1. Actinopterygii
Bony fishes are the dominant fishes today; they consist of two classes, Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes). Ray-finned fishes include most modern bony fishes. Lobe-finned fishes contain few living species. They include tetrapods, lung fishes and coelacanths. Sarcopterygii are considered to as ancestors of amphibians and other terrestrial vertebrates.
The Actinopterygii, the class of ray-finned fishes, also called spiny rayed fishes, are the dominant class of vertebrates including about 30,000 species. This group is among the largest and the most successful groups of fishes and constitutes nearly 50% of all living vertebrates.
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Unfortunately, some of the species are becoming extinct faster than they can be discovered in some parts of the world, for example in the Amazon basin. Actinopterygii are extremely diverse and live in various environments, including lakes, desert springs, subterranean caves, ephemeral pools, polar seas and the depths of the ocean.
Two main groups of ray-finned fishes:
- Chondrosteans (e.g. sturgeons) have heterocercal tails and ganoid scales.
- Neopterygians – one lineage of early neopterygians led to the modern bony fishes (teleosts).
Actinopterygians lost their heavy scales and instead developed spines, which are used as anti-predator devices when a fish cannot use speed to escape. The way the pelvic and pectoral fins are positioned also changed in the process of evolution. Instead of pelvic fins behind the pectoral fins, actinopterygians developed the pelvic fins positioned just below or even a little in front of the pectoral fins; in this position they help in increased maneuverability instead of being used simply as stabilizers, as in predecessors of modern actinopterygians. Symmetrical, homocercal, tail allows increased speed.
Fins of Teleosts, for example, are diversified for a variety of functions: camouflage, communication, complex movements, streamlining, etc.
Figure 1. Ray-finned fishes
(Source: University of Michigan Museum of Zoology)
Caudal propulsion, used in cruising and sprint swimming, acceleration, and fast turns, is very important in Actinopterygian swimming morphology, because it helps fishes to escape from enemies and catch food. The fins of Actinopterygians are very efficient: they allow for powerful swimming and give stability and maneuverability. Most probably, the evolutionary success of these species is mainly due to their morphological adaptation.
a. Species that are pelagic (where fin is used for swimming only)
Pelagic fishes are usually fast-swimming and often migrate over large distances in the sea. Some of the pelagic species include billfish, tunas, dorado and sharks.
Actinopterygians’ adaptations for high speed include:
Primarily axial locomotion:
- Anterior to posterior series of contractions of myomeres
- Muscles on one side contract while those on the other side relax.
Minimizing drag / maximizing thrust:
- Viscous drag: due to friction with water.
- Favors thicker stouter body to minimize surface area.
- Small scales or no scales.
- Less flexible to reduce drag (not anguilliform).
- Inertial drag: turbulence created by displacement of water; increases with increasing speed.
- Favors streamlined (teardrop) shape.
- Force transferred from slender, muscular caudal peduncle.
- Fast swimmers have caudal fins with high aspect ratio (dorsal-ventral length/anterior-posterior width) to maximize thrust.
Below some Actinopterygian pelagic species will be discussed.
Albacore Tuna is a large oceanic fish, which can have very high speed. Its pectoral fins, located on each side of the body, are extremely long; deeply-forked tail fin helps create the power required to maintain the albacore tuna’s remarkable speeds. The pectoral fins of the albacore are very long, almost half of the fish’s length. The dorsal spines are 8 to 10 in number, and well forward of the rays of the dorsal fin. The anterior spines are much longer and provide a concave outline to the spiny part of the dorsal fin.
Swimming of big-belly seahorse, also called Hippocampus abdominalis, is supported by the quickly oscillating dorsal fin; they steer using the fins on both sides of the body, the pectoral fins.
b. Species that are bottom dwelling aquatic (fin/limb used for “walking” under water)
Spotted Handfish (Brachionichthys hirsutus) is bottom dwelling fish that can swim, but is usually moves across the bottom of the ocean crawling on a pair of modified pectoral fins. It has highly adapted pectoral fins, which look like hands and allow it to walk on the sea floor. Spotted Handfish uses both the pectoral and ventral fins to scrawl. For swimming through the water, it uses the unpaired or ‘median’ fins – the tail and anal fin.
Red-lipped Batfish (Ogcocephalus darwini) does not swim well. This fish use their pectoral fins to “walk” on the ocean floor. It has a shorter disk margin compared to porrectus but higher modal pectoral fin ray count.
Atlantic Halibut (Hippoglossus hippoglossus) is the largest flatfish in the world. Flatfish show exceptional and characteristic anatomy that is adapted to their life on the sea floor – they are flattened sideways and usually lie on one side of their body, instead of being flattened from top to bottom like many others of the sea bed.
2. Sarcopterygii
Sarcopterygians, the class of lobe-finned fishes, have lobed-paired fins and these fins are connected to the body by one bone. The fins evolved into legs of the amphibians. They also had two dorsal fins with separate bases, unlike the single dorsal fin of actinopterygians. Sarcopterygians have lobe in their fins. Unlike other fish, sarcopterygian fish has a central appendage in their fins with many bones and muscles. Their fins are very flexible and can support their body on land, as in lungfish and tetrapods. The lobe fins of sarcopterygii are fleshy and similar to the limbs of land animals, unlike the fins of Actinopterygii. Sarcopterygii are not represented only by the coelacanth, six species of lungfishes and four species of Protopterus.
Lobed-finned fishes include 2 species of coelacanths and 6 species of lungfishes. This group was much more plentiful during the Devonian period. All early sarcopterygians had lungs as well as gills and a heterocercal tail.Later sarcopterygians have a continuous flexible fin around the tail. They have fleshy, paired lobed fins, which allowed to scuttle along the bottom.
a. Species that are pelagic (where fin is used for swimming only)
Coelacanth (Latimeria chalumnae) have limb-like, lobed pectoral and pelvic fins and a distinctive tail consisting of three separate lobes. They are closest link between fish and the first amphibians. Coelacanths are not like many other existing fishes. They have an extra lobe on the tail, paired lobed fins, and a vertebral column that is not fully developed. Coelacanths have paired fins, which move in a similarly to human arms and legs.
b. Species that are bottom dwelling aquatic (fin/limb used for “walking” under water)
Lungfish that exist nowadays have an extended body with fleshy paired pectoral and pelvic fins and a single unpaired caudal fin replacing the dorsal, caudal, and anal fin of most fishes. Lungfishes are the closest living relative of tetrapods. Lungfishes are bottom dwelling predators. The lungfishes developed the first proto-lungs and proto-limbs. The lungfishes adapted to use their stubby fins (proto-limbs) to walk on land and find new water if it is necessary, and use their lungs to breathe air. Lungfish attacks almost everything that moves.
c. Terrestrial (fin/limb used for walking on land)
Some lobe-finned fish have developed several characteristics, which indicate that they were ancestors of the amphibians. One of these characteristics was presence of skeletal components in the dorsal fins that probably evolved into tetrapod limbs. The early amphibians already possessed feet and limbs for walking. Primitive tetrapods developed from Sarcopterygii. Most probably the earliest tetrapods, for example Acanthostega, lived only in water and were not adjusted to the life on land at all. As some scientists suggest, after a pond or lake where tetrapods lived in dried out, they had to adjust to the new environment and evolved legs and lungs in order to adapt to the terrestrial locomotion.
The ray-finned fish have fins supported by slim rays and do not contain muscles. The advanced characteristics that make ray-finned fish very successful evolved during the Triassic. These advanced traits include hydrostatic air bladders, streamlining of body and fins and reduction of scales. Actinopterygii, compared to Sarcopterygii, possess increasing mobility of fins. Overall, all the fish species had to solve the locomotion problem and did it in different ways adjusting to the environment.
Bibliography:
Moyle, P. and Cech, J. (2004). Fishes: An Introduction to Ichthyology – fifth edition. Upper Saddle River, NJ: Prentice-Hall, Inc.
Jonna, R. Kids’ Inquiry of Diverse Species. Retrieved on May 21st, 2009 from
http://www.biokids.umich.edu/critters/1810/Actinopterygii/specimens/.
Cloutier, R. and P.E. Ahlberg. (1996). “Morphology, characters, and interrelationships of basal sarcopterygians.” pp 445-479. In: M.L.J Stiassny, L. R. Parenti, and G.D. Johnson. (eds.) Interrelationships of Fishes. San Diego and London: Academic Press.
Webb, P.W. (1982). Locomotor Patterns in the Evolution of Actinopterygian Fishes. American Zoologist, 22(2), p.329-342.
Arratia, G., Wilson, M. V. H. and Cloutier,R. (eds.) (2004). Recent Advances in the Origin and Early Radiation of Vertebrates. München, Germany: Verlag Dr. Friedrich Pfeil.
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