Bluestreak cleaner wrasse
and Remoras
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We have all heard of these fish that live in association (usually symbiotic) with larger fish, taking on the task of ridding them of dead skin, mucus and parasites. These cleaner fish mainly belong to two groups: cleaner wrasse and remoras. These are the best known, but there are in fact a large number of cleaner fish1.
Both belong to the same superclass in the classification of living organisms: the Actinopterygii. Whilst the former refers to a specific species – Labroides dimidiatus – the latter refers to several species (five) of the same genus: the genus Remora.
Both have a similar distribution: they live in the warm reef waters of the Indian Ocean, off the African coast, and in the western Pacific Ocean, in the reefs of northern Australia. Wrasse inhabit coral-rich areas and lagoons. Remoras have a wider distribution than wrasse: they are also found along the coasts of the equatorial western Atlantic29.
Remoras remain, however, little studied in comparison with wrasse. The mutualistic nature of the relationships between remoras and some of their hosts is also sometimes called into question2. This is the case with dolphins, for example, where remoras are thought to make their swimming more energy-consuming and where their attachment may irritate their skin; the dolphins would therefore derive little benefit3.
Schema. Simplified classification of wrasses and remoras (based on WORMS, World Register of Marine Species, https://www.marinespecies.org/).
Differences and similarities between wrasse and remora
Cleaner wrasses (Labroides dimidiatus)
First appearance in the fossil record at the end of the Cretaceous – beginning of the Palaeogene, around 62 million years ago. Most of the lineages leading to modern groups already existed at the beginning of the Miocene, around 23 million years ago4.
The first occurrence of cleaning behaviour in Labridae is estimated to be at the end of the Miocene, around 10 million years ago5.
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Lives at depths of between 2 and 30 metres
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Average size: 10 cm (max. 14 cm)
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Lives during the day (it builds a protective mucous cocoon every night)
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Can change gender
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Lives in cleaning stations within coral reefs
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Remora (Remora)
First appearing in the fossil record as Opisthomyzon at the start of the Oligocene, around 30 million years ago6.
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Capable of following its hosts down to bathypelagic depths! Observations have recorded common remoras at depths of 1,460 metres in waters with temperatures of around 3.4 °C7.
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Average size of 40 cm, but some species can grow up to 1 m
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Travels with its hosts
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Remora remora Original drawing.
A elongated body, lacking a dorsal fin, which has evolved into a sucker composed of several transverse lamellae, enabling them to attach themselves to their hosts.
Labroides dimidiatus Original drawing.
A slender body, recognisable by the broad black longitudinal stripe running across its side and eye. Its back and belly are white (the back is sometimes slightly yellowish). This white colour transitions to a bright blue on the animal’s underside, whilst the black stripe widens towards the tail. Juveniles, meanwhile, are black with a bright blue line.
Ecology and recent scientific discoveries
Cleaner wrasse are among the few species in which a positive response to the mirror test has been demonstrated. Thus, they are capable not only of recognising themselves in the mirror’s reflection, but also of forming a mental image of what their face looks like and possessing self-awareness8-10! It has therefore been suggested that self-awareness was already present in the common ancestor of modern vertebrates11. Wrasse are also capable of recognising clients with whom they are familiar12. These fish possess advanced cognitive functions and a complex brain, which are likely to be permanently affected by increasingly frequent and intense heatwaves13. The impact of sleep on their cognitive abilities has also been tested, demonstrating a significant decline in their cognitive abilities following sleep deprivation14,15.
They remain in “cleaning stations”, where larger fish come to be cleaned. These cleaning stations are occupied by a pair of adults, a group of juveniles, or a group of females accompanied by a dominant male (one of the females then becoming a male in the event of the latter’s death or disappearance)16,17–29. Their behaviour differs slightly from one region to another18.
Studies have shown that clients choose the individuals they approach to be cleaned. For instance, longer black stripes increase the number of visitors to the cleaning station19, and a vibrant blue colour is thought to indicate a higher quality of cleaning service20.
A recent study conducted in French Polynesia highlights the importance of using video recordings for ecological studies of wrasse, as human presence disrupts interactions between the wrasse and their clients21.
These mutualistic relationships allow wrasse to feed whilst making their presence beneficial to larger fish: the latter are rid of their parasites and dead skin. They thus play an important ecological role in reef environments. Their experimental exclusion from the environment has led to changes in the behaviour and size of client fish22 as well as in their abundance23.
It should be noted that, like any mutualistic relationship, this one can turn into parasitism when the host no longer derives any benefit. This occurs when wrasse begin to ‘cheat’ by feeding on the skin tissue of the fish they are cleaning, but they choose which clients they cheat on12,24.
Remoras are most commonly associated with sharks and whale sharks25, but they are also found on other large fish, sea turtles26, cetaceans, rays27 or even boats29. Remoras usually form relationships with a specific species. They possess receptors that enable them to know when to attach themselves to their hosts28.
Remoras need a constant flow of water through their gills to breathe, which their host provides, along with protection, transport and food. They can remain attached to their host for up to three months. We have little information about their reproductive process, but pairs of remoras have sometimes been observed attached to the same host.
Bibliography
1. Côté, I. M. Evolution and ecology of cleaning symbioses in the sea. in Oceanography and Marine Biology: an Annual Review vol. 38 p311-355 (R.N. Gibson and Margaret Barnes, 2000).
2. Gayford, J. H. The multidimensional spectrum of eco‐evolutionary relationships between sharks and remoras. Journal of Fish Biology 105, 4–9 (2024).
3. Wingert, N. et al. Relationships Between Common Bottlenose Dolphins (Tursiops truncatus) and Whalesuckers (Remora australis) at a Remote Archipelago in the Equatorial Atlantic Ocean. Aquat Mamm 585–598 (2021) doi:10.1578/AM.47.6.2021.585.
4. Cowman, P. F., Bellwood, D. R. & Van Herwerden, L. Dating the evolutionary origins of wrasse lineages (Labridae) and the rise of trophic novelty on coral reefs. Molecular Phylogenetics and Evolution 52, 621–631 (2009).
5. Baliga, V. B. & Law, C. J. Cleaners among wrasses: Phylogenetics and evolutionary patterns of cleaning behavior within Labridae. Molecular Phylogenetics and Evolution 94, 424–435 (2016).
6. Friedman, M., Johanson, Z., Harrington, R. C., Near, T. J. & Graham, M. R. An early fossil remora (Echeneoidea) reveals the evolutionary assembly of the adhesion disc. Proc. R. Soc. B. 280, 20131200 (2013).
7. Fontes, J., Castellano‐González, G., Macena, B. C. L. & Afonso, P. Hitchhiking to the abyss. Ecology and Evolution 13, e10126 (2023).
8. Kohda, M. et al. Cleaner fish recognize self in a mirror via self-face recognition like humans. Proc. Natl. Acad. Sci. U.S.A. 120, e2208420120 (2023).
9. Kobayashi, T., Kohda, M., Awata, S., Bshary, R. & Sogawa, S. Cleaner fish with mirror self-recognition capacity precisely realize their body size based on their mental image. Sci Rep 14, 20202 (2024).
10. Sogawa, S. et al. Rapid self-recognition ability in the cleaner fish. Sci Rep 15, 41882 (2025).
11. Kohda, M., Sogawa, S. & Bshary, R. On the mirror test and the evolutionary origin of self-awareness in vertebrates. Phil. Trans. R. Soc. B 380, 20240312 (2025).
12. Tebbich, S., Bshary, R. & Grutter, A. Cleaner fish Labroides dimidiatus recognise familiar clients. Anim Cogn 5, 139–145 (2002).
13. Pereira, B., Cascalheira, L., Rosa, R. & Paula, J. R. Alteration of cleaner wrasse cognition and brain morphology under marine heatwaves. Functional Ecology 39, 1894–1905 (2025).
14. Sowersby, W., Kobayashi, T., Awata, S., Sogawa, S. & Kohda, M. The influence of sleep disruption on learning and memory in fish. Journal of Sleep Research 34, e70005 (2025).
15. Yoshida, M., Izumi, A., Sogawa, S., Awata, S. & Kohda, M. Characteristics of the sleep structure in the cleaner wrasse Labroides dimidiatus. Zoological Lett 12, 3 (2026).
16. Sakai, Y., Kohda, M. & Kuwamura, T. Effect of changing harem on timing of sex change in female cleaner fish Labroides dimidiatus.
17. Triki, Z. & Bshary, R. Sex differences in the cognitive abilities of a sex-changing fish species.
18. Bansemer, C., Grutter, A. S. & Poulin, R. Geographic Variation in the Behaviour of the Cleaner Fish Labroides dimidiatus (Labridae). Ethology 108, 353–366 (2002).
19. Stummer, L. E., Weller, J. A., Johnson, M. L. & Côté, I. M. Size and stripes: how fish clients recognize cleaners. ANIMAL BEHAVIOUR 68, 145–150 (2004).
20. Cacela-Rodrigues, I., Trigo, S., Araujo-Silva, H., Saraiva, J. L. & Soares, M. C. Client reef fish prefer more blue-saturated cleaner wrasses Labroides dimidiatus. Journal of Experimental Biology 228, jeb249694 (2025).
21. Cryan, D. M., Freeze, J. & Osenberg, C. W. Comparing remote and hand‐held video observation methods for quantifying a cleaning mutualism. Journal of Fish Biology 106, 775–783 (2025).
22. Waldie, P. A., Blomberg, S. P., Cheney, K. L., Goldizen, A. W. & Grutter, A. S. Long-Term Effects of the Cleaner Fish Labroides dimidiatus on Coral Reef Fish Communities. PLoS ONE 6, e21201 (2011).
23. Grutter, A. S., Bejarano, S., Sun, D. & Mumby, P. J. Indirect effects of cleaner fish Labroides dimidiatus on fish grazing per reef area and benthic community structure. Mar Biol 169, 135 (2022).
24. Wilson, A. D. M., Krause, J., Herbert-Read, J. E. & Ward, A. J. W. The Personality Behind Cheating: Behavioural Types and the Feeding Ecology of Cleaner Fish.
25. Norman, B. M., Reynolds, S. D. & Morgan, D. L. Three-way symbiotic relationships in whale sharks. Pacific Conservation Biology 28, 80–83 (2022).
26. Sazima, I. & Grossman, A. Turtle riders: remoras on marine turtles in Southwest Atlantic. Neotrop. ichthyol. 4, 123–126 (2006).
27. Nicholson-Jack, A. E., Harris, J. L., Ballard, K., Turner, K. M. E. & Stevens, G. M. W. A hitchhiker guide to manta rays: Patterns of association between Mobula alfredi, M. birostris, their symbionts, and other fishes in the Maldives. PLoS ONE 16, e0253704 (2021).
28. Cohen, K. E., Flammang, B. E., Crawford, C. H. & Hernandez, L. P. Knowing when to stick: touch receptors found in the remora adhesive disc. R. Soc. open sci. 7, 190990 (2020).
29. WORMS, World Register of Marine Species, https://www.marinespecies.org/
March 2026
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