The Sea Slug That Refused to Be Sensible

In Investing in 3.8 Billion Years of Research, we began with three marine creatures: the sea urchin, the octopus and the jellyfish. Together, they revealed what industry might learn from biological approaches to material formation, distributed intelligence and energy-efficient movement. Our next candidate is the glorious nudibranch.

More specifically, the Spanish dancer: a large, brightly coloured sea slug found across the Indo-Pacific, including the Red Sea at its western edge. Nudibranchs are gastropods, the group of molluscs that includes snails and slugs, but their ancestors abandoned the adult shell and pursued a more adventurous form of engineering.

The Spanish dancer feeds on sponges, acquires defensive chemicals from them, concentrates those chemicals in vulnerable parts of its body and transfers them to its eggs. When threatened, it unfurls the broad edges of its mantle and swims through the water like a flamenco dancer working a skirt. It also occasionally carries an emperor shrimp around on its back.

Recent science has made the creature more interesting still. For more than 200 years, most Spanish dancers were treated as one highly variable species, Hexabranchus sanguineus. Its scientific name means something close to “blood-coloured six-gills”, which is accurate in spirit, although the number of gill branches is not always six.

A 2023 study combining molecular evidence, anatomy, colour patterns and taxonomic history concluded that the familiar Spanish dancer was not one species but a complex containing at least five. Science had not found a Spanish dancer. It had found a troupe. A Spanish dancer? No! A troupe of dancers

This matters for more than zoological tidiness. Different members of the group may feed on different sponges, possess different chemistry or produce subtly different structures and movements. Before translating a biological capability, we need to know which organism possesses it. Therefore it’s important to identify the dancer before copying the dance.

What the Spanish dancer can do

Giving up the shell initially looks like a poor exchange. A shell provides protection, somewhere to retreat and a substantial obstacle to being eaten. Abandoning it is rather like selling the house because the front door is inconvenient.

But the shell also carries weight and imposes constraints. Without it, nudibranchs turned their exposed tissues into highly functional surfaces supporting movement, respiration, warning colouration and chemical defence. The Spanish dancer did not replace its shell with one new form of protection. It assembled a system from flexibility, chemistry, signalling and ecological relationships.

Turning softness into movement

Most of the time, the Spanish dancer crawls along the reef with the edges of its mantle folded around its body. When disturbed, it releases the mantle and combines pitching of the whole body with travelling waves passing through the flexible tissue.

Researchers have modelled this movement using computational fluid dynamics. Their simulations indicate propulsive efficiencies of up to approximately 57%, with simulated terminal speeds of 1.33 body lengths per movement cycle. Its overall body shape also appears to improve its swimming performance. Swimming without a spine

The animal does not have a propeller, articulated fins or a separate transmission. Its body becomes structure, actuator and control surface at the same time.

This extends the principle we encountered through the octopus in Investing in 3.8 Billion Years of Research. The octopus distributes sensing and control through its soft body and nervous system. The Spanish dancer shows how geometry and compliant material can contribute directly to propulsion. In both cases, part of the intelligence resides in the body rather than entirely in a central processor.

That principle is central to soft robotics. Conventional underwater machines can struggle around reefs, aquaculture systems, cables and confined spaces. A more compliant machine could absorb impacts, adapt to irregular structures and move without exposed propellers.

It would not need to resemble a mechanical sea slug. Biomimicry becomes limiting when it copies an animal’s appearance rather than extracting the principle that makes it work. There is little value in constructing a robotic nudibranch merely to inconvenience both robotics and nudibranchs. The more useful question is whether material and geometry can perform part of the control function normally assigned to motors, sensors and software.

Turning dinner into defence

Nudibranchs have developed a remarkable range of chemical defences. Some manufacture compounds themselves. Others acquire them through their diets, retaining, concentrating or modifying chemicals produced by their prey.

The Spanish dancer feeds on chemically defended sponges. Research published in 1988 identified closely related oxazole-containing macrolides in the sponge, the nudibranch and its egg ribbons. Feeding trials found that tissue from the animal’s mantle was rejected by fish and crab predators. The defensive compounds were concentrated where the animal was most exposed to attack and within tissues involved in digestion and reproduction. Defensive chemistry of the Spanish dancer

The most abundant macrolide found in the sponge was absent from both the nudibranch and its eggs, suggesting that the compound was modified during digestion. The animal was not simply eating chemical protection and carrying it around. It appeared to be selecting, processing, transporting and redeploying it.

This raises questions with obvious industrial relevance. How does the animal identify useful compounds in a chemically complex meal? How does it absorb them without poisoning itself? How are they transported and safely stored? How and why are some modified?

Marine organisms already provide important leads for pharmaceutical and biotechnology research. Compounds associated with nudibranchs have demonstrated cytotoxic, antimicrobial, antifungal and other biological activity. In the Spanish dancer and its relatives, these include macrocyclic compounds such as kabiramides and halichondramide derivatives. Bioactive compounds from marine heterobranchs

This does not make the Spanish dancer a pharmaceutical company. Activity in a laboratory is only the beginning of a long process involving toxicity, selectivity, dosage, synthesis and clinical evidence. The more distinctive opportunity may lie not in one molecule but in the animal’s approach to selecting, transforming and safely storing complex chemistry. Industry expends considerable energy separating useful chemicals from complex mixtures and applying them to products after fabrication. The sea slug eats a sponge.

Protecting the next generation

The Spanish dancer lays elaborate ribbons containing thousands of eggs. These structures form in seawater, remain physically coherent, permit development and gas exchange, and discourage predators.

The eggs cannot crawl away, hide or deploy the emergency flamenco, so chemical protection is incorporated into the ribbon. The 1988 study found defensive macrolides at substantially higher concentrations in the egg masses than in either the sponge or the adult animal. The Spanish dancer appears to acquire chemistry from another organism, process it, concentrate it and transfer it into material protecting the next generation.

Egg masses attributed to Hexabranchus sanguineus have also yielded ulapualides with cytotoxic activity against selected cancer cell lines. This is not evidence of a treatment, but it reinforces the significance of the egg ribbon as a chemically active structure rather than simply decorative marine spaghetti. Ulapualides from a Spanish dancer egg mass

Protective industrial materials are often fabricated first and coated later, using multiple processes and considerable energy. The Spanish dancer forms the structure and incorporates its protection together, in seawater and at ambient temperature. That is interesting materials science, even before anything hatches from it.

Colour, integration and the shrimp

The Spanish dancer’s conspicuous red, orange and white patterns appear to warn predators that taking a nibble may be a mistake. The signal benefits both parties: the nudibranch avoids injury, while the predator avoids an unpleasant meal.

Human risk communication often works in the opposite direction. Information is added until disclosure has technically occurred, even though the resulting message has become almost impossible to notice or understand. That may sound uncomfortably familiar.

The biological principle is straightforward. A warning should be recognisable before the dangerous interaction occurs and correspond honestly with the underlying risk. The Spanish dancer’s colour is part of its defence system. It was not added by the marketing department.

That integration runs throughout the animal. Its soft body supports movement. Its diet supplies chemical protection. Its tissues concentrate the compounds where they are needed. Its colour communicates the risk, while its egg ribbon combines reproduction, material formation and defence.

Then there is the emperor shrimp! These small crustaceans are sometimes found travelling on Spanish dancers and other large marine animals. The shrimp gains transport, access to food and a degree of protection, apparently without causing significant harm to its host.

The Spanish dancer has therefore combined soft propulsion, chemical procurement, defensive signalling, reproductive materials engineering and a limited public transport service.

More importantly, the shrimp reminds us that biological capability rarely belongs to one organism alone. The nudibranch’s chemistry begins inside a sponge. Its movement depends on the properties of water. Its warning works because predators recognise the signal. Its passenger exploits the mobility and protection created by the larger animal. The relevant unit of research may not be the organism. It may be the relationship or the ecosystem.

Applying the two maps

The investment thesis developed in Investing in 3.8 Billion Years of Research begins with two maps. The first maps biological capabilities: the structures, chemistry, functions and behaviours that living systems have developed through 3.8 billion years of research and development. The second maps the problems that industry is spending money trying to solve. The opportunity lies where the maps intersect.

The Spanish dancer’s biological map includes compliant propulsion, embodied control, selective chemical acquisition, safe storage of defensive compounds, ambient formation of protected materials and visible risk signalling.

The industrial map includes low-disturbance underwater inspection, soft robotics, efficient actuation, pharmaceutical discovery, molecular separation, protective materials and clearer communication of risk.

Several intersections deserve attention. Compliant propulsion could support underwater vehicles capable of working safely around fragile or complex structures. Chemical acquisition could inform discovery platforms or selective separation processes. Egg-ribbon formation could contribute to active materials and protective coatings produced under ambient conditions.

The maps identify possibilities. They do not establish that any of them should receive investment. For that, the candidate must face the questions.

Putting the candidate through the investment process

A serious biomimetic investment process tests at least six things.

First, does the capability address an important industrial problem? Underwater inspection, low-disturbance observation, chemical discovery and protective materials represent real requirements and potentially valuable markets.

Second, can the principle be separated from the organism? A machine does not need to resemble a Spanish dancer to use compliant undulatory propulsion. A chemical-selection process does not need to contain a nudibranch to learn from molecular uptake, modification and storage.

Third, is the science mature enough to translate? The locomotion has been modelled, and underwater soft robotics is advancing. The defensive chemistry has been studied for decades. Important gaps remain between describing what the animal does and reproducing the function reliably.

Fourth, can protectable intellectual property be created? The biological observation belongs to the living world. Protectable value is more likely to arise through new materials, actuators, control systems, biosynthetic methods, separation processes and particular applications.

Fifth, can the technology scale economically and sustainably? A robot that performs one impressive laboratory undulation is not yet an underwater inspection business. A bioactive compound isolated from an egg ribbon is not automatically a safe, manufacturable drug. Harvesting sea slugs or the sponges they consume would be ecologically unacceptable and commercially absurd.

Finally, does the solution offer a material advantage over existing alternatives? It must be safer, cheaper, more efficient, more adaptable or capable of doing something that conventional technology cannot. Nature-inspired is not itself a competitive advantage.

Against those tests, the Spanish dancer produces a mixed but interesting result.

Its soft propulsion sits at the intersection of an established industrial requirement and an advancing field of engineering, but direct commercial translation remains early. Its chemistry has substantial scientific value and possible pharmaceutical relevance, although the development risks are considerable. The more distinctive opportunity may lie in the methods through which useful molecules are selected, modified, transported and incorporated into materials. Its egg ribbons, warning system and ecological relationships provide valuable research questions, but not yet obvious standalone investments. The Spanish dancer is not one finished company. It is a portfolio of capabilities requiring structured investigation.

The verdict

The Spanish dancer makes the research portfolio. It does not yet make the venture portfolio.

A conventional investment fund waits for companies to arrive with a product, management team, intellectual property and identifiable market. By that stage, most of the biological search and scientific translation has already taken place, if it happens at all.

The model proposed in Investing in 3.8 Billion Years of Research begins earlier. It sits somewhere between a research institute, a venture studio and a specialist investment fund. It starts with the two maps, identifies promising intersections and brings together the biologists, chemists, materials scientists, engineers, entrepreneurs and industrial partners required to test them.

It would not begin by funding “the Spanish dancer”. It might establish separate translation programmes around compliant underwater propulsion, selective molecular capture and the ambient formation of protected materials. Each would have its own scientific milestones, industrial partners and investment gates.

Some would fail. The underlying function might prove impossible to reproduce, too expensive to manufacture or no better than an existing technology. That is not a failure of the investment model. It is the purpose of the process: to answer difficult questions before committing large amounts of capital to an attractive biological story.

Successful programmes could become companies, licensing platforms or technologies embedded within existing industries. The others would return knowledge to the biological and industrial maps.

Nature contains an immense stock of functional intelligence, while industry contains an equally large collection of expensive problems. Very few institutions are organised to search systematically across both. The Spanish dancer again demonstrates why one might be needed.

The glorious nudibranch

The Spanish dancer may be less commercially mature than the sea urchin, octopus or jellyfish. It may also provide the richer research programme because its value does not reside in one obvious function.

It discarded armour and turned flexibility into propulsion. It converted dinner into chemical defence, moved that protection to the tissues most likely to be attacked and concentrated it further around the next generation. It formed an active material in seawater and communicated danger using a signal that predators could actually understand. And, occasionally, it gave a shrimp a lift.

There is a broader lesson here. Capability is easily overlooked when it arrives in an unfamiliar form. Strength is expected to be rigid. Intelligence is expected to sit inside a brain or processor. Protection is expected to be manufactured internally. Serious engineering is not expected to resemble a brightly coloured sea slug leaving the scene in a flamenco dress with a crustacean on its back.

The Spanish dancer violates those expectations. Its apparent vulnerability became the starting point for a different combination of capabilities. We may never build a machine that looks exactly like it, and there is little reason to try. The opportunity is to understand how a soft-bodied animal turns flexibility into movement, food into protection, chemistry into material and colour into communication, then test where those principles might solve problems our rigid machines and fragmented industries still cannot.

The Spanish dancer may or may not make the final portfolio. But after several hundred million years without a shell, it has certainly earned an interview.

Our next candidate kept its armour, put eyes in it and built self-sharpening teeth from iron.

This essay began as a line of enquiry developed through the work on Value2Society. ChatGPT was used as a thinking and editorial partner, testing the argument, challenging its edges and helping shape it for publication.