Biology and ecology of a sawfish
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Biology and Ecology of a Sawfish. Taxonomy. Phylum: Chordata Class: Chondrichthyans (sharks, rays and chimeras) Subclass: Elasmobranchii Superorder : Batoidea (rays). Order: Pristiformes Family: Pristidae. Biology. Life history strategy K-selected life history strategy

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Taxonomy l.jpg
Taxonomy

  • Phylum: Chordata

  • Class: Chondrichthyans (sharks, rays and chimeras)

  • Subclass: Elasmobranchii

  • Superorder: Batoidea (rays)

  • Order: Pristiformes

  • Family: Pristidae


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Biology

Life history strategy

  • K-selected life history strategy

    • Long-lived

      • 25-30 years

    • Large size

      • Up to 7m

    • Late sexual maturity

      • Approx. 10 years old

    • Low fecundity

      • 8-24 pups


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Distribution and Habitat

  • Global distribution in tropical and sub-tropical regions

  • Distribution is currently fragmented with most populations thought to be depleted

  • Occur in marine, freshwater and coastal habitats

  • Tolerance to freshwater dependent on species

  • Preference for shallow, inshore coastal and freshwater habitats but can occur to at least 80 m depth


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Description

  • Closely related to rays with modified bodies similar to sharks

  • Also similar to and often confused with sawsharks

  • Distinguished by presence of gills on the underside of head


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Behaviour

  • Use rostrum for feeding by

    • Stunning slow moving schooling fish with slashing motion

    • Locating and dislodging prey buried in sand or mud

  • Locate buried prey by using sensors on rostrum

  • Will also eat dead prey and strike at lures

  • Sawfish are nocturnal

    • Sleep in the day

    • Hunt at night

  • Sawfish are top order predators

  • Actively seek out prey items such as

    • Fish

    • Crustaceans

    • Squid


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Anatomy

  • Rostrum can also be used to;

    • rake through sediment to find prey

    • Stun prey with slashing motion

    • Protect from predators

  • Also used to distinguish species

    • Number of teeth

    • Shape of rostrum

Rostrum

  • Most distinguishing feature

  • Long flat and saw-like

  • Made of cartilage

  • Contains pairs of teeth on sides

    • Modified scales

  • Used to detect and catch prey using motion and electro-sensors

    • Allow detection from long distance and under sediment


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Anatomy

Eyes

  • On top of head so can see when partially buried

  • Eyesight good but often not useful due to murky habitat

  • Make up for this with sensitive sense organs in rostrum

Spiracle

  • Located behind the eye

  • Used to take in water when lying on the bottom


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Anatomy

Fins

  • The sawfish has 4 different types of fins all with different uses

    Pectoral

  • Pair of pectoral fins located behind the head

  • Used to lift fish when swimming

  • Helps with steering

  • Consists of cartilage spread out in fan shape

    • Does not reach edge of fin

    • Makes fin pliable increasing manoeuvrability

Dorsal

  • Dorsal means back or upper surface

  • 2 dorsal fins

  • Used for stabilisation

    Pelvic

  • Pair of pelvic fins located on side behind the pectoral fins

  • Used for stabilisation while swimming

    Caudal

  • Also known as the tail fin

  • Shape differs between species

  • Used to generate power and speed in swimming


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Anatomy

Dorsal

  • Dorsal means back or upper surface

  • 2 dorsal fins

  • Used for stabilisation

Pelvic

Pair of pelvic fins located on side behind the pectoral fins

Used for stabilisation while swimming

Caudal

Also known as the tail fin

Shape differs between species

Used to generate power and speed in swimming


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Anatomy

Internal Anatomy

  • Sawfish don’t contain swim bladder for buoyancy

  • Instead have an oil filled liver like sharks and rays

Underside of Sawfish

  • Like rays the flat ventral or underside of sawfish contain

    • Nostrils (nares)

    • Mouth

    • Gill slits

  • Jaw contains thousands of dome shaped teeth

    • Used to crush small fish and crustaceans e.g. Crabs and shrimp


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Reproductive Biology

  • Reproduce through internal fertilisation

    • Male contain claspers

      • Modification of pelvic fin

      • Used to insert sperm into female

  • Ovoviviparous

    • Eggs are fertilised and grow within mother

    • Hatch within womb and mother gives birth to fully developed pups


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Reproductive Biology

  • Average of 8 pups are born at around 65-90cm

  • Gestation period (fertilisation to birth time) is approx. 5 months

  • Number and size of pups increase with maturity of birthing mother

  • Pups born with a toothed rostrum which is covered by a enclosed membrane designed to protect mother from teeth

  • Growth rate

    • Rapid in first year of life

    • Slows considerably as maturity reached



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  • Saw fish are threatened throughout the world

  • All Australian species are on the IUCN Redlist of threatened species as critically endangered

  • Because they prefer an inshore benthic habitat for breeding ,the juveniles are vulnerable to human interference

  • They are also known to actively seek out dead baits or strike at lures making them vulnerable to recreational fishing


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Hunting

  • Sawfish are hunted for their

    • Rostrums

    • Fins

    • Flesh

    • Other body parts

  • These are sold for food and use in traditional Asian medicines

  • Rostrums are sold as valuable souvenirs, trophies or curios

  • Sawfish may also be collected as live specimens for aquariums


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Habitat modification and pollution

  • Loss of soft bottom feeding and breeding habitats also poses a threat

Building of weirs and barrages can restrict movement of sawfish in rivers

  • Other problems include increased pollution from:

    • Land clearing - increased sediment runoff

    • Use of fertilisers - increased nutrient runoff

    • Use of pesticides - reduced health and death of organisms

    • Urban and industrial development


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Entanglement in fishing nets

  • Sawfish are vulnerable to entanglement as the rostrum is easily entangled in nets and fishing line

  • The sawfish thrashes around to get free causing further tangling

  • When unable to swim the sawfish is unable to move water over gills and may drown

  • Entanglement may occur in

    • Trawling nets

    • Shark nets

    • Recreational fishing line


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