Anguilla

INTRODUCTION

Anguilla, and are elongated fish of snake-like bodies, with long dorsal, caudal and anal fins forming a continuous fringe. They are catadromous, spending their adult lives in freshwater, but migrating to the ocean to spawn.

Eels are an important food fish and some species are now farm-raised, but not bred in captivity. Many populations in the wild are now threatened, and Seafood Watch recommend consumers avoid eating anguillid eels.

PHYSICAL DESCRIPTION

•  Adult freshwater eels are elongated with tubelike, snake-shape

• They have large, pointed heads and their dorsal fins are usually continuous with their caudal and anal fins, to form a fringe lining the posterior end of their bodies. They have relatively well developed eyes and pectoral fins compared to saltwater eels that they use to navigate and maneuver through river bottoms and shallow water.

• Unlike most eels, freshwater eels have not lost their scales, and instead have soft, thin, scales that are embedded in the epidermis.

•   Additionally, freshwater eels possess small, granular teeth arranged in bands on the jaws and vomer.

• They do exhibit size-dependent sexual dimorphism. Male anguillids invest more energy into mating with as many females as he can, than they do into growth. Therefore, female anguillids are usually larger, ranging from 1.5 – 3 feet, while male anguillids rarely get larger than 1.5 feet long. Adult anguillidae can vary in color, but normally are brown, olive or olive-yellow, and can be mottled. Coloration matches the floor of rivers and lakes which prevents the eels from being seen by predators while in clear or shallow water. Freshwater eels go through physical changes in their bodies when going to and from the ocean for different stages of life.

COMMERCIAL IMPORTANCE

Anguillid eels are important food fish. Eel aquaculture is a fast-growing industry. Important food eel species include longfin eel, Australian long-finned eel, short-finned eel, and Japanese eel. Most eel production historically has been in Japan, Korea, and Taiwan, but in recent years, the greatest production has been in China.

ECOLOGY

•  Freshwater eels are aquatic and live in various habitats, including freshwater, estuaries, and saltwater/marine habitats, and occupy the roles of both predator and prey, and evidence has been found of nematode parasitism in some species. Some eel species have been observed consuming the eggs of predatory fish such as trout, aiding in population control in these systems.

•  Juvenile eels occupy small spaces in between rocks, in crevices or mud. Freshwater eels are widespread and are catadromous, meaning they spend most of their life in freshwater (rivers mainly) and migrate to the ocean to breed.

• Leptocephali (larval) migration can range from months to up to almost a year. Temperate eels migrate on average for approximately 6–10 months, while tropical eels undergo shorter migrations between approximately 3–5 months on average.

REPRODUCTION AND LIFE CYCLE

• Anguillid eels are semelparous, meaning they only live to reproduce once, as they die after reproduction.

  •   However, these eels do not necessarily reproduce every year; they will sometimes wait until conditions are right in order to migrate and breed. These conditions may include fat content, water quality or temperature, prey availability, river height, water flow rate, etc.

  • This variability allows some eels to live even 50–70 years; however, the lifespan of freshwater eels is not well documented. Very little is known about the mechanics of fertilization and spawning, and the time it takes these eels to hatch from their eggs is variable.

  • Members of this family spend their lives in freshwater rivers, lakes, or estuaries and return to the ocean to spawn. All eels pass through several stages of development in their life cycle.

  •  Anguillid eels undergo morphological changes during these developmental stages that are associated with environmental conditions and aid in preparing them for further growth and finally reproduction.

  • ·Anguillid eels begin their life as an egg in the ocean, and once hatched, enter a larval stage called leptocephali. The young eel larvae live only in the ocean and consume small particles called marine snow.

  • Anguillid eels lay adhesive demersal eggs (eggs that are free-floating or attached to substrate), and most species have no parental care.

  •  These planktonic (free-floating) eggs and translucent, leaf-like larvae are dispersed via ocean currents and migrate sometimes thousands of miles. They grow larger in size, and in their next growth stage, they are called glass eels.

  •   At this stage, they enter estuaries; upon returning to freshwater growing habitat, the eels become pigmented and develop through the elver and yellow eel stages. The yellow and silver eel stages are named aptly for the coloration of the underbelly of the eel during these developmental stages.

  •        Elvers travel upstream in freshwater rivers, where they grow to adulthood. Finally, anguillids transition through the silver eel stage into adulthood and migrate to the oceanic breeding grounds to reproduce and begin the cycle anew.

BEHAVIOR

•     Eels in the Anguillidae family are primarily solitary and do not communicate socially or school.

•     Large groups of elvers can form in response to environmental conditions.

•     These eels are generalists and opportunistic feeders, consuming various prey including crustaceans and fish.

•     They lack paired appendages and use axial-based lateral undulation for locomotion, similar to snakes.

•     Their high trunk maneuverability aids hunting in complex habitats like reefs.

•     Some species burrow into sediment for foraging and anti-predatory behavior.

•     Freshwater eels face predators such as large fish and birds.

•     Much remains unknown about their behavior and origins due to observation challenges, especially in reproduction, social interactions, and migration.

SENSORY

•        Anguillidae possess a fully developed lateral line along their trunk, enabling them to sense their environment through water displacement, crucial for predation and hunting as they are primarily nocturnal generalists.

•        Olfactory senses in Anguillidae are highly developed, with olfactory cells capable of detecting extremely diluted chemicals, aiding in nocturnal activities and migration by using terrestrial odors, low salinity, and colder temperatures as cues.

PHYSIOLOGY

•    Paired frontal bones in the skull contribute to a strong cranium, assisting Anguillidae in burrowing through mud and navigating terrestrial obstacles encountered at the water's bottom, where they spend most of their daylight hours.

•        Ventral lateral gill slits facilitate eighty-five percent of gas exchange and efficiently convert between salt and freshwater, distinguishing freshwater eels from others with internal gill chambers.

•        Many Anguillidae species exhibit variegated skin, adapting their coloration to blend with their environment for optimal camouflage.

•        The fused dorsal, anal, and caudal fins, along with the lack of pelvic fins, contribute to Anguillidae's streamlined form and highly skilled swimming abilities, crucial for migration and hunting.

OTHER FEATURES

•        Cutaneous respiration accounts for about 15% of oxygen intake in Anguillidae, but they can receive up to 50% of oxygen through gas exchange via their skin when out of water, aiding in moving between bodies of water and burrowing in mud.

•        During dry periods, Anguillidae burrow into mud, entering torpor to lower metabolic rate and body temperature, enhancing survivability until rain arrives.

•        Anguillidae swim proficiently both forward and backward due to their muscle attachment and myomeres, using this ability for efficient feeding by biting, twisting, and tearing food.

•        Population sex ratios in Anguillidae vary based on egg abundance, influencing the ratio of males to females.

•        Mucous cells in Anguillidae aid in predation and moisture retention, with higher concentrations on the body's dorsal and ventral sides.

•        Metamorphosis in Anguillidae prepares them for migration, with adaptations such as increased gas bladder pressure tolerance, fat reserves for oceanic food scarcity, and changes in eye structure for deep-sea vision.

• Captivity experiments reveal Anguillidae's innate drive to migrate, evidenced by attempts to escape toward freshwater or saltwater sources.