Ichthyology Less_1_General Features PDF

1. An overview on the course: Fish facts  What is a fish? Define a “fish” is a unrealistic attempt, given the diversity of shape and adaptation that characterizes the thousands of species alive today (e.g., hagfishes, lampreys, sharks, rays, ratfishes, coelacan...

1. An overview on the course: Fish facts  What is a fish? Define a “fish” is a unrealistic attempt, given the diversity of shape and adaptation that characterizes the thousands of species alive today (e.g., hagfishes, lampreys, sharks, rays, ratfishes, coelacants, lungfishes, sturgeons and all the advanced ray-finned fish) Hovewer, one can define a fish as: “An aquatic chordate with appendages developed as fins, whose main respiratory organs are gills, and whose body is usually covered with scales” More simply: “A fish is an aquatic vertebrate with gills and fins”  Fish are vertebrates (L.: Vertebrata) They have a vertebral column (or spinal column, or backbone) (bony or cartilaginous) and a skull (known for this as craniates, L.: Craniata) (bony or cartilaginous too) The backbone is a segmented series of rigid elements (vertebrae), separated by mobile joints (intervertebral discs) Nota bene: few living ancient fish (hagfish) do not possess a true vertebral column  Fish are vertebrates (L.: Vertebrata) They have a vertebral column (or spinal column, or backbone) (bony or cartilaginous) and a skull (known for this as craniates, L.: Craniata) (bony or cartilaginous too) The backbone is a segmented series of rigid elements (vertebrae), separated by mobile joints (intervertebral discs) Nota bene: few living ancient fish (hagfish) do not possess a true vertebral column  Fish have gills A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide (and other wastes) We find different organization and Gill pockets (blind background) shape in the different groups of fishes (e.g., Lampreys) (we’ll see it later in the course) Gill slits (e.g., Sharks) Operculum (e.g., Bony fish)  Fish have fins Fins are the most distinctive feature of a fish, composed of rays protruding from the body with skin covering them Their principal function is to help the fish swim: moving forward, turning, keeping an upright position, stopping, etc. (we will see it later in the course) Nota bene: apart from the caudal fin, fins have no direct connection with the vertebral column, and are supported by muscles only.  We distinguish two main kinds of fins in fish, based on the position in regard to the dorso-ventral axis: - Paired fins, i.e the pectoral and ventral (also called pelvic or abdominal) fins, corresponding to the fore (arms) and hind (legs) limbs of the tetrapods, respectively - Median or unpaired fins, which include the caudal fin, the dorsal fin(s) on the back, and the anal fin(s) on the belly a Nota bene: caudal fin shapes aid the identification of the main groups of fishes.  Fish have (as a rule) scales Scales cover the body of almost all fish species: they serve mainly for protection and for aiding swimming Nota bene: scales types aid the identification of the main groups of fishes.  In living fish, there are three basic types of scales: 1) The placoid scales of sharks and rays (Chondrichthyes): very rough, pointed, covered by enemel 2) The ganoid scales of ancient, non-teleosts bony fish (Chondrostei/Holostei): thick, flattened, jointed, covered by “ganoine” (enamel-like substance) 3) The leptoid scales of advanced bony fish (Teleostei) - Leptoid scales show two forms: a) Cycloid scales: thin, imbricate, smooth edge b) Ctenoid scales: thin, imbricate, rough edge A fourth type, 4) Cosmoid scales [green box]: very thick, with lamellar bone and “cosmine”, was present in extinct Sarcopterygii Nota bene: extant coelacanths and lungfish show reduced cosmoid scales, similar to leptoid scales  Numbers, habitat and life cycle of fish Numerically, valid scientific descriptions exist for over 30 thousand living species of fishes (50% of vertebrates) Nota bene: Diadromous fish s.s.: 1) Anadromous fish When broken down by major habitats, about 40% of species live in fresh water, 2) Catadromous fish 60% in sea water, and less then 1% move between fresh water and the sea (or vice Diadromous fish s.l.: versa) during their life cycles (diadromous 3) Oceanodromous fish fish s.s.). 4) Potamodromous fish  They are born in freshwater, then migrate to the ocean as juveniles where they grow into adults before migrating back into freshwater to spawn.  They are born in saltwater, then migrate into freshwater as juveniles where they grow into adults before migrating back into the ocean to spawn.  They are born near spawning grounds, then drift on ocean currents as larvae before settling as juveniles to grow into adults in feeding grounds before migrating back to spawning grounds. (Modern/Living) Fish are divided into three main groups: I. Agnates (L.: AGNATHA): primitive jawless fish e.g. Lampreys, Hagfishes II. Cartilaginous fish (L. CHONDRICHTHYES): (L. GNATHOSTOMA) jawed fish with cartilaginous skeletons e.g. Sharks, Rays They are born in upstream III. Bony fish (L.: OSTEICHTHYES): freshwater habitats, then (L. GNATHOSTOMA) jawed fish with (as a rule) bony skeletons migrate in downstream habitats (still e.g. Coelacants, Lungfish, Sturgeons, Trouts, Basses, in freshwater) Salmons, as juveniles to grow into Pikes, Groupers, Tunas, Swordfish, Gobies... adults before migrating back and many more upstream to spawn.  Systematics (Living) fish are divided into three main groups: I. Cyclostome fish (L. CYCLOSTOMATA): Primitive jawless fish (L.: AGNATHA) Myxiniformes Petromyzontiformes II. Cartilaginous fish (L. CHONDRICHTHYES): Jawed fish (L. GNATHOSTOMA), with cartilaginous skeleton Elasmobranchii Holocephali III. Bony fish (L.: OSTEICHTHYES): Jawed fish (L. GNATHOSTOMA), with bony skeleton Sarcopterygii Actinopterygii I. CYCLOSTOMATA – Features They have no jaws: in place there is an oral sucker They are predators/scavenger, using the mouth to rasp preys’ skin/tissues They are eel-like shaped, with a very simple and flexible cartilaginous skeleton They possess a smooth scaleless skin, gill pouches, a primitive protocercal caudal fin (we’ll talk about the different types of caudal fins later) They live in fresh and saltwaters, some are anadromous Extant cyclostomes comprise: 1.Myxiniformes and 2.Petromyzontiformes Types of Cyclostomes 1. Myxiniformes Hagfish Lampreys 2. Petromyzontiformes II. CHONDRICHTHYES – Features They have upper and lower jaws, often with a significant array of teeth They have a cartilaginous skeleton Their skin is covered with tough dermal denticles (placoid scales), with a texture similar to sandpaper  They have a heterocercal caudal fin, 1. Shark more or less developed 2. Ray They have five to seven pairs of gill slits 3. Rat fish Extant cartilaginous fish comprise: 1.Sharks and 2.Rays (Elasmobranchii) 3.Rat fishes (Holocephali)  1.Sharks (Elasmobranchii: Selachii or Pleurotremata) Group of fish characterized by: Five pairs of gill slits (exception: family Hexanchidae, six/seven pairs) Snout (It. “pescecane”) and spiracles (the last involved in respiration) Real heterocercal caudal fin (the dorsal portion is larger than the ventral one) Claspers (It.: missipterigi) in the male (separate sexes, internal fertilization) Two dorsal fins (exception: family Hexanchidae, with one dorsal) Almost exclusively distributed in seawater (with very few exceptions)  Hexanchus griseus Carcharinus leucas 2.Rays (Elasmobranchii: Batoidea or Hypotremata) Group of fish characterized by: Flattened bodies, enlarged anteriorly (discus) Five gill slits pairs on the ventral surface Spiracles (dorsally) Modified heterocercal caudal fin, with varying degree of reduction Claspers (It.: missipterigi) in the male Almost exclusively distributed in seawater (with numerous exceptions) Rays sensu latu comprise: - Stingrays - Skates - Torpedos 2.Rays (Elasmobranchii: Batoidea or Hypotremata) Group of fish characterized by: Flattened bodies, enlarged anteriorly (discus) Stingray Five gill slits pairs on the ventral surface Spiracles (dorsally) Modified heterocercal caudal fin, with varying degree of reduction Claspers (It.: missipterigi) in the male Almost exclusively distributed in Skates seawater (with exceptions) Rays sensu latu comprise: - Stingrays - Skates - Torpedos Torpedo  3.Rat fish (Holocephali) Chimaeras are a group of marine fishes characterized by: Elongated bodies Tentaculum and pelvic claspers in the male Gill slits covered by a soft operculum (but not bony !) Often reduced heterocercal caudal fin Dorsal fin with a (venomous) spike  Inset: some species of Chimaeridae (ratfishes) Harriotta haeckeli Neoharriotta pinnata Rhinochimaera atlantica Hydrolagus affinis Chimaera monstrosa  Inset: some species of Callorhinchidae (elephantfishes) Callorhinchus milii Callorhinchus capensis III. OSTEICHTHYES – Features Also called bony fish (not always truly «bony»), they are a big and divesified group of fish characterized by: A very complex calcified (often, but not always) skeleton Soft and hard rays sustaining the fins  The most distinctive feature is a bony operculum (pl. opercula), that covers the gills at each side (see lampreys and sharks for a comparison) Lamprey for comparison Shark for comparison Bony fish typically (but not always) have a swim bladder (the green organ in the x-rayed fish), which helps the body to create a neutral balance between sinking and floating (buoyancy) (we talk about this later) Most have scales, that generally can be: - Cosmoid-like (some primitive bony fish) - Ganoid (other primitive bony fish) - Cycloid/Ctenoid (advanced bony fish) Bony fish typically (but not always) have a swim bladder (green organ in the x-rayed fish), which helps the body to create a neutral balance between sinking and floating (buoyancy) (we talk about this later) Most have scales, that generally can be: - Cosmoid-like (some primitive bony fish) - Ganoid (other primitive bony fish) - Cycloid/Ctenoid (advanced bony fish) However: - Herrings have deciduous scales, which are easily shed and aid in escaping predators (“glitter” effect) - Moray eels and blennies do not have scales (mucus serves as lubricant to move within rocks and crevices) - Different groups of fish have evolved a number of modified scales to serve various functions (e.g. seahorses have scales transformed in a though armour) Seahorse Moray eel Herring Blenny Extant bony fish are divided into: 1) Lobe-finned fish (Sarcopterygii), that possess fleshy, lobed, paired fins - Pectoral and pelvic fins have articulations resembling those of tetrapod limbs (arms and legs), used to «walk» (lizard-like) - The living sarcopterygians comprise: a) Coelacanths b) Lungfish 2) Ray-finned fish (Actinopterygii), that possess lepidotrichia or “fin rays” - Their fins are formed by webs of skin supported by rays, used to swim - The living actinopterygians comprise: a) Chondrostei ( + Cladistia) b) Holostei c) Teleostei a b c  1a. Lobe-finned fish (Sarcopterygii): Coelacanthiformes The coelacanths constitute a now rare order of marine Sarcopterygii fish, that includes two extant species in the genus Latimeria: - The West Indian Ocean coelacanth (L. chalumnae) - The Indonesian coelacanth (L. menadoensis) Latimeria sp. Distribution of the species of the genus Latimeria  They possess a leptocercal (or modified diphycercal) caudal fin Their body is covered by primitive cosmoid–like scales, very thinned Coelacanths are opportunistic feeders, hunting cephalopods and other fish found in deep water They often feed and rest upside down !  Unique to extant coelacanths is the presence of a "fatty lung" (vestigial lung), homologous to the lungs of tetrapods, and to the swim bladder of teleosts fish Nota bene: the presence of fat and oil probably helps flotation, due to their lower specific weight in respect of water; furthermore, the ventral location of the fatty lung favours the overturn of the fish while feeding or resting  1b. Lobe-finned fish (Sarcopterygii): Ceratodontiformes Known as Dipnoi ( = double breathing) or lungfish ( = fish with lungs), they also belong to the living lineage of Sarcopterygii fish As coelacanths, Dipnoi possess: - A leptocercal (or modified diphycercal) caudal fin, reduced to a protocercal - A body covered by cosmoid–like scales  Lungfishes live only in freshwater of Africa, South America and Australia Although the present range seems to represent an ancient distribution limited to the Mesozoic Gondwana, nonetheless: - The fossil records suggest that lungfish had a widespread freshwater distribution (also in the north hemisphere) - The current distribution thus reflects extinction of many lineages, which remained localized only in the south hemisphere  Most extant lungfish species have two lungs (but one is often atrophied) The lungs connect to the pharynx; they are homologous to the lungs of tetrapods, but are moved on one side: it serves as respiratory organ  The last phylogenetic tree of Sarcopterygii Nota bene: from a phylogenetic viewpoint, Sarcopterygii forms with tetrapods a monophyletic group.  2a. Ray-finned fish (Actinopterygii): Chondrostei Chondrostei ( = half bones and half cartilage) are “cartilaginous” fish (not in the systematic sense), showing only a partial ossification The ancestors are thought to be bony fish, but the characteristic of an ossified skeleton was lost in later evolutionary development They are divided among two orders: 1) Polypteriformes (bichirs) (African continent) (recently moved to the taxon Cladistia) 2) Acipenseriformes (sturgeons and Polypterus sp. paddlefish) (North hemisphere) (now the «true» Chondrostei) Acipenser sp. Polyodon spathula The last phylogenetic tree of Actinopterygii (CLADISTIA) (20.000) 1) Polypteriformes (Cladistia) Bichirs are elongated fish, with a unique series of dorsal finlets instead of a single dorsal fin, which vary in number (7 to 18) They are the only fins with hard spines The body is covered by thick, bonelike, rhombic ganoid scales Bichirs also present fleshy pectoral fins, superficially similar to those of lobe-finned fishes; they possess a hemihomocercal caudal fin 2) Acipenseriformes (Chondrostei s.s.) Once, this group was classified together with chondrichthyans Sturgeon Paddlefish The similarities are obvious: - As sharks, chondrosteans mostly lack bone in the skeleton - Both groups have similar scales: a) Ganoid scales in chondrosteans, covered with ganoin b) Placoid scales in chondrichthyans, also covered with enamel - They shared a snout, and spiracles (openings on the head which lead to the respiratory system) behind eyes Sturgeon Shark  - Chondrichthyans and chondrosteans also share a heterocercal caudal fin Paddlefish Shark Sturgeon However, fossil records and molecules suggest that Sturgeon chondrosteans have more characters in common with bony fish, than their external appearance might suggest operculum In particular, they share with bony fish bony opercula  2b. Ray-finned fish (Actinopterygii): Holostei Holostei are bony fish that still retain very primitive characteristics There are formed by a dozen of North and Central American species, divided among two orders: 1. Amiiformes, represented by a single living species (Amia calva) 2. Lepisosteiformes, with few living species (Lepisosteus spp.)  Basal (primitive) features are: - The skeleton only lightly ossified: a thin layer of bone covers a mostly cartilaginous skeleton - The presence of ganoid scales (ganoin: an enamel-like substance) - They possess a hemihomocercal caudal fin The last phylogenetic tree of Actinopterygii (20.000)  2c. Ray-finned fish (Actinopterygii): Teleostei Teleostei are the third group of Actinopterygii: it includes over 20 thousand extant species ! They have a homocercal caudal fin (equally lobed) Most teleosts have a swim bladder (for buoyancy) A) Primitive, soft-ray teleost (e.g., the herring) B) Advanced, spiny teleost (e.g., the yellow perch) They also possess overlapping leptoid (cycloid or ctenoid) scales  2c. Ray-finned fish (Actinopterygii): Teleostei Teleostei are the third group of Actinopterygii: it includes over 20 thousand extant species ! They have a homocercal caudal fin (equally lobed) Most teleosts have a swim bladder (for buoyancy) A) Primitive, soft-ray teleost (e.g. the herring) B) Advanced, spiny telost (e.g. the yellow perch) They also possess overlapping leptoid (cycloid or ctenoid) scales  A distinguishing feature of the teleosts is a mobile premaxilla (in red); the premaxilla is unattached to the braincase (skull); it plays a role in protruding the mouth together with the maxilla (in yellow) Nota bene: due to the high diversity of species/shapes/adaptations, most part of the course is dedicated to teleosts The last phylogenetic tree of Actinopterygii (20.000)  Inset: a short note on some phylogenetic terms 1) A monophyletic group is a group of organisms that consists of all the descendants and their common ancestor 2) A paraphyletic group consists of all of the descendants, the common ancestor, minus one (or more) monophyletic group(s) 3) A polyphyletic group is a set of organisms grouped together, without sharing an immediate common ancestor  Inset: «peculiar» sensory systems in fish (Deepenings in Appendix 8) To relate to their surroundings, fish use many senses common to other terrestrial vertebrates, in particular vision and chemoreception, with similar organs (e.g., eyes, nostrils, barbels) However, the sensory environment of a fish is quite different than what experienced by terrestrial animals: -Vibrations may travel for long distances under water, so fish can rely on the 1.Mechanoreception -Water’s conductive properties surround fishes with electric impulses, making possible the 2.Electroreception  1.Mechanoreception: the lateral line The lateral line is a sense organ used to detect movements and vibrations in the surrounding water The sensory ability is achieved via neuromasts, which respond to displacement of stereocilia (“hair cells” protected by a cupula) caused by motion of the surrounding water, and transduce these signals into electrical impulses to the sensory neuron Neuromasts are in a canal beneath the scales of the side (the “ trunk lateral line”) and in dermal bones of the head (the “cephalic lateral line”), which open to the surrounding water via small pores Nota bene: the lateral line, as organ, is present in most of bony fish and chondrichthyans  1.Mechanoreception: the «Weberian apparatus» It is an anatomical structure that connects the swim bladder to the auditory system in some fishes called «Otophysi» This structure consists of a set of minute bones (Weberian ossicles) that physically connect the inner ear to the swim bladder The Weberian apparatus acts as an amplifier of sound waves, that would otherwise be only slightly perceivable by the inner ear structure alone (due to the similar density of both fish tissues and water)  2.Electroreception: the ampullae of Lorenzini The ampullae of Lorenzini of chondrichthyans are sensing organs called electroreceptors, forming a network of jelly-filled pores, used to detect minimal electric fields Each ampulla consists of a jelly-filled canal opening to the surface by a pore in the skin, and ending blindly in a cluster of small pockets, also full of jelly  2.Electroreception: the rostral organ The rostral organ of coelachants is a sensing organs, similar to the ampullae of Lorenzini of sharks and rays, also used to detect minimal electric fields of prey A great advantage for fish that often prey in dark water !