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Osteichthyes

Bony Fishes

The clade Osteichthyes may at first seem misnamed as it includes not only what are readily recognized as “bony fishes” but also the entire lineage of tetrapods. However, Osteichthyes refers to the ancestor and all descendants (i.e., a monophyletic group) of a lineage that is the sister group of the cartilaginous fishes. These two great lineages of fishes differ in several fundamental features, most notably in the composition of their skeleton. Osteichthyans have a bony skeleton while chondricthyans have a skeleton formed entirely of cartilage. In addition, the neurocranium of osteichthyans has evident sutures (sutures are absent in chondrichthyans); their fin rays (if present) are segmented and derived from the dermis (termed “lepidotrichia”), while those of chondrichthyans are unsegmented and epidermal in origin (termed “ceratotrichia”). Finally, osteichthyans typically have a gas bladder or its derivative (lungs in tetrapods), a structure lacking in chondrichthyans. The Osteichthyes comprises two large lineages, the Sarcopterygii, or lobe-finned fishes and tetrapods, and the Actinopterygii, or ray-finned fishes (Stiassney et al., 2004).

SARCOPTERYGII—Lobe-finned Fishes

The Sarcopterygii is a major group of vertebrates that includes the coelacanths, lungfishes, and tetrapods and comprises over 25,000 species. These “lobe-finned fishes” are characterized by the presence of enamel on the teeth, a unique skeletal support for the paired fins (or limbs) that includes a central axis of bone, and autostylic jaw suspension in which the upper jaw is fused with the skull. We treat the two most “fish-like” orders of sarcopterygians, the Ceratodontiformes, or lungfishes, and the Coelacanthiformes, or coelacanths. The tetrapods are not considered here.

COELACANTHIFORMES : LATIMERIIDAE—Coelacanths

DIVERSITY: 1 family, 1 genus, 2 species

REPRESENTATIVE GENUS: Latimeria

DISTRIBUTION: Indian and western Pacific oceans, off southern Africa and Indonesia

HABITAT: Marine; tropical to temperate; continental shelf and continental slope, demersal on deep rocky reefs

REMARKS: The living coelacanths represent a group thought to have become extinct 80 million years ago. An extant member of this unique group of fishes was first discovered in 1938 in the western Indian Ocean, while a second species was first captured by scientists in 1998 in Indonesia (Holder et al., 1999; Pouyaurd et al., 1999). Coelacanths have been and continue to be studied extensively. They are the only vertebrates with an intracranial joint, possibly allowing vertical movement of the head in order to increase the size of the gape. They are piscivorous and utilize an electroreceptive sense to enhance their predation on small fishes. Coelacanths are unusual in having a rectal gland and high levels of urea in the blood. They are internal fertilizers (Smith et al., 1975), may be monogamous (Lampert et al., 2013), and the females give birth to 5–26 well-developed young. Their entire genome was recently sequenced (Amemiya et al., 2013). They are endangered as a result of their low reproductive potential and small geographic range; Latimeria chalumnae is listed as critically endangered by the IUCN. The stories of the discovery of both species are full of drama and intrigue (Nelson, 2006; Smith, 1956; Thomson, 1991).

REFERENCES: Amemiya et al., 2013; Bruton, 1995; Cloutier and Ahlberg, 1996; Forey, 1980, 1991, 1998; Holder et al., 1999; Lampert et al., 2013; McCosker and Lagios, 1979; Musick et al. 1991; Pouyaurd et al., 1999; Smith et al., 1975; Smith, 1940, 1956; Thomson, 1991.

COELACANTHIFORM CHARACTERISTICS:

1) second dorsal, anal, pelvic, and pectoral fins lobe-like

2) first dorsal fin with hollow spines

3) caudal fin diphycercal and in three lobes

4) double gular plate between left and right sides of lower jaw

5) large, bony, cosmoid scales

6) intracranial joint

ILLUSTRATED SPECIMEN:

Latimeria chalumnae, SIO 75–347, 950 mm TL

CERATODONTIFORMES—Lungfishes

DIVERSITY: 3 families, 3 genera, 6 species

REPRESENTATIVE GENERA: Lepidosiren, Neoceratodus, Protopterus

DISTRIBUTION: Sub-Saharan Africa (Protopterus), South America (Lepidosiren), and Australia (Neoceratodus)

HABITAT: Freshwater; tropical to subtropical; benthic to demersal over soft bottoms

REMARKS: The three families of living lungfishes, also called the Dipnoi, have one or two lungs used for either facultative or, in some cases, obligate air breathing (Graham, 1997). The African lungfishes (Protopteridae) are characterized by the presence of slender, elongate pectoral and pelvic fins, small scales, and six gill arches, while the montotypic South American Lungfish (Lepidosirenidae) also has slender pectoral and pelvic fins, small scales, and paired lungs, but only five gill arches. In contrast, the monotypic Australian Lungfish (Ceratodontidae) has paddle-like pectoral and pelvic fins, larger scales, and a single, unpaired lung. The African lungfishes are the sister group of the single species of South American Lungfish, and are placed together in the order Lepidosireniformes by some authors. These two lineages are the sister group of the single extant Australian Lungfish (Ceratodontidae). Nelson (2006) includes all three families in the order Ceratodontiformes. African lungfishes are large, with at least one species reaching lengths up to 1.8 m TL. During the dry season, individuals burrow into the mud and aestivate inside mucous cocoons. The pelvic fins of reproductive males of the South American Lungfish become highly vascularized and feather-like, infusing oxygen into the water where young are developing. Lungfishes are omnivorous, feeding on other fishes, frogs, mollusks, and in some cases, plant material, including seeds.

REFERENCES: Bemis et al., 1987; Cloutier and Ahlberg, 1996; Graham, 1997; Nelson, 2006.

CERATODONTIFORM CHARACTERISTICS:

1) pectoral and pelvic fins slender (A and B) or flattened (C)

2) gular plate absent

3) premaxilla and maxilla absent

4) one or two lungs

5) five or six gill arches

ILLUSTRATED SPECIMENS:

A) Protopterus aethiopicus, CAS 46377, 854 mm TL (Protopteridae—African lungfishes)

B) Lepidosiren paradoxa, CAS 14001–7, 549 mm TL (Lepidosirenidae—South American Lungfish)

C) Neoceratodus forsteri, CAS 18189, 748 mm TL (Ceratodontidae—Australian Lungfish)