tide, Crail, Scotland, UK, July.

Nick Upton / Sea lemon (Archidoris pseudoargus) sea slug among Common Barnacles (Semibalanus balanoides) attached to rock exposed on a low spring tide, Crail, Scotland, UK, July

Robert Thompson / Sea lemon (Archidoris pseudoargus) Strangford Lough, County Down, Northern Ireland.

Adrian Davies / Sea lemon nudibranch {Archidoris pseudoargus} South coast, England, UK

Jane Burton / Sea lemon nudibranch (Archidoris pseudoargus) Atlantic coast, captive

Alex Mustard / Yellow nudibranch / Monterey sea lemon (Archidoris montereyensis) crawls over kelp. Browning Pass, Port Hardy, Vancouver Island, British Columbia, Canada. Queen Charlotte Strait, North East Pacific Oc...