Natural History of Sharks, Skates, and Rays

Early Chondrichthyes MARE 380 Dr. Turner

Defining Elasmobranchs

What kind of Thyes?...Chondrichthyes

What kind of Fish?...Cartilagenous Fish What kind of Brates?...Vertebrates… What kind of Dates?...Chordates…

Phylum Chordata

3 subphylum:

Subphylum Tunicata

– tunicates, sea squirts

Subphylum Cephalochordata

– lancelets

Subphylum Vertebrata (Crainiata)

– fishes, amphibians, reptiles, birds, mammals

Subphylum Tunicata

– Sessile, feeding 4 chordate traits: Dorsal hollow nerve chord , notochord , pharyngeal gill slits , post anal tail – Mobile, non-feeding

Subphylum Cephalochordata

4 chordate traits: Dorsal hollow nerve chord, notochord, pharyngeal gill slits, post anal tail

Who did the what now?

“Well, whenever I'm confused, I just check my underwear. It holds the answer to all the important questions.”

– Grandpa Simpson

Neoteny

is the retention of juvenile traits in an adult Specifically,

paedomorphosis

is the developmental process by which these changes take place

Subphylum Vertebrata

“Crainiata” Characteristics that distinguish vertebrates: Extensive skull Backbone - a dorsal row of hollow skeletal elements ( vertebrae ) which enclose and protect nerve (spinal) cord

Fishes

“I wish, I wish I did not kill that fish”

– Homer Simpson Simplest & oldest of all living vertebrates •Sharks (400-500 MYBP) Most abundant vertebrates (by # & species) ~29,500 living spp of fishes (>482 families) 58% Marine; 1% diadromous  fish travel between salt & fresh water

Fishes

Class Agnatha (jawless fish) Subclass Myxinodea (hagfish) Subclass Petromyzontida (lamprey)

Infraphylum Gnathostomata

(jawed fish-cart) Class Chondrichthyes (cartilaginous fish) Class Osteichthyes (jawed fish-bony) Subclass Actinopterygii (ray-finned fish) Subclass Sarcopterygii (lobe-finned fish)

Class Agnatha

Subclass Myxinodea

(hagfish) - Cartilage skull - Lack jaws & vertebrae - All Marine (30 spp.) - Scavengers - Produce slime

Class Agnatha

Subclass Petromyzontida

(lamprey) - Cartilage skull - Lack jaws & vertebrae - Freshwater & Marine (35 spp.) - Parasites – attach to host - Rasping tongue - Some

diadromous

Infraphylum Gnathostomata

Class Chondrichthyes

(sharks, skates, rays, ratfish)

Placoid scales

- Cartilage skeleton (jaws & vertebrae) - Marine & few FW (750 sp.) - Traces of bone in scales & teeth - Buoyancy via liver – squaline oil

Spiral valve

– corkscrew intestine  SA:V ratio; compact - Internal fertilization of eggs

Claspers

– modified pelvic fins ♂

Infraphylum Gnathostomata

Class Chondrichthyes

(sharks, skates, rays, ratfish)

Skates differ from Rays:

Skates have a more muscular tail, two dorsal fins & often a caudal fin, lay eggs

Skates & Rays differ from Sharks:

Enlarged pectoral fins that attach to side of head, no anal fin, ventral gill openings, dorsal eyes & spiracles

Ratfishes (Chimaeras):

Possess an operculum or gill cover over gill slits, adults have no scales, ♂ clasper on head

Shark Skate Ray Ratfish

Early Vertebrates

Earliest vertebrates - early Cambrian 530 mybp (million years before present)..

Early relatives of agnathans (jawless fishes) first 500+ mybp

Early Fishes

Early Fishes

1. Ordovician (505-438 mybp) 2. Silurian (438-408) 3. Devonian (408-360) 4. Carboniferous (360-290) 5. Permian (290-240) 6. Triassic (240-205) 7. Jurassic (205-138) 8. Cretaceous (138-63) 9. Cenozoic (63-24) 10. Quaternary (24-0) A. Hemicycapsis, B. Pterapsis, C. Cyathapsis, D. Drepanapsis, E. Coccosteus, F. Helodus, G. Cladoselache, H. Raja, I. Chimaera

Early Fishes

Conodonts

– (550 mybp) – known from small (<2mm) teeth found in fossil deposits Large eyes and eel-like bodies, notochord Closer to jawed fishes than lamprey & hagfishes

Early Fishes

Hagfishes

- (Subclass Myxinodea) – (550 mybp?) – marine, jawless, eel-like fishes; scavengers Single nostril, rudimentary eyes, ventral mouth, tongue with rows of keratinized teeth

Early Fishes

Lampreys

– (Subclass Petromyzontida) – anadromous or freshwater, lawless, eel like fishes Predatory & non-predatory forms Keratinized teeth on buccal funnel & tongue 360 mybp

Early Fishes

Ostracoderms

– some of the earliest fishes Lack jaws, have paired fins, bony armor, cartilaginous skeleton, heterocercal tail (460 mya)

Early Fishes

Acanthodii

– “spiny sharks” – some of the oldest known jawed fishes (440 mybp) Small (<20cm), large eyes, streamlined bodies, dentine-tipped scales

Early Fishes

Placoderms

– also some of the earliest jawed fishes Heavy bony skeletons No special affinities with modern fishes 380 mybp?

Placoderms

Most likely sister group to the combined lineages of Acanthodii, Chondrichthyes, and Osteichthyes; share: 1) jaws with common structure 2) Two pairs of paired fins w/ bony girdles 3) three semicircular canals in inner ear

Early Fishes

Osteichthyes

– bony fishes – loosely defined group Defined by common structures and lack of characters that define chondrichthyes 420 mybp

Early Fishes

Chondrichthyes

–

What are they Doctor?

Sharks, rays, & skates…

But that’s not important right now…

Early Cartilagenous Fishes

Chondrichthyan fishes

most successful measured by historical endurance; ability to survive extinctions

Defined by

cartilagenous skeleton mineralized by calcifications (tesserae) and modification of mixopterygia (claspers) in ♂

Early Cartilagenous Fishes

Two sister taxa:

Elasmobranchii

(sharks, rays, skates) &

Holocephali

(chimeras)

Evidence of Early Chondrichthyans

Easier group to define than bony fishes: 1) only approximately 850 species 2) fossil groups are poorly known

Evidence of Early Chondrichthyans

Scales & spines from early chond. Identified in Lower Silurian (430 mybp) to Devonian (350 mybp) Difficult to nail down due to similar morphology among scales & spines of thelodonts & acanthodians (spiny sharks) at this time

Evidence of Early Elasmobranchii

True sharks - appeared in middle Devonian (350 mybp); Rays appeared during Jurassic (200 mybp) Few well preserved specimens; difficult to piece together evolution Two early forms: cladoselachian & xenacanth sharks

Evidence of Early Elasmobranchii

Two early forms: Cladoselachian Xenacanth (350 mybp)

Order Cladoselachiformes

Lacked:

claspers, an elongate skull, amphistylic jaw suspension, no anal fin,

Had:

triangular, paired fins, multicusped teeth Predator in marine systems

Jaws…Then & Now

Devonian shark: Snout typically short and rounded; jaws longish and located at the front of the head Modern sharks: Snout typically longish and pointed; jaws shorter and located underneath the head

Jaws…Then & Now

Long jaws are structurally weaker than short ones and less able to produce a powerful bite Early sharks may have plucked prey from the bottom or with forceps-like delicacy

Jaws…Then & Now

Early sharks' upper jaws were fixed to the braincase at both the front and the back (

amphistylic

) form of jaw suspension Most modern sharks the upper jaw is fixed to the braincase at the back only (

hyostylic

) jaw suspension. Ancient sharks may have been less able to protrude their jaws than modern sharks, reducing their ability to suck prey into their mouths and restricting the size of their food

Order Xenacanthiformes

Had:

2 anal fins, tail diphycercal (pointed) Predator in freshwater systems

Order Hybodontiformes

Ancestral to modern sharks Appeared during Permian (260 mybp) Fed on large, active invertebrates – first with large, sharp teeth

Order Chimaeriformes

Appeared during Devonian (350 mybp) with Elasmobranchs Modern forms during Jurassic (170 mybp)

Modern Cartilagenous Fishes

Monophyletic group Common origin – distinct from bony fishes

Modern Cartilagenous Fishes

Beyond cartilage have several traits in common….

1. Simple box-like cranium 2. Upper jaws (palatoquadrate cartilage) not fused to cranium ; lower jaw is a single element (Meckel’s cartilage) 3. 4-7 internal & external gill openings 4. Vertebral column is notochord ; becomes supported by calcified vertebrae

Modern Cartilagenous Fishes

5. Pectoral & pelvic fins are supported internally by a girdle skeleton ; externally by rays (lepidotrichia) of flexible connective tissue 6. Basal skeleton on ♂ anal fins – claspers (paired copulatory organs) 7. Most have covering of small placoid scales (dermal denticles)

Modern Cartilagenous Fishes

S-U-C-C-E-S-S

Success due to adaptive characteristics: 1) buoyancy 2) respiration 3) external covering 4) feeding 5) movement 6) sensory systems 7) osmoregulation 8) reproduction

Buoyancy

-no swimbladder -combination of methods to reduce density Cartilage less dense than bone (1.1 vs 2.0) Large, oil-filled liver (0.8) (water 1.0) Hydrodynamic lift from heterocercal tail & pectoral fins

Respiration

3 basic means of respiration 1) Two-pump system (like teleosts) – pump O 2 water across gills in slow-moving, bottom oriented sharks 2) Ram ventilation – push water across gills during swimming; fast-moving sharks 3) Spiracles – used to bring water across gills; small round opening precede gills on lateral sides of head – on top of head in rays – almost absent in pelagic sharks

Spiracle

small round opening precede gills on lateral sides of head

External Covering

All have placoid scales Rays – few rows on back; sometimes modified into spines Sharks – skin overlapping into lightweight, protective coat fast-swimming sharks have channels between scales to minimize turbulence

External Covering

Slow moving sharks more “armored” dorsal spines Rays – barb/sting Skates – denticles Teeth are modified placoid scales

Feeding

Most are specialized predators – teeth dictate type of feeding Triangular, blade-like teeth – large fish & marine mammals Long, thin, pointed whole fish – Rows of small, sharp teeth – small inverts

Feeding

Flattened, pavement-like teeth – hard-shelled inverts Pointed in front/flattened in back – small inverts (grasping and crushing)

Feeding

Teeth continually shed & replaced; may loose 30,000 in lifetime Jaws loosely attached to cranium - can throw jaws or create suction Large stomach & spiral valve intestine

Movement

Large heterocercal tail Counter-current heat exchangers in pelagic sharks (Lamnidae) Pectoral fins (Mylobatidae) – fly through the water

Movement

Homocercal Heterocercal

Sensory Systems

Odor – olfaction detects dissolved chemicals in the water Low frequency sounds – inner ear and lateral line system; “hear” and “feel” sound waves respectively Ampullae of Lorenzini – pit organs filled with an electrically conductive gel used for detecting weak electrical currents & magnetic fields

Ampullae of Lorenzini

Osmoregulation

Osmoregulators – regulate internal salt concentration approximately 1/3 seawater Utilize large quantities or organic salts (urea & trimethylene oxide) Invade marine, estuarine (brackish) and freshwater systems

Reproduction

Osmoregulatory and and reproductive systems likely evolved simultaneously; Long gestation periods of embryos (in egg or ♀) would not be possible without ability to withstand high concentrations of waste

Reproduction

Unlike most bony fishes (teleosts) put most energy into relatively small number of large, active, young (ecological term?) Wide variety of means to this end: egg laying ( oviparity ) to live bearing ( viviparity ); all stages in-between