Beautiful seaslugs of New Zealand

by Dr J Floor Anthoni (2009)
Seaslugs are like the butterflies amongst the flowers, highly specialised and colourful, often with fantastic shapes. They belong to the molluscs, the snails, clams and inkfish, yet they look so different. They slide around on their feet like snails do, but they do not have a hard shell for protection. They are highly visible and colourful, yet are not eaten. They are highly specialised in what they feed on, which makes them impossible to keep in marine aquariums. Their scientific name is nudibranch, meaning naked gill, because they do not hide their gills inside a shell or sack. But some can withdraw their gills into their bodies. See also our page about sea hares.
Keywords: NZ, New Zealand, underwater, sea, ocean, environment, mollusc, sea slug, seaslug, nudibranch
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New Zealand's underwater scene is that of kelp forests, usually exposed to the open ocean. Sessile animals like sponges, bryozoa, hydrozoa and anemones are not numerous, and where they are found, there is often too much wave action for nudibranchs. Nudibranchs are delicate feeders specialised in eating sponges or the polyps of tiny coral-like animals called bryozoa (moss animals). Because these tiny polyps live inside secure boxes, nudibranchs have mouths specialised to reach the polyps inside. Even so, one can find sea slugs specialised in sucking plant sap, crunching hard sponges, and even ones predating on other sea slugs.
For a very interesting article about seaslugs, visit
f050511: sea slugs may have originated from sea hares like this black-tail sea hare (Aplysia dactylomela), which still has a thin shell hidden under its skin. Although its gills are on its back, they are not exposed but hidden under two layers of skin folds.
f050525: the spotted side-gill slug (Pleurobranchia maculata) is not considered a nudibranch, because it is hiding its gills on the right hand side under its mantle. Here the gills can be seen poking out from the rear end of its mantle.
f039237: ornate berthella side-gill slug (Berthella ornata?), which comes out in the night.
f019307: ornate berthella side-gill slug (Berthella ornata), photographed at night.
f040422: this green sap-sucking blue-spotted elysia (Elysia sp.) has no gills at all but its elaborate wings act like gills. It is a very small sea slug (15mm).
f018701: the archidoris (Archidoris wellingtoniensis) is a large sea slug, weighing up to half kilogram. It feeds on certain kinds of sponges and here it has just flattened a yellow nipple sponge (Polymastia croceus).
f039911: the archidoris has various colours. Here we encountered one during a night dive while it was feasting on a salmon coloured sponge. The scatter in the photo is live zoo plankton.
f040530: the variable aphelodoris (Aphelodoris luctuosa) is a true nudibranch which is highly variable in colour. All sea slugs are both male and female at the same time (hermaphrodites) and they can fertilise themselves.
f039834: two mating aphelodoris nudibranchs. Nudibranchs mate top to toe as their mating organ is on the righthand side. Because their shapes are flexible, they can grow or shrink their sexual organs to match.
f003621: although this aphelodoris looks quite different, it is possibly just another Aphelodoris luctuosa.
f005125: a white nudibranch (Atagema carinata) with a sandpapery skin, found eating a sponge in a degraded harbour.
f037835: the gem nudibranch (Dendrodoris dennisoni, Dendrodoris gemmacea) has blue spots on a brown background and a beautiful skirt all around. Its gills are also spectacular.
f039736: a reddish-brown variant of the gem nudibranch.
f050207: a young gem nudibranch is still mainly white but already showing its characteristic blue spots.
f050534: the black doris (Dendrodoris nigra) is always jet black with a thin red margin. Its horns and gills can hardly be distinguished.
f038822: the goldribbon doris (Chromodoris aureomarginata) is a small but elegant sea slug, always completely white with a gold margin.
f017214: an unidentified doris (Doris granulosa?).
f051020: the clown doris (Ceratosoma amoena) has orange polka dots on a purple-white skin, with purple horns and purple gills. It is relatively common. These two are mating and their sex organs are visible.
f038829: a very large clown doris at over twice its normal size, found in a degraded harbour where apparently their food was plentiful.
f050227: a lemon doris (Dendrodoris citrina) is a small seaslug with a warty skin and coloured from pale yellow to pale orange.
f050601: an orange lemon doris (Dendrodoris citrina) found in a rock pool.
f050228: an unidentified doris thought to be a lemon doris, but this one has a smooth skin and carries its skirt in a different way.
f040034: an unidentified doris with very wide mantle flaps and a smooth skin.
f040523: an unidentified doris (Rostanga rubicunda?) feeling robust to touch.
f038806: two flat doris (Platydoris formosa), one orange and one brown, partly overlapping (or mating?) while eating an orange carpet sponge.
f040935: Willans cadlina (Cadlina willani) is only 2 cm long, here feeding on bryozoan firs.
f051418: unidentified seaslug, perhaps a lemon doris, but well outside its habitat.
sandpaper doris (Alloiodoris lanuginata)
1011152: sandpaper doris (Alloiodoris lanuginata) has a sandpapery hard back but is otherwise pliable as any seaslug. Found near Leigh during a night dive.

Tambjas have their gills on the middle of their backs, and most are unable to withdraw them inside.
They are found in the cleanest of waters where fragile bryozoa and hydroid firs abound.
f041308: fine lined tambja (Tambja mullineri). These dark green sea slugs are always small.
f029405: long lined tambja (Tambja tenuilineata) on left and on right a much smaller fine lined tambja, feeding on lace-like hydroid firs.
f029804: two fine lined tambjas of two different species. Note that the green crust is a sponge, not a seaweed.

f024329: a morose tambja (Tambja morosa) lost on an orange crater sponge, must find its way up the cliff again towards the bryozoan firs on which it feeds.
f037833: morose tambja (Tambja morosa) always has bright blue marks on a dark black or greenish body.
f029424: the morose tambja prefers to eat bryozoa like these orange stick bryozoa and the orange bryozoan firs below.
f040924: three Vercoes tambja (Tambja verconis) huddling together. They prefer to feed on the blue-green bryozoan firs. These sea slugs are very sociable.
f029425: two beautiful Vercoes tambja tail-gating on a patch of green-blue bryozoan firs.
f037817: tambja gills are placed in the middle of their backs and consist of five 'hands'. These are the gills of Vercoes tambja.
f024321: the sea tiger (Roboastra luteolineata) is green with thin yellow/orange stripes and it hunts other sea slugs like fine-lined tambjas.
Sea slugs with gills all over
f018028: beautiful jason (Jason mirabilis) lives only from the polyps of the hydroid tree (Solanderia ). It is a staunch climber, its foot designed to hold on tight to the narrow branches.
f013137: two beautiful jasons finishing off the last polyps, after which they lay ribbons of eggs in the same colour as their bodies.
f012406: this hydroid tree still has a large number of uneaten polyps but the jason seaslugs make sure that this won't last.
f040927: a Mokohinau janolus (Janolus mokohinau) named after the Mokohinau islands, is small and rare.
f028924: the fire janolus (Janolus ignis) comes out only at night and prefers these whitish bryozoan firs.
f037905: a fire janolus (Janolus ignis) on hydroid firs.
f041334: this warty janolus (Galeojanolus ionnae). Its skin is made up of sticky tubercles that stick to any attacker.
f018320: a tritonia (Tritonia incerta) has two rows of gills. It is a finger-long seaslug that feeds on the tough deadmans finger soft corals (orange). Here the coral has withdrawn its polyps in defence.
f032525: a beautiful tritonia (Tritonia incerta) in search of soft coral, here passing edible colonial seasquirts.
Tritonia incerta, young
f053613: an interesting observation illustrated by four images. Here a small Tritonia incerta sniffing its way to its prey, about 20cm away. It knows precisely where to go.
Tritonia approaching an Alcyonium deadman's finger
f053616: The Tritonia now approaching its prey, a deadman's finger Alcyonium cf aurantiacum. It appears an easy kill.
Tritonia pausing before attacking alcyonium
f053619: but before attacking, the Tritonium hesitates and pauses for almost one minute.
Tritonium after its sudden lunge forward
f053620: the Tritonia suddenly lunges forward in 1/4 second, but equally quickly the polyps withdraw and the seaslug is left without a meal. But bigger Tritonia can chew through the tough crust of the deadman's finger.
0904037: a young (3 cm) tritonia on a red seaweed, or is it a different species?
f040324: unidentified 2cm seaslug (Aeolidia sp.?).
f224634: tiny 5mm unidentified seaslug on a Callyspongia ramosa thin finger sponge.
f052012: tawny speckled chelidonura (Chelidonura fulvipunctata), about 3cm, comes out at night.
f036911: Gardiners philinopsis (Philinopsis gardeneri) is a 2-3cm small slug, found in rock pools by day. It is sociable and often found together or tailgating one another.These two are joined head to tail.

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