Title photo by Todd Aki

• Nudibranchs are hermaphrodites (having both male and female sex organs), but they still require a mate in order to reproduce as they cannot self fertilize

Above nudibranch reproductive system by Carissa Shipman, found here

• The reproductive organs are usually next to each other inside the nudibranchs body, and the exterior reproductive opening being on its right lateral side

Above, 'External anatomy of a Tambja verconis nudibranch (by wadeangeliart) found here

• Being simultaneous hermaphrodites increases their opportunities to find a mate, as both partners will transfer sperm, and lay eggs, via reciprocal reproduction…although there are some exceptions as we will discover!

Above photo Bennett’s nudibranchs Mating’ by John Turnbull

• Nudibranchs will follow the scent trail left by potential partner. When they catch up with them they start courtship which involves the gentle touching of each other

Above photo by ilan Lubitz

• Depending on the species, the nudibranchs will then orientate their bodies so that their reproductive openings are facing, allowing their swollen gonopores to connect

Above photo of Nembrotha chamberlaini mating, by Jim Greenfield

Above photo of Nembrotha purpureolineata, by Brian Mayes

• Nudibranchs will take every opportunity they can to mate when finding a partner. One quirk of their anatomical development whilst maturing into adults is that the male reproductive organs will grow, and be functional before their female organs. This is called protandry…

Above photo of Hypselodoris bullocki mating, by Coppertane…a possible protandry mating, due to size difference

• They can still take the opportunity to mate though as a ‘male’… It has been suggested that any sperm passed to the ‘male’ partner, can be stored within their bodies until their female organs have matured. It will then use the stored sperm to fertilize its eggs!

Above photo of pelagic nudibranchs mating by Rajiv Bhambri

• Mating duration depends on species, and can vary from brief encounters to several hours!

Above photo by Tony Wu

• There are 3 basic mating positions depending on species- right side to right side… (see photo below)

Above, Nembrotha chamberlaini, by Andrew Wu

• …Head to head…(see photo below)…

Above, Lobiger viridis, from here

• …And head to tail, which is either a reciprocal, or a unilateral process depending upon flexibility of the species, or even partner aggression…(see photo below)…

Above, Mariaglaja inornata, from here. The genitals are separate in this species- the penis is in the head, and the vagina in the tail

[Above] The gastropterid sea slug Sagaminopteron ornatum will sometimes form a circle of two to achieve reciprocal mating, or at other times as illustrated here, act unilaterally as a male by approaching from behind to copulate with another acting as a female. source

• Hypodermic Insemination is the preferred method used by some species. The penis has a sharp point which is used to stab the partner in order to deliver a packet of sperm. This can be done either as a mutual act, or happening unilaterally with one nudibranch taking advantage to inseminate another…This can occur amongst some of the sacoglassans

Above photo of Costasiella usagi, by eunice khoo…not as cute as I originally thought…

• Goniobranchus reticulatus is an unusual nudibranch as after it has mated the external part of its penis detaches! And within 24 hours it grows back…!

Scientists think this mating strategy has evolved so the sperm of rival nudibranchs stored in the vagina of their mate will not accidentally get passed on to future mates source

Above photo of Goniobranchus reticulatus by Bernard Picton…with or without penis…(Schrodinger’s penis?)

…And finally a shrimp jockey on a pair of mating nudis…

Above photo by Ludovic

Information from-

here

here

and the fantastic Nudibranch Domain

As always, I’m not an expert…any errors let me know in the comments, and I’ll edit my post!