Fishes: A Guide to Their Diversity. Philip A. Hastings
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these major lineages of vertebrates have been discussed for decades and a consensus has been reached (Meyer and Zardoya, 2003). The lobe-finned and ray-finned fishes form a monophyletic group (called the Osteichthyes or “bony fishes”); they together with the cartilaginous fishes make up the “jawed vertebrates” or the Gnathostomata. The jawless fishes are the sister group of all other extant vertebrates. This book covers all major lineages of the Vertebrata with the exception of the Tetrapoda.
Hypothesized phylogenetic relationships of the major lineages of vertebrates.
AGNATHA (CYCLOSTOMATA)
Jawless Fishes
As their name implies (a = without; gnathos = jaw), agnathans lack jaws, and instead possess a rounded mouth, a fact reflected in the older term for the group, the Cyclostomata (cyclo = round; stoma = mouth). Extant members lack pelvic fins, have pore-like rather than slit-like gill openings and an elongate, eel-like body. Agnathans have a well-developed notochord; a rudimentary vertebral column is present only in the lampreys. The group has a rich fossil record, and many of the extinct members had a bony external skeleton that is lacking in living representatives whose entire skeleton is cartilaginous. Extant agnathans include two major lineages, the hagfishes (Myxiniformes) and the lampreys (Petromyzontiformes). Analyses of morphological features imply that the lampreys, though not the hagfishes, are the sister group of the jawed vertebrates (e.g., Forey, 1995; Janvier, 1996). However, extensive molecular data (e.g., Heimberg et al. 2010; Kuraku and Kuratani, 2006) overwhelmingly support the sister-group relationship of hagfishes and lampreys and thus the monophyly of the Agnatha. This finding implies that the extant representatives, especially the Myxiniformes, are reductive in a number of features, confounding efforts to reconstruct their phylogenetic relationships based solely on morphology. Their biology was summarized by Hardisty (1979).
Phylogenetic relationships depicting a monophyletic Agnatha (left), and lampreys as the sister-group to the Gnathostomata (right) (after Heimberg et al., 2010).
MYXINIFORMES : MYXINIDAE—Hagfishes
DIVERSITY: 1 family, 6 genera, 76 species
REPRESENTATIVE GENERA: Eptatretus, Myxine, Nemamyxine
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine; tropical to temperate; inshore to deep sea, benthic, in or on soft substrates
REMARKS: The single family of hagfishes is one of two groups of living jawless or agnathan fishes. In addition to the features listed above, they are characterized by a single nostril, and two features, one semicircular canal, and body fluid isosmotic with seawater, unique among the Vertebrata. Their eyes are degenerate, lacking a lens and extrinsic eye muscles. Their conspicuous slime glands contain both mucous and thread cells and serve to thwart predators. Hagfishes are known to prey on benthic organisms but generally are considered scavengers. They are able to remove chunks of flesh from carcasses using the paired tooth plates on the tongue, gaining leverage by tying their body in a knot. Hagfishes have a few very large eggs and, lacking a larval phase, the hatchlings resemble small adults (Jorgensen et al., 1998). Hagfishes are utilized by the fish leather industry (Grey et al., 2006).
REFERENCES: Fernholm, 1998; Fernholm, in Carpenter, 2003; Fernholm and Paxton, in Carpenter and Niem, 1998; Grey et al., 2006; Jorgensen et al., 1998; Kuo et al., 2003; Kuraku and Kurtani, 2006; Wisner and McMillan, 1995.
MYXINIFORM CHARACTERISTICS:
1) body eel-like, naked
2) paired fins and dorsal fin absent
3) lateral line absent in adults
4) one to sixteen pairs of external pore-like gill openings
5) oral barbels distinctive
6) numerous mucous pores on body
ILLUSTRATED SPECIMEN:
Eptatretus stoutii, SIO 87–125, 145 mm TL
Inset: Tooth plates of Myxine capensis, showing keratinous cusps, SIO 92–107
PETROMYZONTIFORMES—Lampreys
The 46 species of living lampreys are found in temperate areas of both hemispheres. The monotypic Geotriidae and the three species of Mordaciidae are found in the Southern Hemisphere, while the more diverse and well-known Petromyzontidae is restricted to the Northern Hemisphere (Renaud, 2011).
REFERENCES: Gill et al., 2003; Renaud, 2011.
PETROMYZONTIFORMES : PETROMYZONTIDAE—Northern Lampreys
DIVERSITY: 8 genera, 42 species
REPRESENTATIVE GENERA: Ichthyomyzon, Lampetra, Petromyzon
DISTRIBUTION: North America, Europe, and Asia
HABITAT: Freshwater lakes, rivers, and streams or anadromous; temperate; demersal, or benthic on soft substrates
REMARKS: Lampreys are characterized by two semicircular canals, an otic capsule anterior to the first branchial opening, and body fluid hyposmotic to seawater. Unlike hagfishes, lampreys lay numerous small eggs; their larva, called an ammocoete, filter-feeds on detritus. Lampreys include both parasitic and free-living species. In general, the 22 free-living species (called brook lampreys) remain in small streams and rivers throughout life, though they cease feeding after metamorphosis. Parasitic species have a similar lifestyle in their young stages, but as adults they migrate to the ocean or large lakes where they use their round mouth to attach to other fishes and their rows of teeth to rasp away flesh. Transitions between these life history patterns have occurred repeatedly, as several pairs of closely related species include a parasitic and a free-living form (Potter, 1980). Gill et al. (2003) explored the phylogeny of lampreys based on morphological features and, more recently, Renaud (2011) reviewed their systematics and biology.
REFERENCES: Gill et al., 2003; Hardisty and Potter, 1971; Potter, 1980; Renaud, 1997, 2011.
PETROMYZONTID CHARACTERISTICS:
1) body eel-like, naked
2) no paired fins, one or two dorsal fins
3) lateral line absent
4) seven pairs of external pore-like gill openings
5) oral barbels absent
6) oral disk and tongue bearing rows of teeth
7) single nostril located between eyes, anterior to pineal eye
8) cloaca located under anterior half of second dorsal fin or posterior lobe of single dorsal fin
ILLUSTRATED SPECIMEN:
Petromyzon marinus, SIO 74–134, 124 mm TL
GNATHOSTOMATA
Jawed Vertebrates
The Gnathostomata is an extraordinarily successful lineage of over 60,000 species characterized by upper and lower jaws that are derived from modified gill arches. Most gnathostomes also possess paired pectoral and pelvic limbs, vertebral centra, slit-like gill openings at some stage of development, and three semicircular canals (Forey, 1995; Nelson, 2006). These and a host of other features mark them as active, mobile predators with more powerful sensory abilities than agnathans (Gans, 1987; Gans and Northcutt, 1983; Shimeld and Holland, 2000). Living gnathostomes include the Chondrichthyes (sharks and rays), Sarcopterygii (lobe-finned fishes and tetrapods), and Actinopterygii (ray-finned fishes). Phylogenetic relationships of gnathostomes were summarized