Remoras, also known as suckerfish, are a family of ray-finned fish best known for their ability to adhere themselves to larger marine animals using a suction disk on top of their heads. Remoras most commonly attach to sharks, whales, turtles, and rays, hitching free rides around the ocean while feeding on scraps leftover from the host animal’s meals.
If you’re short on time, here’s a quick answer to your question: Remoras exhibit some parasitic behaviors, but most scientists do not consider them true parasites, instead classifying them as symbiotic organisms that benefit their hosts as well as themselves.
In this approximately 3000 word article, we’ll take an in-depth look at the relationship between remoras and their hosts to understand whether remoras should technically be classified as parasites or not.
We’ll examine remora anatomy and behaviors, the costs and benefits to both remoras and their hosts, and insights from scientific research on this unique relationship.
Remora Anatomy and Attachment
Suction Disk for Attachment
Remoras, also known as suckerfish, have a unique oval-shaped suction disk on top of their heads that allows them to attach themselves to larger marine animals like sharks, rays, sea turtles, and whales (Florida Museum).
This suction disk consists of a modified dorsal fin made up of a series of parallel laminae or ridges with tissue in between. By manipulating the angles of these bony ridges and altering the amount of suction, the remora can quickly and strongly attach itself to smooth surfaces.
Studies have shown that the remora’s suction disk allows it to withstand forces over 300 times its own body weight (Fulcher and Motta, 2019). The rim of the disk also contains small spinules or tooth-like projections that help provide additional grip.
This incredibly effective means of attachment likely evolved as an adaptation to allow the smaller remora to safely hitch rides on larger hosts for transportation, protection, and even access to parasites or prey leftovers from the host.
Other Adaptations
In addition to their signature suction disk, remoras also have other key anatomical and behavioral adaptations that facilitate their commensal relationship with hosts:
- A slender, streamlined body shape reduces drag and turbulence when swimming attached to fast marine animals.
- Coloration that matches host animals provides camouflage from predators.
- The ability to rapidly flash their white undersides is thought to communicate with hosts.
So while the remora’s suction disk is its primary means of hitching a ride on larger marine creatures, the fish also possesses secondary adaptations that maximize its chances of survival through this unusual symbiotic lifestyle (O’Toole, 2002).
Remora Diet and Feeding Behaviors
Feeding on Host’s Leftovers
Remoras have developed an ingenious feeding strategy by attaching themselves to larger marine animals like sharks, rays, sea turtles and whales. As the host swims around, remoras get to enjoy any scraps of food left over from the host’s meals.
They use their sucker-like dorsal fins to hang onto the host, allowing them to snatch bits of prey as the host feeds. Studies have shown around 28% of a remora’s diet comes directly from scavenging on host leftovers of meals.
This gives remoras easy access to food without expending much energy finding and capturing prey themselves. The downside is they have to eat whatever their host happens to be dining on that day, like crustaceans, squid, fish or mollusks.
Some key benefits remoras receive from feeding on host leftovers include:
- Easy access to a regular food supply without hunting
- Energy savings since they don’t have to chase down prey
- Safety from predators by hiding near a larger host
While it’s true remoras benefit from this association by getting free leftovers, they don’t actually harm the host in the process. Their food stealing is relatively minor compared to the host’s total food intake.
So this commensal relationship is beneficial to the remora without really costing the host much.
Stealing Parasites from Hosts
In addition to scavenging scraps, some species of remora have developed an ingenious technique for catching parasites on their host’s body! By darting around the host and nibbling at their skin, remoras get a tasty meal of parasites like copepods, isopods and cymothoid fish lice.
This cleaning symbiosis helps keep the host parasite free and gives remoras access to parasites as a unique food source. Research by marine biologists has found parasite removal accounts for around 19% of a remora’s total feeding time and diet.
Some key benefits of remora parasite feeding include:
- Access to parasites as an abundant food source
- The host gets cleaned of pesky parasites
- Builds goodwill between remora and host by providing a service
While remoras gain nutrition from eating parasites, their feeding provides the valuable service of parasite removal for the host animal. So this mutualistic relationship is beneficial for both species involved. The host gets cleaned while the remora gets fed!
Costs and Benefits to Remoras
Transportation and Food Access Benefits
Remoras, also known as suckerfish, have formed a symbiotic relationship with larger marine animals that provides them with substantial benefits related to transportation and food access (Mulvany, 2020).
By attaching themselves to sharks, whales, turtles and other large hosts with their suction-cup like dorsal fin (NOAA, 2022), remoras gain free access to food-rich locations as they hitch a ride on their host’s migrations across the ocean.
This allows remoras to expend less energy swimming and foraging on their own while still gaining access to ample prey in the form of leftovers from the host’s meals and organisms stirred up in their wake (Allen, 2002). Some statistics on the transportation benefits remoras receive:
- Remoras attached to whale sharks have been recorded traveling over 3,000 miles migrating between rich feeding areas (Schmidt et al., 2009).
- Individual remoras may spend upwards of 30 consecutive days attached to the same dolphin host during its travel and foraging (Silva Jr & Sazima, 2003).
- Computer modeling suggests remoras gain a 10-15% energetic benefit from hitching rides versus self-propulsion ( Weihs et al., 2007).
In terms of food access, remoras excel at snatching up scraps of prey stirred up by larger hosts as well as bits of food dropped by messy eaters like sharks and groupers. Their connection to their host also provides safety in numbers by deterring potential predators.
Scientists have documented over two dozen species of remora around the world benefiting from transportation and feeding access provided by host associations (Strasburg, 1959).
Predation and Environmental Risks
While remoras benefit tremendously from their relationships with hosts, there are also substantial costs and risks associated with their parasitic lifestyle. Remoras must constantly balance their need for food and transportation with the risk that their host may attempt to dislodge or even consume them (O’Toole, 2002).
Cases of large pelagic fish like tuna and dolphinfish that have choked to death on accidentally swallowed remoras have been well-documented (Haensly et al., 1982). Sharks have also been known to intentionally prey on remoras when food is scarce or simply to rid themselves of an irritating hanger-on (McKibben & Nelson, 1986).
In addition to predation by hosts, remoras also face higher risks from other predators because their host associations make them more conspicuous. Interestingly, while sharks gain a measure of parasite protection by allowing cleaner fish like wrasses to remove troublesome pests, they often quickly chase off remoras that attempt to hide in these “cleaning stations” to avoid being eaten themselves (O’Toole, 2002).
There is also danger for a remora if it fails to quickly detach from its host in time to avoid being beached or trapped in shallow water habitats. Overall, while remoras benefit tremendously from their hosts, their lifestyle has substantial risks that prevent totally carefree exploitation of other marine organisms.
Costs and Benefits to Hosts
Cleaning Services Benefits
Remoras provide valuable cleaning services to their hosts by feeding on parasites, dead tissue, and other debris (Vaughan et al., 2016). Studies have shown that fish with remoras have lower parasite loads and less dead tissue compared to fish without remoras (Sazima et al., 2012).
The cleaning services help maintain the health and hygiene of the host’s body surface.
According to one estimate, a single remora can remove over 3000 parasites per hour from a host, providing significant sanitary benefits (Strasburg, 1962). Additionally, cleaning symbioses facilitate social behaviors in reef fish communities and may help control disease transmission between cohabiting fish species (Vaughan et al., 2016).
Small Metabolic Costs
Despite the benefits, remoras do impose some metabolic costs on hosts by increasing drag during swimming. However, multiple studies have quantified these costs and found them to be fairly minimal (Sazima et al., 2012).
For example, one study on sailfish found that attached remoras increased drag by only 2-4% and had negligible impacts on gill ventilation and recovery rates after exercise (Hyatt, 2019). Similarly, research on sharks with suctioned remoras showed they expended only ~10% more energy while swimming compared to shark controls without remoras (O’Toole, 2002).
So while remoras can negatively impact hydrodynamics, most evidence indicates the costs are outweighed by the significant cleaning services they provide. Their presence facilitates cleaner and healthier oceans by forming commensal and mutualistic partnerships with marine megafauna.
Scientific Classification as Symbiotic, Not Parasitic
Remoras, also known as suckerfish, have long been associated with sharks, rays, and other large marine animals. Their distinctive sucking disc allows them to attach themselves to hosts for extended periods.
This relationship was long assumed to be parasitic, with remoras simply hitching a free ride and stealing food from their hosts. However, scientific research has shown the relationship is much more complex and mutually beneficial, classifying remoras as symbiotic organisms rather than parasitic ones.
The key distinction between parasitic and symbiotic relationships is whether one organism benefits at the expense of the other. Parasites obtain all benefits of the association, often impairing hosts and even resulting in death.
Symbiosis describes close, prolonged interactions between two species in which both derive benefit. Research has shown the remora-host relationship fits the latter definition.
Far from being freeloaders, remoras provide several advantages to their hosts. For example, they feed on dead skin and other debris on the host’s body, providing a cleansing service. Their presence may also deter parasites or even remove them directly.
Remoras have been observed feasting on parasites attached to whales, relieving irritation to the host.
In return, remoras gain transportation, protection from predators, and access to food. As hosts move through the ocean, remoras can capitalize on scraps of prey. One study of sharks with attached remoras showed the remoras scavenged up to 80% of prey remains floating after the shark fed.
This cleaning symbiosis enhances survival of both species.
Scientists have identified specific adaptations that enable this advantageous bond. Remoras possess a uniquely structured suction disc that allows long-term attachment without harming the host. Their bodies are flattened and streamlined to reduce drag.
Many species have dorsal fins modified for adhesion rather than propulsion. All of these specializations facilitate hitching a ride with larger, faster marine creatures.
Molecular studies support remoras’ classification as symbiotic organisms. Genetic analysis shows remoras are more closely related to perciform fish like groupers and mackerels than to vertebrate parasites. Their physiology and lifecycle are similarly distinct from true parasites.
Remoras do not display adaptations seen in parasitic species, like simplified digestive systems or direct dependence on the host for nutrients.
Conclusion
In summary, while remoras exhibit some behaviors that resemble those of true parasites, such as hitching free rides and stealing food scraps from hosts, they also provide cleaning services that benefit their hosts. The costs to the host animal are generally small.
Most researchers thus classify remoras as symbiotic organisms rather than parasites, since their relationship appears to be mutually beneficial. While not completely one-sided, the relationship leans further toward commensalism or even mutualism than parasitism when both costs and benefits are weighed.
So in answer to the original question, while remoras walk a fine line and display some parasitic qualities, most scientists do not consider them true parasites that only benefit themselves at significant cost to their hosts.
