Fish Lab Info Flashcards
Agnatha
Agnathans are jawless vertebrates, including a complex and poorly understood radiation of armoured forms that began 520 million years ago.
These ancient fishes, collectively known as ostracoderms (‘bony skin”), are primarily bottom-dwellers that were covered by elaborate head shields and bony plates on the outside, but possessed little internal bone.
Hagfishes (Myxinoidea) and lampreys (Petromyzontoidea) are the two living groups of Agnatha.
Besides lacking jaws, both groups lack paired fins, bone and scales. Hagfishes, however, are thought to be much nearer the base of vertebrate phylogeny than lampreys.
Gnathostomes
jawed vertebrates, evolved from one of the many groups of ostracoderms.
Placoderms
Placoderms are first known from the fossil record from 410 million years ago and all are extinct.
Placoderms may have been the first jawed fishes; they displayed many unique and primitive features.
Like ostracoderms, external bony plates covered them.
Although some were bottom dwelling, other species were large, mid-water predators.
Acanthodians
Acanthodians are the first jawed fishes to appear in the fossil record, 440 million years ago.
They possessed a variety of advanced features, possibly associated with a more active lifestyle in the water column. Acanthodians were covered by a combination of small bony plates and scales, including a bony gill cover (operculum).
They had peculiar fins supported by stout spines and a well-developed internal skeleton of bone.
Although the stem lineages are extinct, fossil acanthodians were part of the lineage that gave rise to Chondrichthyes.
Chondrichthyes
The Chondrichthyes or cartilaginous fishes, which form a monophyletic clade with the extinct acanthodians, are first found in the fossil record 410 million years ago, but they are widely viewed as being significantly older based on recent fossil evidence and many anatomical features of living forms.
Sharks, rays, and chimeras all have an internal skeleton composed of cartilage, internal fertilization, and lack a swim bladder.
Sharks have filled the role as top predator in aquatic systems for millions of years and have undergone a series of changes in dentition and locomotory structures during this time.
Sarcopterygians
Also called flesh-finned or lobe-finned fishes, this group includes the coelacanths, lungfishes, and the “fishy” ancestors of tetrapods, as well as the tetrapods themselves according to cladistic classification.
Sarcopterygians first appeared in the fossil record 410 million years ago.
They share a variety of features with the Actinopterygians including an internal skeleton of bone and primitive lungs (swim bladder).
The robust, paired fins of the Sarcopterygians, which contain a bony axis, connective tissue and muscle, are one distinct feature of this group.
Actinopterygians
The ray-finned fishes are the most successful group of living vertebrates in terms of numbers of species and individuals, and they are the dominant fishes in all freshwater and marine habitats.
They are characterized by paired and unpaired fins composed of bony rays that are linked by webs of skin. Ray-finned fishes first appeared 410 million years ago, and successive groups have displayed changes in tail morphology, scale types and patterns, swim bladder structure and function, and jaw mechanics.
Among living fishes, the sturgeon and paddlefish represent the most ancestral grade (collection of morphological features).
An intermediate grade of evolution, originating about 200 million years ago, is represented by the bowfin and gar.
The thousands of other species of ray-finned fishes are teleosts and display advanced morphology.
Trends in Fish Evolution
Development of jaw with increasing mobility
Differences in the attachment of the jaw bones (particularly the maxilla and premaxilla) across different groups of bony fishes have allowed for differences in gape (gape = transverse opening of the mouth).
The differences in gape and jaw flexibility also permit different feeding styles. For example, maxillary in gape (Amia sp.) and (Perca sp.).
Trends in Fish Evolution
Progressive lightening of the skeleton for greater speed and mobility.
- A reduction in repeating parts; i.e., rays and vertebrae.
- In the evolution of fish the dermis of gave rise initially to dermal / cosmoid bone or plates that gave rise to dermal scales.
Changes in scale types from cosmoid plates (dermal bone), to cosmoid scales or placoid scales, to ganoid scales, to cycloid scales, to ctenoid scales, to the complete loss of scales.
Trends in Fish Evolution
Movement of the position of the fins for greater control of motion.
- Pelvic fins from abdominal region to thoracic region.
-abdominal
-thoracic - Pectoral fins from low to high
- Change from rays only to rays and spines in some fishes.
-rays only
-rays and spines
Trends in Fish Evolution
Lungs and swim (gas) bladders
Lungs were initially paired lateral organs that developed in series with pharyngeal pouches.
These ancient organs shifted ventrally to form the respiratory structures of lungfishes, some primitive ray-finned fishes and tetrapods.
In most ray-finned fishes the ancient, paired organs either shift dorsally to merge over the gut and become a single bladder, or one lung is lost leaving the other to shift around the gut.
The trend in teleost evolution is from physostomous (bladder with duct attached to esophagus) to physoclistous (no attachment to gut).
Trends in Fish Evolution
Tail evolves from being a buoyancy device to a locomotory structure.
Evolved heterocercal (elasmobranchs, ancestral actinopterygians) to either diphycercal
(lungfishes) or homocercal (advanced teleosts).
Body Form
Whole body form
The shape of the body is highly adapted to the environment and mode of life of the fish.
- Fusiform forms (torpedo-like) are usually highly active, fast swimming species; i.e., a shark or mackerel.
- Compressed shapes (laterally flattened) are generally found in still-water species, especially those forming schools and those living in reef habitats. Specialized forms of a laterally compressed fish are the flounders and soles.
- Bottom-dwellers tend to be depressed (dorsoventrally) and in extreme cases they may be flat like a skate or a ray.
- Other species may show considerable variation such as eels, seahorses, needlefish, puffers, anglerfishes, etc.
Body Form
Mouth position
- Terminal mouths are positioned at the anterior-most end of the skull.
—This is the ancestral condition and is used by generalist feeders and some types of specialized feeding. It allows fish to eat food items in front of them. - Superior mouths are angled upward.
—This mouth position is an adaptation for feeding at the surface of the water, above the body position of the fish. - Inferior are angled downward or even positioned underneath the head.
—This mouth position is an adaptation for bottom feeding, eating food that is located below the position of the fish.
Protective Mechanisms of Bony Fishes
Scales and other surface armour
Generally, most fish depend on fast swimming, powerful jaws and good sensory systems. However, subsidiary protective measures have been developed, especially in those fishes that have given up the fast-swimming habit.
- Protective armour of the Paleozoic fishes, formed by thick, overlapping bony plates. Present day trunkfishes also have bony armour.
- Cycloid and ctenoid scales form a covering of thin overlapping bony plates.
