Marine Biology

15. Fish diversity, adaptions, and life histories

The general classification of fish:

• Jawless fish
• Cartilaginous fish
• Bony fish

Fish are the simplest and oldest of all living vertebrates. They are also the most abundant Ð the 22,000 estimated species make up to half of all species of vertebrates. About 60% of fish are marine.

Jawless fish (Class Agnatha) lack jaws and so feed by suction. They are the most primitive living fish. The body is cylindrical and elongated. They lack paired fins and scales. Examples include hagfish (Mixine, Eptatretus) and lamprey (Petromyzon).

Cartilaginous fish (Class Chondrichthyes) include sharks, rays, skates and ratfish. They have a skeleton made of cartilage and possess movable jaws mostly with powerful teeth. The mouth is located ventrally (under the head). The presence of paired lateral fins allows for efficient swimming. The skin is covered with small scales that have the same composition as teeth.

Bony fish (Class Osteichthyes) include the great majority of fish (approx. 21,500 species, roughly 98% of all fish). They have the following characteristics:

• Skeleton made (at least partially) of bone
• Thin, flexible, overlapping scales along the body
• A flap of bony plates and tissue known as the gill cover, or operculum, that protects the gills
• Protruding jaws with more freedom of movement than cartilaginous fish
• The presence of a swimbladder Ð helps buoyancy and compensates for the relatively heavier bony skeleton
• Highly manoeuvrable fins

Body shape, evolution and life histories were also introduced and examples provided. Main identification features, reproduction, strategies and general species biology were discussed.

Some sharks lay eggs (all skates and ratfish do), but most are ovoviviparous(all rays are). The young develop with their yolk sacks within the mother, but without a placenta or umbilical cord. Some sharks (the Great White) are oviphagous (or oophagous); the young eat the other developing young and embryos inside their mother and only the fiercest is born! A few sharks (hammerheads and reef sharks) are viviparous; like mammals, the young are nourished with a placenta within the mother. The gestation period is around 22 months and 2-80 pups are born per litter. Because most sharks are ovoviviparous or viviparous, they do not produce mass numbers of young like other fish do.

Fish reproductive strategies include:

1. Oviparity - Lay undeveloped eggs, external fertilisation (90% of bony fish), internal fertilisation (some sharks and rays). Cod are seasonal, coastal spawners (Feb-April). They have separate sexes and reproduce at 5 yrs or so (>45 cm). An adult female lays between 4-6 M eggs at a single spawning! Fertilisation external.
2. Ovoviviparity - Internal development- without direct maternal nourishment - Advanced at birth (most sharks + rays) - Larval birth (some Scorpaeniformes - rockfish).
3. Viviparity - Internal development - direct nourishment from mother - Fully advanced at birth (some sharks, surf perches). Coelacanths are live egg bearers; the eggs are large (8-9 cm in diameter) and hatch internally.

In fish, parental care is very rare as most fish are broadcast spawners, but there are a few instances of parental care, e.g. male gobies guard the eggs in a nest until they are born; the male yellowhead jawfish actually guards the eggs by holding them in his mouth; and after an elaborate courtship, the female seahorse deposits eggs (up to 300!) into the male's abdominal pouch where they are fertilised; the male nourishes and protects the developing embryos until they are developed; young are release all at once.

Hermaphroditism

Some fish individuals are both males and females, either simultaneously or sequentially. About 21 families of fish are hermaphrodites.

• Simultaneous hermaphrodite:. There are some instances where being a member of both sexes could have its advantages. In the deep sea, the low light levels and limited food supply make for a very low population density; meaning that potential mates are few and far between. Members of the fish family Serranidae (hamlets) are simultaneous hermaphrodites; they can spawn with any individual encountered.
• Sequential hermaphrodite (sex reversal). They may change from being males to females (protandry; e.g. anemone fish) or females to males (protogyny; e.g. wrasses and parrotfish).

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