Nudibranch / Sea Slugs Defence Systems

Nudibranchs / Sea Slugs Identification


In the realms of human fantasy, nudibranchs have the ultimate sex lives.

After all, they are born with male and female sex organs and no eyes, so any mature adult of the same species with the same smell, is a sure thing.
This type of reproductive biology is known as functional simultaneous hermaphrodism, which seems such a long term for such small creatures. 
However, a nudibranch's life is not just fun and fancy free, indeed, it is very serious business, as are all of nature's reproduction strategies.


Risbecia godeffroyana_nuzzling

Nuzzling behaviour by two Godeffroy's Risbecias Risbecia godeffroyi,
another courting procedure. Loloata Island, Papua New Guinea.
(Photo: Neville Coleman)

With a life expectancy varying from a few days to a few months (depending on the species) their lives are hectic merry-go-rounds to say the least.  Upon emerging from their eggs they must 'home' in on their particular food source, eat, grow and smell out a mate or mates, copulate and lay their egg mass or masses, all in the space of a week to several months.  Easy to say in a few lines but, in reality, the odds are immense.  The oceans are gigantic and strong currents are everywhere. The veliger planktonic young of tropical species may be widespread across thousands of kilometres of oceans, yet the species remain viable.


Phyllidia picta
Common across a wide range of the Asia/Indio-Pacific region, black-rayed Phyllidia Phyllidia picta can be found snuggling up and even mating but their egg masses remain a mystery. ( Santo,Vanuatu) ( photo: Neville Coleman)


Sexual behaviour

Sexually mature nudibranchs have reproductive pores on the right-hand sides of their upper bodies in the vicinity of the neck. Once two members of the same species have recognised each other by chemoreception, contact of the body or rhinophores is made.
In  some species it appears that there is a certain amount of 'getting to know you' or courting and manoeuvring around in opposite circles to get their reproductive pores aligned.  


Nembrotha chamberlaini_mating

Mating specimens of Chamberlain's Nembrotha Nembrotha chamberlaini
showing sperm exchange (white wavy substance inside penis). Anilao, Philippines.
( Photo: Neville  Coleman)

The courting may appear to be simple and straightforward to humans, but to nudibranchs it is often much more. In some cases, one might follow the other around touching on the edge of its tail for some time before the one in front gets the message. This behaviour is sometimes referred to as 'tailgating' (from the human behaviour of driving too close to the back of another car in traffic).


Risbecia tryoni

Thought to be a courting behaviour these Tryon's Risbecias
Risbecia tryoni are displaying the common term of "tailgating", one following the
other around by staying on its tail. Bali, Indonesia. ( photo: Neville Coleman)

Some species have very distinctive courting procedures by rubbing themselves on each other, nuzzling, heads and rhinophores and  crawling all over and around each other. There are those that are known to stroke their mate with their cephalic mouth tentacles, others fight each other and then there are those whose hunger overrides their sex drive, so they swallow the first potential mate and with a full belly 'chat up' their next mate, or meal.

Mating can only take place when each nudibranch has manoeuvred into a head to tail position with their 'necks' touching.
On contact, each penis everts from each neck and seeks out the female genital duct of the other (humans did not invent 'necking', nudibranchs did!)


Hypselodoris bennetti

Bennett's Hypselodoris  Hypselodoris bennetti mating. They have very wide
and inflated sperm transfer organs. Sydney, New South Wales, Australia.
( photo: Neville Coleman)

Various species of nudibranchs may form communal mating, or spawning groups, while others just pair up, swap sperm and move on to lay their respective egg ribbons.  Intercourse times vary from a few minutes to a few days and during copulation, staying 'hooked up' is the most important thing.  
To ensure against interruptions or such, some species have a modified penis with multiple hooks which act as a stimulator and an anchor, while others have a spike-like penis, for deep penetration.   Some female vaginas are lined with chitinous spines to ensure against premature withdrawal when mating occurs in strong currents, or swell conditions.


Egg ribbons


Jorunna funebris

The egg ribbons of nudibranchs come in an amazing array of colours; some are reasonably drab, while others are extreme in their bawdy hues. Not every species lays an individually distinctive egg ribbon and without verification of a nudibranch laying an egg ribbon, or an egg ribbon found on a known food source of a particular species, it is often difficult to determine which species laid which egg ribbon.

However just as we have come a long way forward with nudibranch 'identification' due to underwater photography, the same methods can be used to record egg laying and egg ribbon recognition in the future.

In many cases egg ribbons are laid directly on or near a food source and the colour and form are generally characteristic of each species.
Egg ribbons are generally laid in a counter clockwise open-ended circle.  Some begin their egg ribbon at the centre and work out and others begin at the outer edge and work in. 

Most species abandon their egg mass once it is completed, but the serpent Pteraeolidia Pteraeolidia ianthina practices brood protection, with one or more individuals staying with the eggs for a certain time.  

The egg ribbons of the funeral Jorunna Jorunna funebris are always white and frilly and laid on or near their food source blue sponge. (Kimbe Bay, Papua New Guinea)
 ( photo: Neville Coleman)


Rostanga sp

An undescribed species of white speckled Rostanga Rostanga sp. first eats away its food sponge and then lays its identically-coloured egg ribbons on the rocky substrate underneath.Milne Bay, PNG. ( photo: Neville Coleman)


Phestilla lugubris 

Very distinctive in appearance the egg ribbons of the Porites Phestilla
Phestilla lugubris are cone shaped. (Milne Bay, Papua New Guinea) 
( photo: Neville Coleman)

Biochemical protection

The egg girdles  laid by nudibranchs are rarely attacked and eaten by predators even though many are layed out in the open and appear extremely vulnerable. The only known predators seem to be other nudibranchs.

Favorinus japonicus Feeding on Eggs

Very few creatures are able to eat nudibranch eggs but they are feasted upon by other nudibranchs. Here a Japan Favorinus Favorinus japonicus has been feeding on the eggs of an unknown nudibranch (notice that the predator is the same colour as its prey). Milne Bay, Papua New Guinea. ( photo: Neville Coleman)

Compounds from opisthobranch egg masses are currently under study.  Bioprospecting has revealed a number of toxic chemicals isolated from nudibranchs but most are too toxic to be used on humans, given the present state of our knowledge. Well over 300 natural products have been described from nudibranchs, yet only a small proportion of species have been investigated.
Clearly, they have great potential as biomedical resources, as long as their populations do not become threatened.


Hexabranchus sanguineus Eggs

The Spanish dancer Hexabranchus sanguineus lays a bright red or pink rosette of eggs out in the open. These eggs are thought to have toxic properties.
( photo: Neville Coleman)


The majority of tropical nudibranchs have planktotrophic larval dispersal, that is, they produce a large number of small eggs (up to many thousands in each egg ribbon).  Upon hatching, the young (called veligers) actually have a small shell, similar to univalve molluscs as they all belong to the same phylum.  The veligers rise into the water column and are distributed by the currents. After drifting around in the currents for days, or weeks, they cast off their baby shell, metamorphose and settle to the bottom on, or near their food source.  Even at a microscopic size it is believed that each species can smell out its own type of food by chemoreception.


Aphelodoris gigas with eggs

In comparison to its pelagically dispersed tropical relatives, the giant
Aphelodoris Aphelodoris gigas has much larger eggs which hatch by direct development. Exmouth, Western Australia. ( photo: Neville Coleman)


Some temperate water nudibranchs reproduce by direct development. These species lay fewer, but larger eggs in their ribbons and the young emerge as minute crawling juveniles, not necessarily replicas of their parents. Direct development species are generally localised, or endemic to one area.  However, our knowledge is sparse and even though it is known that species within a single genus may employ different methods of reproductive development there is still a long way to go before the several thousand species known to occur in the Asia/Indio-Pacific region become common knowledge.


Glossodoris stellata

The starry Glossodoris Glossodoris stellata are always laid
on the underside of its food sponge.(originally discovered by the author) Mabul, Malaysia. ( photo: Neville Coleman)


The importance of underwater imagery

In a mere 30 years we have taken an obscure group of underwater critters and raised them to a status all humans should employ in their regard to the nature of things.  The diving community really 'discovered' nudibranchs and have taken them to their hearts.


1.Scuba Divers Magnifying Glass looking for nudibranchs

Searching out the many species of nudibranchs from their hiding places in our vast oceans can be assisted by using magnifying glasses, close-up lenses in your mask and a good torch. Great Barrier Reef. ( photo: Neville Coleman)

All divers, especially photographers can become  explorers. Every observation and every image adds to our knowledge and all divers should be encouraged in their endeavours to learn more because "learning is the greatest adventure".

The World of Water is all around us
and the challenge is worthy of our participation.


Snorkel Nudibranchs

Nudibranchs can also be found by snorkelling around in the shallows.
Heron Island, Great Barrier Reef. ( photo: Neville Coleman)

Copyright Neville Coleman


Nudibranch / Sea Slugs Defence Systems

Nudibranchs / Sea Slugs Identification


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- Customer's Comments on Nudibranchs Encyclopedia -

"The book is fantastic … a real credit to Neville's commitment to both the underwater world and dedication in the study of Nudibranchs.
One would have to consider this book a must in the library of each and every diving enthusiast.
It is a great reference tool."

- John Natoli –

"We just received our copy of the Nudibranchs Encyclopaedia – WOW! it is really terrific –
I know that it will become the premier reference book in our home! It is lovely, congratulations!
a lot of work, time and effort have obviously gone into this book, and the results look terrific.
Thank you so much for putting it together – I can hardly wait to go diving and start identifying!
It has already helped me with some of the photos I took on our last dives.
Again, Thanks for a terrific book. "

- Sonja Rigby-Jones -

"I have just received your Nudibranchs Encyclopedia book. In one word, magnificent.
In my opinion it is the best book your have published to date, although Marine Life of the Maldives comes a close second.
The pictures of the nudibranchs are nice and large and that will make identification so much easier.
The quality of the paper and the quality of the printing is first class.
Congratulations, all your hard work has been well rewarded with this superb book."

- Garry Dyer –

"I received NUDIBRANCHS ENCYCLOPEDIA that I ordered.
This book is wonderful, and I think this is my bible of nudibranchs.
I boast of this to my diving friends."

- Kouji Yamada -

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