Cleaning Symbiosis: Underwater Hygiene with the Ocean’s Favourite Cleaning Experts

Photo Story Published : May 13, 2021 Updated : Sep 24, 2023

Cleaner wrasses and cleaner shrimps feed on parasites for a living. This, in turn, helps animals that want parasites removed from their bodies, and results in a healthier ecosystem

What does it take for animals to survive in the wild? The most obvious factors that come to mind include being able to forage or hunt, while successfully evading predators. A critical factor we often forget to think about, perhaps due to its seemingly subtle and less exciting nature, is the ability of wild animals to stay clean. Some of the most fascinating interactions and behaviours to behold in the wild are not always the sensational hunting sequences, animals devouring their kill, or their elaborate mating rituals. It is the extent to which some animals will go to maintain good hygiene. This is especially fascinating, and easy to observe in the ocean where animals of all sizes come together in the quest to be and help others be disease-free.

Parasites latch onto fish of all sizes, getting between their scales, gills, and other soft tissue. They steadily drain their hosts’ resources and increase their chances of contracting diseases. Parasites can literally weigh their hosts down. Some of the most unlikely examples of cooperation between species in the ocean have evolved to address this threat to an animal’s fitness.

“Cleaning symbiosis” is a fascinating, mutually beneficial therapeutic exchange between animals that want parasites removed from their bodies, and animals that feed on those parasites for a living. Humans are not the only ones to understand the value of “spa treatments”; fish and aquatic invertebrates also partake in the stress-relief and hygienic benefits of being cleaned and massaged, often by unrelated species. Cleaners like cleaner wrasses and cleaner shrimps offer these fish (their clients) the service of carefully getting rid of their parasites, nipping them in the bud. A win-win situation to all parties involved in the cutthroat race for survival.

Cleaner wrasses (Labroides dimidiatus) are small and slender fish that dedicate their lives to cleaning other animals on a coral reef. These wrasses are not doing other fish any favours but are working towards a clear goal. Each parasite removed is food in their bellies. Cleaner wrasses can usually be seen working in pairs, either scouting the perimeters of their ‘cleaning stations’ in small hops or making sprinted swims, as freelance agents, soliciting potential clients from around the reef. Whether in-depth cleaning treatments for larger carnivores like groupers, or quick inspections for schooling fusiliers who cannot stop for too long without increasing predation risk, wrasses will customise their treatments for different client species. Video: Vikas Nairi Subba
Cover photo: Umeed Mistry

Client species have a variety of different behaviours and signals to communicate effectively with cleaner wrasses. Parrotfish tilt their bodies almost entirely and become vertical to indicate that they are ready for a clean-up while rabbitfish gradually turn a darker hue. Cleaners must pay attention to these, often subtle, signals. Which part of its body is the stingray flapping for a clean? Is the squirrelfish signalling for a gill check yet? Which fin is the sweetlips twitching? Video: Umeed Mistry

A cleaning station is meant to be a space for cleansing. Cleaners play a vital role in creating an atmosphere where competitors and predators leave their aggression at the door, making it a safe space for small fish. Cleaning stations are often visited by regular clients that are comfortable and satisfied with the service offered by specific wrasses. Wrasses also benefit by remembering individual client fish and their past visits, especially if they went poorly. Gentle strokes on the back of a client fish (tactile stimulation using fins) can often assuage unhappy client fish and make up for mistakes (an accidental bite for instance). Video: Samuel John

Infrequent but not altogether rare visitors to cleaning stations are migratory and pelagic species. Long-distance travellers like manta rays, turtles, and sharks are important high-value customers because of the potential load of parasites they bring. They are, however, too large-scale an operation to undertake for any one pair of cleaner wrasses. Cleaners can often be seen taking on several collaborators opportunistically. These can include a variety of reef fish from damsels, bannerfish, butterflyfish, surgeonfish, to other wrasses. Such interactions are a wonderful example of fish taking real-time decisions, evaluating the cost-benefit of sharing and cooperating with other species over a common resource. Photo: Umeed Mistry

Even though this relationship is rooted in cooperation and trust, it is governed by best practices to ensure that clients are satisfied, and cleaners do not cut corners. Clients trust that cleaners will eat only their parasites while cleaners trust that they will come out alive after cleaning. This sets a solid framework to work with on most days but there are instances when cleaners cannot resist and cheat. While parasites are a cleaner’s main prey, scales, mucous and tissue are nutritious too. The cost cheaters bear can range from aggressive chases, loss of a client permanently, or if the breach is with a predatory fish — death! Video: Samuel John

Even if we’re convinced of the importance of cleaning in an individual animals life, what is truly staggering is the inadvertent impact that this interaction can have on the health of an entire ecosystem. A study conducted off Lizard Island in the Great Barrier Reef, Australia found that a single cleaner wrasse can have over 2,000 client fishes visiting them within a span of a four-hour working day. Reef patches where cleaning stations were experimentally removed for 18 months showed that fish species plummeted to half and their abundance reduced to one-fourth of the original population. Photo: Umeed Mistry

The cleaner-client symbiosis is straightforward in the benefits it offers but remains quite complex in how it functions. While scientists are still in the process of understanding the nuances of these relationships, a visit to the nearest reef is enough to witness this magnificent and action-packed phenomenon unfold. People often wonder why cleaning symbioses are compared to spa treatments because humans are not necessarily looking to have parasites removed while on a massage table. Studies of their hormones show that fishes exposed to high levels of stress solicit cleaning even when they seem to have no parasites. The physical contact that comes with cleaning immediately relaxes them. Photos: Umeed Mistry

Cleaner wrasses are the easiest cleaning agents to observe underwater, they swim around boldly and sport bright colours. Cleaner shrimps are just as active and popular cleaning agents in the market but are harder for us to find. They gather in large numbers around dark reef crevices. This choice of location allows client fish to receive their treatment with more privacy and ensures better safety. Cleaner shrimps help fish eliminate small parasites but even merely walking over their bodies seems to provide sufficient tactile stimulation and relaxation. Video: Chetana Babburjung Purushotham

The 21st century is turning out to be a period of catastrophic change. Coral reefs are being devastated by the impacts of coral bleaching, ocean acidification, overfishing, and marine pollution. At times like these, we need small-scale relationships like cleaning mutualisms to be sustained. The seemingly simple act of getting cleaned could go a long way in helping fish populations cope with the monumental stress of seeing their habitats disappear in their lifetimes. Photos: Umeed Mistry