Lampreys

Lampreys (sometimes inaccurately called lamprey eels) are a group of jawless fish comprising the order Petromyzontiformes, sole order in the class Petromyzontida. The adult lamprey is characterized by a toothed, funnel-like sucking mouth. About 38 extant species of lampreys are known, with around seven known extinct species. They are classified in three families – two small families in the Southern Hemisphere (Geotriidae, Mordaciidae) and one large family in the Northern Hemisphere (Petromyzontidae).

Modern lampreys spend the majority of their lives in the juvenile “ammocoete” stage, where they burrow into the sediment and filter feed. Adult carnivorous lampreys are the most well-known species, and feed by boring into the flesh of other fish (or in rare cases marine mammals) to consume flesh and/or blood; but only 18 species of lampreys engage in this predatory lifestyle (with Caspiomyzon suggested to feed on carrion rather than live prey). Of the 18 carnivorous species, nine migrate from saltwater to freshwater to breed (some of them also have freshwater populations), and nine live exclusively in freshwater. All noncarnivorous forms are freshwater species. Adults of the noncarnivorous species do not feed; they live on reserves acquired as ammocoetes.

Lampreys live mostly in coastal and fresh waters and are found in most temperate regions. Some species (e.g. Geotria australis, Petromyzon marinus, and Entosphenus tridentatus) travel significant distances in the open ocean, as evidenced by their lack of reproductive isolation between populations. Other species are found in land-locked lakes. Their larvae (ammocoetes) have a low tolerance for high water temperatures, which may explain why they are not distributed in the tropics.

Anatomy

Adults superficially resemble eels in that they have scaleless, elongated bodies, with the largest species, the sea lamprey having a maximum body length around 1.2 m. Lacking paired fins, adult lampreys have one nostril atop the head and seven gill pores on each side of the head.

The brain of the lamprey is divided into the telencephalon, diencephalon, midbrain, cerebellum, and medulla.

Lampreys have been described as the only living vertebrates to have four eyes, having a single pair of regular eyes, as well as two parietal eyes, a pineal and parapineal one (the exception is members of Mordacia). The eyes of juvenile lampreys are poorly developed eyespot-like structures that are covered in translucent skin, while the eyes of adult lampreys are well developed. Accommodation is done by flattening the cornea, which pushes the lens towards the retina. The eye of family Mordaciidae possess just a single type of photoreceptor (rod-like), the family Petromyzontidae possess two photoreceptor types (a cone-like and a rod-like), and the family Geotriidae possesses five types of photoreceptors.

The buccal cavity, anterior to the gonads, is responsible for attaching the animal, through suction, to either a stone or its prey. This then allows the tongue to make contact with the stone to rasp algae, or tear at the flesh of their prey to yield blood.

The last common ancestor of lampreys appears to have been specialized to feed on the blood and body fluids of other fish after metamorphosis. They attach their mouthparts to the target animal’s body, then use three horny plates (laminae) on the tip of their piston-like tongue, one transversely and two longitudinally placed, to scrape through surface tissues until they reach body fluids. The teeth on their oral disc are primarily used to help the animal attach itself to its prey. Made of keratin and other proteins, lamprey teeth have a hollow core to give room for replacement teeth growing under the old ones. Some of the original blood-feeding forms have evolved into species that feed on both blood and flesh, and some who have become specialized to eat flesh and may even invade the internal organs of the host. Tissue feeders can also involve the teeth on the oral disc in the excision of tissue. As a result, the flesh-feeders have smaller buccal glands as they do not require the production of anticoagulant continuously and mechanisms for preventing solid material entering the branchial pouches, which could otherwise potentially clog the gills. A study of the stomach content of some lampreys has shown the remains of intestines, fins and vertebrae from their prey.

The unique morphological characteristics of lampreys, such as their cartilaginous skeleton, suggest they are the sister taxon of all living jawed vertebrates (gnathostomes). They are usually considered the most basal group of the Vertebrata. Instead of true vertebrae, they have a series of cartilaginous structures called arcualia arranged above the notochord. Hagfish, which resemble lampreys, have traditionally been considered the sister taxon of the true vertebrates (lampreys and gnathostomes) but DNA evidence suggests that they are in fact the sister taxon of lampreys.

The heart of the lamprey is anterior to the intestines. It contains the sinus, one atrium, and one ventricle protected by the pericardial cartilages.

The pineal gland, a photosensitive organ regulating melatonin production by capturing light signals through the photoreceptor cell converting them into intercellular signals of the lamprey is located in the midline of its body, for lamprey, the pineal eye is accompanied by the parapineal organ.

One of the key physical components to the lamprey are the intestines, which are located ventral to the notochord. Intestines aid in osmoregulation by intaking water from its environment and desalinating the water they intake to an iso-osmotic state with respect to blood, and are also responsible for digestion.

Close to the jaws of juvenile lampreys, a muscular flap-like structure called the velum is present, which serves to generate a water current towards the mouth opening, which enables feeding and respiration.
Studies have shown that lampreys are among the most energy-efficient swimmers. Their swimming movements generate low-pressure zones around the body, which pull rather than push their bodies through the water.

Different species of lamprey have many shared physical characteristics. The same anatomical structure can serve different functions in the lamprey depending on whether or not it is carnivorous. The mouth and suction capabilities of the lamprey not only allow it to cling to a fish as a parasite, but provide it with limited climbing ability so that it can travel upstream and up ramps or rocks to breed. This ability has been studied in an attempt to better understand how lampreys battle the current and move forward despite only being able to hold onto the rock at a single point.

Many lampreys exhibit countershading, a form of camouflage. Similarly to many other aquatic species, most lampreys have a dark-colored back, which enables them to blend in with the ground below when seen from above by a predator. Their light-colored undersides allow them to blend in with the bright air and water above them if a predator sees them from below.

Lamprey coloration can also vary according to the region and specific environment in which the species is found. Some species can be distinguished by their unique markings – for example, Geotria australis individuals display two bluish stripes running the length of its body as an adult. These markings can also sometimes be used to determine what stage of the life cycle the lamprey is in; G. australis individuals lose these stripes when they approach the reproductive phase and begin to travel upstream. Another example is Petromyzon marinus, which shifts to more of an orange color as it reaches the reproductive stage in its life cycle.

Lifecycle

The adults spawn in nests of sand, gravel and pebbles in clear streams. After hatching from the eggs, young larvae—called ammocoetes—will drift downstream with the current till they reach soft and fine sediment in silt beds, where they will burrow in silt, mud and detritus, taking up an existence as filter feeders, collecting detritus, algae, and microorganisms. The eyes of the larvae are underdeveloped, but are capable of discriminating changes in illuminance. Ammocoetes can grow from 8–10 cm to about 20 cm. Many species change color during a diurnal cycle, becoming dark at day and pale at night. The skin also has photoreceptors, light sensitive cells, most of them concentrated in the tail, which helps them to stay buried. Lampreys may spend up to eight years as ammocoetes, while species such as the Arctic lamprey may only spend one to two years as larvae, prior to undergoing a metamorphosis which generally lasts 3–4 months, but can vary between species. While metamorphosing, they do not eat.

The rate of water moving across the ammocoetes’ feeding apparatus is the lowest recorded in any suspension feeding animal, and they therefore require water rich in nutrients to fulfill their nutritional needs. While the majority of (invertebrate) suspension feeders thrive in waters containing under 1 mg suspended organic solids per litre, ammocoetes demand minimum 4 mg/L, with concentrations in their habitats having been measured up to 40 mg/L.

During metamorphosis the lamprey loses both the gallbladder and the biliary tract, and the endostyle turns into a thyroid gland.

Some species, including those that are not carnivorous and do not feed even following metamorphosis, live in freshwater for their entire lifecycle, spawning and dying shortly after metamorphosing. In contrast, many species are anadromous and migrate to the sea, beginning to prey on other animals while still swimming downstream after their metamorphosis provides them with eyes, teeth, and a sucking mouth. Those that are anadromous are carnivorous, feeding on fishes or marine mammals.

Anadromous lampreys spend up to four years in the sea before migrating back to freshwater, where they spawn. Adults create nests (called redds) by moving rocks, and females release thousands of eggs, sometimes up to 100,000. The male, intertwined with the female, fertilizes the eggs simultaneously. Being semelparous, both adults die after the eggs are fertilized.

Research on sea lampreys has revealed that sexually mature males use a specialized heat-producing tissue in the form of a ridge of fat cells near the anterior dorsal fin to stimulate females. After having attracted a female with pheromones, the heat detected by the female through body contact will encourage spawning.

Source: Wikipedia