Hopkin’s Rose Nudibranch

Jan 20

Ceratodoris rosacea

Hopkin’s rose nudibranch is a sea slug that lives in the tidepools along North America’s West Coast. It gets its rosy-pink pigment by eating pink bryozoans — tiny, colonial animals that form larger plant-like structures. Despite looking like bubblegum, its frilly pink appearance is thought to deter predators.

The Tidal Museum of the West Coast

Tidal zones are among the most dynamic habitats in the world. With the twice-daily changing of tides, coastal inhabitants are either swept along to sea or, where waves have sculpted pitted landscapes of rock, are left stranded — immobile organisms cling to rocks, encased in their natural armour, safe from the open air, while more lively and soft-bodied critters shelter in pools of water left behind by the outgoing tide. For the marine enthusiast, these tidal pools are pockets of sea preserved along the coast; temporary museum exhibits curated by the rocks and waves that one can explore and examine — with due caution and gentleness — and marvel at the ocean's oft-hidden lifeforms. And few museums boast such a rich collection of marine creatures as North America's West Coast.

Clockwise from top left; a mossy chiton (*Mopalia muscosa*), chestnut cowry (*Neobernaya spadicea*), sunflower sea star (*Pycnopodia helianthoides*), sandcastle worms (*Phragmatopoma* spp.), California seahare (*Aplysia californica*), warty sea cucumber (*Apostichopus parvimensis*), and a Pacific purple sea urchin (*Strongylocentrotus purpuratus*).

As spring arrives in the North Pacific, seasonal currents sweep great loads of nutrients from the deep ocean towards the surface and the coast. Like the spring blooming of plants in the forest, these nutrients allow phytoplankton — microscopic marine algae that float through the water — to prosper and proliferate. Forming the base of the oceanic food chain, the phytoplankton explosion sets off a cascade of burgeoning life, ultimately resulting in the great biodiversity of the West Coast. One can spend a lifetime documenting the creatures that live in the tidal pools here.

Clockwise from top left; a sunburst nemone (*Anthopleura sola*), aggregating anemones (*Anthopleura elegantissima*), and a pair of giant green anemones (*Anthopleura xanthogrammica*).

The lifeforms range from ordinary striped shore crabs and tidepool sculpins to alienesque sunflower sea stars, which can have anywhere from 15 to 24 undulating arms and grow up to a metre (3.3 ft) across. There are bizarre mossy chitons with armoured bodies encircled by beards of bristly hairs and built-up colonies of sandcastle worm tubes that cover coastal rocks in honeycomb patterns. Cute chestnut cowries peak meekly from beneath their glossy, copper shells and sea hares spew purple ink into the water. Warty sea cucumbers (which incidentally look like budding sweet potatoes) race around, covering nearly 1 metre (3 ft) per 15 minutes, while purple sea urchins stay rooted to their spot, dissuading predators from their tasty insides with spiny exteriors, like hedgehogs of the sea. But perhaps the most eye-catching creatures of the tidal pools are the sea anemones.

From giant green anemones that grow 30 cm (11.8 in) tall and glow near-neon green, to especially sociable aggregating anemones which live together in large numbers and can rapidly clone themselves, to the pale green-purple sunburst anemones and multi-coloured moonglow anemones. They stick to their chosen rocks, seldom moving, their many frilly tentacles swaying hypnotically in the water — tentacles packed with venomous, harpoon-like spines that can paralyze small prey, which the anemones voraciously devour. But anemones aren't the only tidepool creatures bedecked in colourful frills.

Feathers, Frills, and Gills

You bend over to investigate another tidepool and spot something bright pink stuck to the rocky bottom. You haven't seen any creature so garishly rosy in any of the pools before and you inspect it closer. It's no larger than 2.5 cm (1 in) and looks like a tiny sea anemone, covered in copious frilly tentacles faded white at the tips. But it's a lot more streamlined than an anemone, less circular and missing a central mouth, and, upon close scrutiny, you notice that among its many swaying tentacles, it has a pair of long "ears" at one end and a bouquet of feathery appendages at the other. Hidden within its tentacles and easily overlooked, these are signature traits of a nudibranch, also known as a sea slug.

This particular sea slug is Hopkin’s rose nudibranch, and it lives in the warm intertidal zone along the West Coast, ranging from Northern Baja California in Mexico up to the lower Oregon coast, and can often be spotted in tidepools and shallow waters. It travels around on a flat, muscular foot — a signifier of its place among the gastropods (snails and slugs), with the name literally meaning stomach ('gastro') foot ('pod') — although you'd have to be pretty patient to notice its progress, as sea slugs move at about 30 cm (1 ft) an hour if they're really booking it.

The nudibranch's two "ears" are located near its head, just above its mouth, and they are indeed sensory organs, although they aren't used for hearing. The pair of appendages are known as 'rhinophores', and are used to detect dissolved chemicals in the water — closer to elongated noses or tongues than ears — with their feathery texture providing a large surface area for maximizing chemical detection. The name 'nudibranch' is formed from the Greek 'nudi' and Latin 'branch', meaning "naked gills" — describing the appendages at its rear, which are also feathery, but in this case, for more efficient gas exchange. In an unfortunate case of anatomical placement, these naked gills encircle the nudibranch's anus, so you could say that it defecates up its own nose.

But what about all those tentacle-like appendages obscuring its entire body, swaying prettily to the rhythm of water currents, and giving this sea slug the appearance of a stringy piece of bubblegum? Do they help it avoid predators? Can they sting like those of sea anemones? They must be more than just pretty frills — purely aesthetic appendages — right? The answer to all of these questions is yes, but we shouldn't be so quick to discount the advantages of flashy attire. And what's one of the flashiest colours you can think of? Before delving into its many frills, let's first get to the root of this sea slug's rosiness; the source and purpose of its pinkness.

Pretty in Pink

Pink encrusting bryozoa (*Eurystomella bilabiata*).

No other creature along the West Coast looks as cheerful and bubbly as Hopkin’s rose nudibranch, but this pretty pink appearance belies its carnivorous nature. It preys exclusively on an animal which, like a sea anemone, looks more like a kind of plant — and is sometimes called a "moss animal" — the pink encrusting bryozoan. Each individual bryozoan (termed a 'zooid') is minuscule — no larger than 3mm long (0.1 in) — residing within its own shell-like structure, and obtaining nutrients by filter feeding. But these encrusting bryozoans don't live alone; they form large colonies which "encrust" the surfaces of coastal rocks and appear like scaly, pink moss or fungal growths. These living mats are an all-you-can-eat for Hopkin’s rose nudibranch. While the sea slug looks to be peaceably grazing, like a sheep in a pasture,¹ it's actually using its 'radula' — an organ analogous to a tongue but covered in many sharp teeth — to scrape bryozoans from their rock, one by one, and suck up their soft tissues.

The bryozoans' chitinous armour protects them from some predators, but not from the rosy nudibranch's toothy "tongue." Toxic compounds make bryozoans unpalatable to most predators, but, again, not so for their rosy arch-nemesis. And the rosy nudibranch doesn't just survive the bryozoan toxins, it steals them; incorporating them into its own body, concentrating them into its frilly appendages — taking its prey's defences, upgrading them, and making them its own. And that's not all its steals. The rose nudibranch is only rosy because of its rosy bryozoan diet. Like a flamingo accumulates pink pigment in its feathers from the shrimp it eats,² Hopkin’s rose nudibranch gets its pink from its prey in the form of hopkinsiaxanthin — a type of carotenoid first discovered in this sea slug's tissues.

Why would an inch-long critter want to be bright pink? The combination of a flashy appearance and a toxic taste underpin the survival strategy known as aposematism — most notably exhibited by poison dart frogs, which come in a variety of garish colours and patterns that make them highly visible to any potential predators. Alone, this would be disastrous for the frogs, but combined with their toxicity, their colours serve as a warning to predators (rather than a dining invitation) who soon learn that these colourful tidbits aren't very palatable. The rose nudibranch, with its bright pink appearance and toxic frills, relies on the same strategy. Its similarity to sea anemones might even be an example of Müllerian mimicry; when several different animals share the same repulsive trait (such as toxic tentacles), predators learn to avoid that trait more readily. And so frilly tentacles become a functional fashion trend.

A Slow Dating Scene & Productive Reproduction

Nudibranchs are slow and solitary, and adults live for only around 18 months. This poses a bit of a problem, in that nudibranchs don't get many chances at romance. They meet few fellow sea slugs throughout a lifetime, and some of those they do meet will be looking for a meal, rather than a mate — given that the predominant predators of nudibranchs are other nudibranchs. So it is vital that, when two adult nudibranchs of the same species do run into each other, they're able to make the most of their intimate encounter. Their strategy is to forgo becoming a binary sex — male or female — and instead, they just have the best of both worlds.

Nudibranchs are simultaneous hermaphrodites, meaning that every individual has the reproductive organs of both a male and a female. Not only does this allow any nudibranch to mate with any other nudibranch of the same species, but a mating pair can also exchange both sperm and egg cells. The two nudibranchs then go their separate ways to each lay their clutches of fertilized eggs — which are attached to a hard surface, strung together in a long ribbon-like shape, and laid in a curious, clockwise spiral. After a few days, the eggs hatch into larvae that drift around in the water column and, after some weeks of free-floating childhood, they settle on a solid surface and metamorphose into their pretty pink, sea slug forms.

A little leaf-sheep (*Costasiella kuroshimae*).

¹ There is another sea slug species known as the "leaf sheep," which has a white face with two black-dot eyes and rosy cheeks, and a back that looks to have sprouted a bunch of green leaves.

While the leaf sheep is a sea slug, it's not a nudibranch, but rather a member of a distantly related group of gastropods known as the sacoglossans — unlike the carnivorous nudibranchs, the sacoglossan sea slugs are predominantly plant eaters. The leaf sheep grazes on algae and incorporates their chloroplasts into the "leaves" on its back, allowing it to photosynthesise — part animal, part plant.

² Flamingos are born a dull grey colour and only get their bright pink feathers by eating food rich in beta-carotene (a red-orange pigment) — food like algae, brine fly larvae, and brine shrimp.

**Where Does It Live?**

⛰️ Warm coastal waters and tide pools.
📍 Western coast of North America; from the lower Oregon coast to Northern Baja California, Mexico (most common along Southern California).

Size // Tiny
Length // 2.5 cm (1 in)
Weight // N/A
Activity: N/A
Lifestyle: Solitary 👤
Lifespan: ~18 months
Diet: Carnivore
Favorite Food: Pink encrusting bryozoans
Phylum: Mollusca
Class: Gastropoda
Order: Nudibranchia
Family: Goniodorididae
Genus: Ceratodoris
Species: C. rosacea

Hopkin's rose nudibranch — a species of sea slug — grows no larger than 2.5 cm (1 in).
Among its frilly pink tentacles, it has two feather-like "ears" called rhinophores which it uses to detect dissolved chemicals in the water.
It also has a bundle of feathery appendages at its rear end which it uses to breathe (the word 'nudibranch' — from the Greek 'nudi' and Latin 'branch', meaning "naked gills" — refers to these). These gills are unfortunately located around the nudibranch's anus.
This pink sea slug's favourite prey is pink encrusting bryozoans. Each individual bryozoan is no larger than 3mm long (0.1 in) and resides within its own shell-like structure. They live in large colonies that "encrust" the surfaces of coastal rocks. The nudibranch uses its 'radula' — an organ analogous to a tongue but covered in many sharp teeth — to scrape bryozoans from their rock and extract them from their shells.
Like a flamingo eating algae and brine shrimp to get pink feathers, the rosy nudibranch's minuscule pink prey is where it gets its pink pigment (specifically from a carotenoid called hopkinsiaxanthin).
Pinkness isn't all the nudibranch gets from the bryozoans; it also steals their toxins and incorporates them into its many waving appendages. Its garish appearance combined with its toxicity, then, is likely a case of aposematism — wherein it purposefully advertises its unpalatability to predators.
Nudibranchs can move at about 30 cm (1 ft) per hour if they're really determined. The Hopkin's rose nudibranch also only lives for around 18 months. A life both slow and short.
Nudibranchs are simultaneous hermaphrodites, meaning that every individual has the reproductive organs of both a male and a female. When two meet, they exchange both sperm and egg cells, and both leave with clutches of fertilized eggs — which are then laid in long, pink, clockwise spirals.