(i) Definition and origins

.The Actinopterygii are the "ray-finned" fishes. They had there origin sometime in the Devonian Period, but did not flourish until later into the Carboniferous (about 360 mya).

The actinops are the dominant group of fishes (i.e., of the major groups Myxini, Petromyzonti, Chondrichthyes, Actinisti, Dipnoi, and Actinopterygii) constituting about 95% of all fish species. The inter-relationships amongst the major groups within the actinops (e.g., orders and families within orders) are usually quite uncertain and are hypotheses requiring rigorous evaluation (just like the issues of Agnathan mono/paraphyly, closest living relative of tetrapods).

The chief trait that defines (more or less, e.g. some groups lack one or more median fins) the actinops are the possession of well defined lepidotrichia, bony elements (fin rays) that provide internal support for the fins and a connection to the basal elements. In the actinops they extend right to the body such that they attach the fin to basal skeletal supports, the radial bones, which then provide the link between the fins and the internal skeleton (e.g. pectoral or pelvic girdles).

(ii) Composition and diversity

Traditionally, the taxonomic diversity within the actinops has been associated with three major periods of radiation in the Devonian, early Cretaceous, and late Cretaceous periods.

The major groups (usually defined as subclasses) of fish that evolved and reached peak diversity during these times were the: Chondrostei (paleonisciforms (extinct), sturgeons and paddlefishes), Holostei (bowfin and gars), and Teleostei, respectively. Subsequently, the Holostei has been abandoned as a monophyletic group and three groups have been recognized: the Cladistia (bichirs, reedfish), Chondrostei (sturgeons and paddlefishes) and the Neopterygii (all remaining groups).

(iii) The Cladistia

The polypteriforms are a small group of fishes with nine species and two genera in a single family, the Polypteridae. They have never been very diverses (including the fossil record) and are confined to the freshwaters of tropical/subtropical Africa.

The genus Polypterus has eight of the nine species. It has pectoral fins that appear "lobe-like" in external appearence, but they do share the internal structure of actinops. They are most easily recognized by the dorsal "flagfins". Each of the 8-15 dorsal fins has a leading edge spine to which are attached horizontal fin rays (see figure). Bichirs are also faculative air breathers in poorly oxygenated waters and have well vascularized lungs.

The uncertain systematic relationships of the "cladistians" is caused by the possession of some traits that are shared with diverse lineages (e.g., lobe-like external appearence of pectoral fin with sarcops, spiricles with sharks and sturgeons, two gular plates as in the coelacanths, external gills as young just like some lungfish). Some classifications put these fishes in their own subclass (separate from Sarcopterygians and Actinopterygians) called the "Brachiopterygians".

(iv) Chondrostei: sturgeons and paddlefishes

The chondrosts have sometimes been linked with cartilagenous fishes, but are thought to have given rise to modern bony fishes. They possess several ancestral traits that suggest that they are basal to higher bony fishes:

(1) heavy ganoid scales

(2) heterocercal tail

(3) largely cartilaginous skeleton

(4) fin rays are more numerous than their basal supports (radials)

(5) Spiracle (usually)

The cartilaginous skeleton in modern forms is secondarily derived (i.e. not a homologous trait shared with Chondrichthyan fishes) because fossil chondrosteans had bony endoskeletons.

The two other families (in two orders) in the Chondrostei are the Acipenseridae (the sturgeons) and the Polyodontidae (the paddlefish).

The sturgeons are freshwater and anadromous fishes of the Northern Hemisphere with a centre of distribution in central Europe and Asia (about 20 species in total with five in Canada). Sturgeons are best recognized by the five rows of bony "skutes" along the body (remnants of ganoid scales), the ventral mouths and associated barbels - usually in four pairs.

Sturgeons are typically slow-swimming omnivores, but their mouths are highly protrusible and some can and do feed on other fishes.

Strugeons ae generally very long-lived and can reach enormous sizes. For instance, the Russian beluga strugeon (Huso huso) can reach over 100 years of age, 8 m in length, and > 1,300 kg! The large size, delicious cavier (eggs) and their relaively old age at first maturity (> 10 years) means that these animals can be easily overexploited commerically. Such overexploitation coupled with habitat degradation and destruction means that virtually wherever they occur, sturgeons are of some conservation concern. For instance, because they are usually highly migratory withn large river/lake systems, hydroelectic developments that block access to regions used for spawning and feeding has contributed to population declines of the white sturgeon (Acipenser transmontanus) in the Columbia River and portions of the Fraser River (Brown et al. 1992).

White sturgeon from the lower Fraser River, early 1900's

Conservation efforts on white sturgeon are underway for BC populations and white sturgeon support a vigorous catch-and-release sport fishery in the lower Fraser River. The Knowledge Network has a nice video feature, HERE, on conservation efforts for upper Fraser River sturgeon. Do a search on "Saving the Freshwater Giants" in 2008.

The paddlefishes are a less diverse group consisting of two species in two genera. They are a classic example of a relict group with a highly disjunct distribution (see figure below). They are more numerous in the fossil record and distributed throughout freshwaters in the Northern Hemisphere, but the living species are found only in the Mississippi River system (Polyodon spathula) and the Yangtze River system in China (Psephurus gladius). These animals are best known for the spoon or paddle-like rostral extension of the head.

The paddle-like snout may function to direct food into the mouth or, because it is covered in electrosensitive pores (ampullae), for detecting biologically-generated electricity (see Wilkens et al. 1998).

As you may recall from examining the lab specimens, these animals have extremely fine and numerous gill rakers and a very flexible branchial area so the mouth can be opened very wide during feeding. They are also long-lived (up to 30 yr), can get quite large (up to 2-3 m), and are of conservation concern owing to their limited distribution, overexplotation, and habitat destruction.

(v) Neopterygii

The Neopterygii consist of the gars, bowfins, and teleosts. They are typically defined by:

(1) Caudal fin is typically homocercal

(2) 1:1 ratio between fin rays and radials

(3) Maxillary bone is usually "free" from the pre-opercular bones and is attached at its anterior end only (to the skull - see figure). This is an early step in the increasing tendency for the upper jaw to become more flexible and capable of a greater range of movements in teleost fishes.

Variable attachment of maxillary bone: (a) fused to skull and opercular series (fossil actinopterygian fish), (b) free from opercular series, attached at anterior end (Amia calva), (c) largely free at anterior end, ligamentous attachement of maxillary and premaxillary (dark shading) bone (higher teleost fish).

(4) Presence of the symplectic bone (see figure). Forms part of the "suspensorium" or series of bones that support the jaw series.

The neopterygians consist of two major lineages: the Ginglymodi (the gars) and the Halecostomi (bowfin and teleosts).

The Ginglymodi (gars) consist of seven species in a single order and family (Lepisosteifoirmes Lepisosteiidae) and are confined to the freshwaters of the south and central eastern North America with two species that trickle into adjacent parts of Canada (i.e., the gars are another relict group that was mopre widespread in the fossil record - Europe, Asia, Africa). They are characterized by:

(1) The quadrate bone extends to well past the anterior portion of the orbit (eye bone series)

(2) Infraorbital bones that bear teeth ( labelled 1-6 in the figure below)

(3) Heavy ganoid scales

(4) Elongate maxillary and lower jaw bones with heavy dentition

(5) Vertebrae that are "opisthocoelus" (convex at the anterior end, concave at the posterior end). Typically, bony fish have amphicoelus vertebrae (concave at both ends).

They are typically found in slow moving warmish water areas and are classic "sit and wait" ambush predators. Note the posterior positioning of the dorsal and anal fins in these fish.

The arrangement of the fins in this position is though to be efficient for rapid thrust generation during ambush swimming strikes and is found in many distantly related groups of fish (osteolepimorphs, gars, bowfin, pike and other teleosts). Gars can reach 3 m in length and are also facultive air-breathers using the heavily vascularized gas bladder as a lung.

The Halecostomi consist of the bowfin (Halecomorphi) and the teleosts. They are distinguished from the gars by:

(1) Median neural spines that are fused to the vertebrae

(2) Maxillary bone becomes hinged anteriorly only enabling it to "swing down" at its posterior end. This is seen as an advancement in jaw movement and facilitates the creation of a "tunnel-like" opening of the mouth that aids in prey injection.

(3) An interopercular bone (see figure). Again, an example of the evolution of more moving parts in the jaw series that facilitated flexibility in jaw movement and associated feeding mechanisms.

(4) Evolution of a third series of muscles in the jaw series. Actinops generally have two series of muscles that facilitate jaw opening (see figure): (i) the epaxial series which serve to elevate the head, and (ii) the hypaxial series which serve to lower the mandible (lower jaw). The Halecostomi possess a third series (e.g. the dilator operculi) that attach the cranium and lower jaw to the opercular bone series. The significance of this third attachment series was that there were now four ways for fish to open their mouth: elevation of the head via the epaxials, depression of the mandible via the hypaxials, swing-down of the maxillary bone, and contraction of the opercular muscles to open the mouth in a medial direction. This, again, resulted in greater flexibility of jaw opening, greater "suction-like" ability for feeding, and generally a greater range of options for feeding mode which likely promoted ecological dicversification and, ultimately, speciation in teleosts. A recent paper of molecular phylogeny of actinops, however, has challanged the monophyly of the Haleocostomi (see Inoue et al. 2003).

The Halecostomi consists of the Halecomorphi (bowfin) and the teleosts. Bowfin are another relict bony fish consisting of a single order and family with one species (it is a "monotypic" order (Amiiformes, Amiidae, Amia calva) that is, again, restricted to the freshwaters of eastern North America (central Quebec and Ontario to eastern Texas) - do you sense a trend here?? The bowfins were much more diverse from the Triassic to the Cretaceous (11 genera described). They are distinguised from teleosts by having a single, median gular plate and a corocoid process. The latter is a dorso-posterior extension of the lower jaw that serves as an articulation point among the lower jaw, symplectic bone, and the quadrate bone. By contrast, the symplectic bone moves posteriorly and is not involved in the articlulation between the quadrate and the lower jaw (they lack the corocoid process). Bowfin are characterized by heavy bones in the skull (see below) and well developed teeth.

Lateral view of the skull of Amia calva.

Bowfin are inhabitants of slow, weedy waters and have heavy dentition on the jaw bones. They too are ambush predators (note the positioning of the median fins and the size of the dorsal) that can be up to 1 m in length. They are also facultive air-breathers via a vascularized gas bladder. Despite their "primative" characters, bowfin may show sophisticated behaviour, e.g., the males engage in parental care of the young by guarding the young until they are relatively large (10 cm). We'll see more of such behaviour in the more diverse teleosts! In some areas, people are serious about bowfin fishing.

(vi) Summary to date

At this point in the course you should have a good appreciation for:

(1) The major groups of fish and the characteristics that differentiate them, their relative positions in the fish "tree of life" and their relative taxonomic diversities. By major groups I mean at the level of subclasses or (in some schemes) superorders: the major lineages of agnathan fishes, Chondrichthyes, and bony fishes.

(2) The general inter-relationships among the major groups (e.g. like the phylogenetic tree below) and the kinds of traits that are used to derive such hypotheses.

(3) Idea of conflicts in character states, their strengths and weaknesses in phylogenetic reconstruction (i.e. molecules versus morphology, fossils, geographic distribuitons, life history traits). The use of phylogenetic reconstruction in testing evolutionay hypotheses in fish (e.g. origin of freshwater rays, which lineage of fish gave rise to tetrapods?).

(4) Form, function, and ecology. How the general body form of a fish is often functionally related to how it feeds, mates, swims, etc and how such associations have evolved repeatedly in different fish lineages.

(5) The distinction between "primative" lineages and lineages that are highly specialized derivatives of ancestral linages. for instance, there is no doubt that the jawless condition in fish is primativ and ancestral to the jawed condition. Modern representatives of jawless fishes (lampreys and hagfish) are not really "primative" - they have highly specialized and derived features (e.g., metamorphosis in lampreys, complex migrations in lampreys, evolution of freshwater non-parasitic lampreys, slime glands in hagfish).

References:

Billard R. Lecointre G. 2001. Biology and conservation of sturgeon and paddlefish [Review]. [Review] Reviews in Fish Biology & Fisheries. 10: 355-392.

Brown, JR, Beckenbach, AT, Smith, MJ, 1992. Influence of Pleistocene glaciations and human intervention upon mitochondrial DNA diversity in white sturgeon (Acipenser transmontanus) populations. Can. J. Fish. Aquat. Sci., 49: 358-367.

Grande, L, Bemis, WE, 1991. Osteology and phylogenetic relationships of fossil and recent paddlefishes (Polyodontidae) with comments on interrelationships of Acipenceriformes. J. Vert. Paleon.,11(Suppl.): 1-121.

Guenette, S, Rassart, E, Fortin, R . 1992. Morphological differentiation of lake sturgeon (Acipenser fulvescens) from the St. Lawerence River and Lac des Deux Montagnes (Quebec, Canada). Can. J. Fish. Aquat. Sci., 49: 1959-1965.

Helfman, G. et al. 1997. The diversity of fishes. Chapter 13, pp. 211-220.

Jun G. Inoue, Masaki Miya, Katsumi Tsukamoto, and Mutsumi Nishida. 2003. Basal actinopterygian relationships: a mitogenomic perspectiveon the phylogeny of the ‘‘ancient fish’’. Molecular Phylogenetics and Evolution 26: 110–120.

Lauder, GV. 1982. Patterns of evolution in the feeding mechanism of actinopterygian fishes. Amer. Zool., 22:275-285.

Patterson, C, 1982. Morphology and interrelationships of primative actinopterygian fishes. Amer. Zool., 22: 241-258.

Webb, PW. 1982. Locomotor patterns in the evolution of actinopterygian fishes.
Amer. Zool, 22: 329-342.

Wilkens, L.A., D.F. Russel, P. Xing, and C. Gurgens. 1998. The paddlefish rostrum functions as an electrosensory antenna in plankton feeding. Proc. Roy. Soc. Lond. B 264: 1723