The remora, often called the suckerfish, is a family of marine fish found in tropical and warm temperate waters globally. They are best known as “the ocean’s hitchhiker” due to their extraordinary ability to attach themselves to larger marine animals. This behavior is possible because of a specialized evolutionary adaptation on the top of the remora’s head that allows it to secure a free ride.
Physical Characteristics of the Remora
Remoras possess a long, slender, and streamlined body shape. Their coloration is generally dark, typically a uniform black or brown, which helps them blend in against the skin of large marine creatures. Depending on the species, remoras range in length from about 30 centimeters up to 110 centimeters, with the largest individuals weighing over two pounds.
The head is notably long and flattened, with the lower jaw slightly protruding past the upper jaw. The most distinguishing feature is the large, flat, oval-shaped structure located on the dorsal side of the head. This structure is the key to their survival, though the rest of the remora’s body is adapted for life in the open ocean.
How the Suction Disc Works
The remora’s specialized attachment mechanism developed from a highly modified first dorsal fin. This flattened oval disc on the top of the head is the source of the remora’s powerful and reversible adhesion. The disc consists of a series of paired, bony structures called transverse lamellae, which run across the length of the oval plate.
These lamellae are the active component of the disc, and they can be rotated from a flat to an erect position. Hundreds of microscale bony spinules project from the edges of the lamellae, generating friction when they engage with the host’s surface. This frictional grip is effective on rough surfaces like sharkskin, preventing the remora from slipping.
Adhesion is achieved through a combination of a vacuum seal and mechanical friction. A soft, fleshy lip surrounds the perimeter of the disc, creating a watertight seal against the host’s body. When the remora raises the lamellae, it simultaneously creates a low-pressure area, or vacuum, within the disc cavity.
The physics of the attachment are such that the force of the host’s swimming helps to maintain the hold. Water passing over the remora’s streamlined body generates a shear force, which passively enhances the rotation of the lamellae, thereby increasing the frictional contact. This mechanism allows the remora to remain securely fastened even when the host is moving at high speeds or leaping from the water, without requiring the remora to expend much energy.
The Commensal Relationship with Hosts
The relationship between the remora and its host is an example of commensalism, a biological interaction where one species benefits significantly while the other remains generally unaffected. The primary benefit is transportation, or phoresy, which allows the remora to travel vast distances across the ocean without using its own energy.
This hitchhiking behavior also provides protection from predators, as few marine animals approach a large shark or whale. Furthermore, the remora gains access to a constant supply of food associated with its host. The fish can detach and reattach itself as needed, often moving to different spots on the host’s body to feed or adjust its position.
Remoras are frequently found attached to a wide variety of large hosts, including sharks, manta rays, sea turtles, whales, and large bony fish like tuna. They have also been observed clinging to the hulls of oceangoing ships. Some remora species also engage in a cleaning behavior, consuming external parasites and dead skin from the host.
This cleaning activity can be seen as a subtle benefit to the host, sometimes leading the interaction to be classified as mutualism, but the remora is the clear beneficiary. The long-term presence of remoras does not damage the host’s skin or negatively impact its movement.
Diet and Geographic Range
The remora is an opportunistic feeder whose diet is intrinsically linked to the habits of its host. A significant portion of its food comes from scavenging the scraps and fragments of meals left behind by the host, such as when a shark feeds. They also actively consume external parasites, dead skin, and bacteria from the host’s body, effectively acting as a cleaner fish.
When not consuming matter directly from the host, remoras feed passively by filtering small planktonic organisms from the water flowing past them. This diverse diet allows them to sustain themselves easily while traveling across the ocean. The remora’s reliance on large, migratory hosts dictates its broad habitat.
Remoras are found globally in tropical and warm temperate oceans, exhibiting a cosmopolitan distribution. They are considered pelagic species, inhabiting the open ocean water column, often at depths up to 200 meters. Their presence in any given area is determined by the migration patterns of the large marine animals they use for transport.