Vertebrate Natural History — Lecture notes, continued…..

Class Cyclostomata – life with a tripartite brain

Advantages over uro- & cephalo- chordates:

1. true cephalization and a tripartite brain housed w/in cranium

2. increased complexity of sensory organs

3. pharyngeal musculature for increased H₂0 velocity (incr. respiration).

4. bone for incr. sensory and activity levels

Characteristics of Class that distinguish from other vertebrates:

1. jaws absent

2. only one pair of appendages present.

Order Myxinoidea (hagfish)

1. > 40 sp., w/ global distribution

2. vertebrae absent

3. exclusively marine, mostly benthic in habit

4. single nasal opening, mouth surrounded by 6 tentacles and housing a tongue covered in keratinized "teeth;" feed on inverts and dead vert. viscera

5. multiple subQ glands secrete mucous as antipredator defense.

6. isoosmotic w/ seawater; multiple blood sinuses, ea. w/ its own "heart."

Order Petromyzontoidea (lampreys)

1. 41 sp., w/ global distribution (fresh or salt H₂0 except tropics & polar seas)

2. most spp. anadromous – ascend streams to spawn, mature in oceans.

3. funnel-shaped mouth lined with keratinized "teeth" and containing protrusible tongue w/ similar covering; oral gland secretes anti-coagulant.

4. 7 pairs of gills for respiration and assist kidneys w/ osmoregulation; single heart with autonomic enervation.

5. Following construction of canal between Lakes Ontario and Erie, colonized all of upper Great Lakes and impacted commercial fishery (1° trout).

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Gnathostomata – life with jaws

Advantages over Cyclostomata:

1. jaws present

2. two sets of paired appendages.

3. development of postorbital process (segregates musculature for jaws & eyes)

4. distinct ducts linking gonads to excretory duct(s).

5. spleen present

6. lateral line system along trunk w/ specialized scales.

Improved gill ventilation conferred improved mobility & predation.

a. jaws arose from 1^st pharyngeal arch (mandibular) – helped suck prey in

b. enlargement of adductor mandibularis muscle allowed holding onto prey with jaws closed whilst water was forced over gills (in pharynx).

Features for improved mobility:

1. more complete vertebrae w/ attached ribs

2. distinction of epaxial and hypaxial musculature

3. myelinated neurons

4. conus arteriosus receives blood from ventricle

5. fins that controlled guiding body in 3-D environment via keratinized or bony fin rays from pectoral and pelvic girdles.

Class Chondrichthyes – distinguishing characteristics from other vertebrates:

1. cartilaginous skeleton

2. intercalary plates along spinal column protect nervous & circulatory system

3. fused halves of pectoral girdle to aid in jaw protrusion

4. swim bladder absent (but liver oil provides some buoyancy)

5. ampullae of Lorenzini are mucous-filled tubes containing sensory cells, and concentrated in snout region for electromagnetic perception of surroundings/prey.

Groupings of Chondrichthyes:

1. Subclass Elasmobranchii –

a) pleurotremate = gills on the sides (typical shark body form, 360 spp.); mostly active carnivores, but a few passive planktivores.

b) hypotremate = gills underneath (typical ray body form, 460 spp.); mostly benthic detritovores, a few pelagic passive planktivores; a few skates can localize electric field to stun prey.

2. Subclass Holocephali – chimeras w/ only 1 gill opening on each side of head; 30 spp.; mostly demersal, some with a venomous dorsal spine.

Success (sp. diversity and relatively large body size) enabled by variable repro. modes – all w/ internal fertilization:

1. females w/ nidimental glands that secrete proteinaceous "shell" around egg

2. ovoviviparity in many spp., but all nourishment from yolk (lecithotrophy)

3. matrotrophic viviparity (mother provisions, but no circulatory exchange).

4. placentotrophic viviparity (nourishment exchange via vascular yolk)

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Class Osteichthyes – life with a bony skeleton

Advantages over Chondrichthyes:

1. skeleton of endochondral bone (replaces cartilage in juveniles)

2. webbing between bony (dermal) fin rays.

3. operculum covering gill arches

4. enlarged, protrusible maxillae (allowed gape-'n'-suck feeding)

5. reduced dermal armor

6. swim bladder as evagination from gut

Subclass Actinopterygii – ray-finned fishes, Å 24,000 sp.

1. bony rays (more or less parallel) supporting fin membrane, increased fin flexibility and therefore mobility

2. hypural bones from up-turned caudal vertebrae into tail formed homocercal tail that increased mobility

3. flexible skull & cheek increased orobranchial chamber volume, thus increased predatory & respiratory ability

(a) Superorder Acipenseriformes – sturgeons & paddlefish; cartilaginous skeleton, freshwater or anadromous, desirable for caviar.

(b) Superorder Neopterygii – bowfins, gars, all other bony fish; many have Weberian apparatus (combined use of swim bladder and inner ear to increase sensitivity to vibration)

Subclass Sarcopterygii – lobe-finned fishes

1. fleshy fins with bony central axis

2. two dorsal fins

3. enlarged head musculature

(a) Superorder Actinistii – Latimeria (coelacanth, a "living fossil"); marine, carnivorous w/ internal fertilization.

(b) Superorder Dipnoi – lungfish; freshwater w/ poor gill function, so evaginations from gut (former swim bladder) modified into single or pair lungs, aestivate to escape drought conditions

When swimming, species from both of these groups move their pectoral and pelvic fins in a manner similar to tetrapod limb movement, suggesting that this Subclass contained the species that gave rise to all tetrapods.

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Fun-filled facets featuring fish

The largest group of fish is w/in Actinopterygii are in Order Perciformes (e.g., perch, bass, snapper, tuna, cichlid, dolphin); these show a wide variety of life history strategies.

1. Marine:

(a) repro. – most spp. w/ ext. fertilizat'n and pelagic eggs & larvae. Why?

i) removal of larval stage from some predator sp.

ii) predator satiation

iii) rapid, wide dispersal

iv) increased nutrient access in highly-productive pelagic zone

v) reduced spp. vulnerability to environmental perturbation

Exceptions:

i) grunion (Leuresthes): females deposit eggs in sand where they are fertilized.

ii) anglerfish (Liophryne): males feed only during larval stages & are then parasitic on female during/following sexual maturation

iii) seahorse (Hippocampus): male protects eggs/larvae in "pouch"

iv) blue wrasse: sequential hermaphroditism from female to male

v) coelacanth (Latimeria): internal fertilization w/ ovoviviparity

(b) biogeography – decreased light penetration @ depth = decreased photosynthesis = decreased food availability = decreased spp. richness

i) epipelagic fish = those in the photic zone

ii) mesopelagic fish often migrate to photic zone to feed but then retreat to depth to lower predation risk and metabolism

iii) bathypelagic fish (> 1000 m deep) rely either on detritus from upper zones or sympatric species for food; eyes, mouth, & teeth rel. larger b/c food is scarce, some use symbiotic bioluminescent lures.

2. Freshwater:

(a) repro. –most spp. produce rel. few eggs & provide parental care @ nest site; b/c swift-moving water can displace larvae from ideal habitat conditions.

Exceptions: i) pupfish (Cyprinodon): satellite male w/ female color sneaks between territorial male and receptive female to fertilize eggs.

ii) bluegill (Lepomis): males establish colonial nests to reduce predation risk

iii) cichlid (Oreochromis): some spp. are mouth-brooders; many spp. w/ parents secreting nutritious mucous on skin, eaten by larvae.

3. Ontogeny:

Otoliths w/in inner ear, used for orintation, are mineralized records of daily environmental conditions that effect fish growth. Can be used to popln biol.

4. Conservation:

Fisheries management is difficult b/c of unpredictable environment where eggs & larvae develop (esp. marine). Fisheries typically in cool waters w/ higher O₂ content (= higher productivity), but areas are being overfished w/out recruitment. Threats also from pollution, habitat destruction, siltation.

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Tetrapoda – the fin-to-limb transition

Why give up aquatic life?

1. increased food availability

2. increased oxygen availability (req’d evolution of cutaneous respirat’n)

3. decreased predation pressure

4. stable climate – recall biogeographical history: ~ 180 mya continents rejoined (enabling radiation across all available land) and mtns. formed (generating isolating barriers).

How does a land animal evolve in water?

• flexible lobed fins facilitated prey capture for ambush predators.
• If living in water w/ low [O₂], swim bladder that maintained buoyancy could also be used as a means of obtaining O₂ (gulping air)

Who got sick of the water first?

Ancestral group to all tetrapods is the Sarcopterygii ("flesh-finned" fish)

: lobed fins

: dorsally oriented eyes on elongate head

: ventrally-projecting ribs

Specifically, Ichthyostega and Acanthostega

: digits in their fins

: partially-ossified articulating vertebral centra

: enamel on teeth

: linked char’s to fish (opercular bone, caudal fin, lateral line)

1^st 200 mya of tetrapod radiation was vast, the stem group referred to as the Labyrinthodonts (b/c of infoldings of enamel on teeth). This group radiated in two directions –

: Reptilomorpha (further discussion later)

: Batrachomorpha, with a more aquatic life style, gave rise to modern groups of amphibians.

In addition to radiation onto land, diversity of early amphibians included several life styles that returned to fully-aquatic existence –

: dorso-ventrally flattened body

: retention of external gills (paedomorphic)

: elongate, flat snout seen in amphibious fish-eaters

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Amphibia —— Life on land (sort of)

Class Amphibia ≈ 4,600 species in three major groups; shared characteristics:

• all carnivorous
• anamniotic — risk of dessication limits repro.
• 3-chambered heart, with cutaneous artery off aorta
• scaleless skin with common components

• stratum corneum — outer-most layer which is dead, & occasionally shed
• mucous glands secrete mucopolysaccharides to facilitate respiration, temperature regulation, water balance, locomotion, & defense (e.g., some spp. aestivate during dry periods by shedding several skin layers and then coating the inside of the "cocoon" with mucous)
• granular glands — secrete alkaloid poison in defence (lower density)
• chromatophores — pigment cells to alter skin color

Order Caudata (salamanders) ≈ 400 species in 9 Familes

General characteristics:

2. elongate bodies, usu. w/ 4 limbs, always w/ tail
3. mostly small size (< 12 cm SVL, < 10 g), except some Cryptobranchidae

Order Anura (frogs) ≈ 4100 species in 18 Families

General characteristics:

• all spp. have limbs, but lack tails (except in 1 sp.)
• modified skeletal characteristics for saltation (jumping)

• elongate hind limbs w/ fused tibia and fibula
• elongate illia of pelvic girdle
• fused posterior vertebrae into urostyle
• ribs reduced or absent
• flexible pectoral girdle (cartilagenous sternum) to cushions landing

Order Gymnophiona ≈ 100 species in 5 Familes; pan-tropical distribution

General Characteristics:

• adaptations for fossorial/benthic life

1. Limbless, elongate, externally-segmented body.
2. Vestigial eyes and left lung
3. Dermal scales usu. present (co-ossification of skin to bone).
4. Specialized tentacle for chemoreception between nostrils and eyes.

• Reproductive modes

1. < 50% oviparous, with parental care of rel. large clutch
2. 50 % viviparous with clutch size < 10.

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When is a frog not a frog? — Amphibian diversity

An evolutionary look at the diversity of natural history traits

Order Caudata

2. repro. mode — primitive = ext. fert.; derived = int. fert. involving complex courtship behavior called amplexus (male grasping behind forelimbs of female) & male deposition of spermatophore that is collected by female
3. breeding habitat — primitive = lentic water; derived = lotic water
4. repro. output — primitive = many small eggs; derived = few large eggs w/ guarding behavior, sometimes on land (viviparity in only 1 Genus)
5. feeding mechanism — primitive = suction feeding in aquatic spp.; derived = projectile tongue in terrestrial spp. (esp. Plethodontidae: fleshy tongue protruded via squeezing of protractors around hyoid bone and simultaneous contraction of retractor muscle.
6. paedomorphosis — retention of jr. char’s. into adulthood, esp. common in primitive families
7. elongation and limb & lung loss — elongation always precedes loss; lung(s) lost to decrease frictional dragtypically seen in fully-aquatic spp. in well-aerated streams

Order Anura

• Mode of locomotion —

• Saltation enabled by 5 folding parts: pelvic girdle, femur, tibiofibula, tarsal bones, phlanges. Muscles on these parts are larger closest to body, and are not antagonistic. Motion is symmetrical and mid-jump body posture is aerodynamic. Intraspp. variation due to physical condition, temp. Smaller spp. also jump relatively farther than large spp.; but ability also depends on natural history — burrowing < aquatic < arboreal < grass/reed frogs (e.g., Ranidae).
• Swimming facilitated by synchronous movement of hind limbs, dorsoventrally-flattened hydrodynamic bodybody, mucous coat.
• Arboreal movement facilitated by expanded sticky toe pads, moist skin
• Fossorial movement facilitated by short strong hind limbs w/ "spades" on posterior margin of feet.

• Mode of reproduction —

• Resonating chamber for voice formed as 2° sex char.; calls are specific to spp. and can be affected by temp.
• Male-male combat common, winner mates while in amplexus
• Primitive = lay-‘n’-leave in water, typically attached to substrate; derived = laying out of water, direct development, or parental care.

Mode of feeding —

• 1° insectivorous
• Obtain prey by tongue-flipping while lunging at prey; several rows of teeth to secure captured prey.

Order Gymnophiona —General life history

1. Spin-&-shear feeding on invert. prey
2. If viviparous, 1^st 3^dr of development fueled by yolk, remaining period by "uterine milk" secreted in female oviduct; when born, young may be up to 60% of female’s body length (she must feed during gestation).

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Amazing anecdotes about amphibians

Interesting form & function

• Salamander respiration : Cryptobranchidae — retain gill slits, but use lungs; Proteidae & Sirenidae — retain external gills and use lungs; Plethodontidae — lungless
• Salamander appearance : Ambystomatidae — paedomorphosis in some spp., or in some populations w/in spp.; Plethodontidae — cave dwelling in some spp. has led to eye and pigment reduction/loss.
• Frog appearance : tailed frog w/ internal fertilization; spade-foot toads w/ enlarged tubercle on hind feet to burrow into soil; flying frogs w/ large webbed feet rel. to body size; Goliath frog up to 3 ft. SVL

Interesting reproductive strategies

• Pacific giant salamander w/ internal fert., individually-laid eggs that take up to 9 mo’s. to hatch, & aquatic larval stage of up to 6 yrs.
• Midwife toad males wrap strands of fertilized eggs around waist, carry them for weeks, & deposit them into water when ready to hatch into larvae; Suriname toad pairs swim in loops while in amplexus such that fertilized egg rolls onto back of female where it embeds under layer of skin for direct development; Darwin’s frog males guard eggs by placing them in vocal sac; max. clutch size in Ranidae = 47,840 eggs (bullfrog)

Interesting behaviors

• Several frog spp. w/ satellite males that do not call around breeding ponds, but intercept females as they arrive (survivorship advantage in areas w/ high predation?)

Interesting anti-predatory adaptations

• Live where you cannot be found (e.g., fossorial caecilians, cave salamander)
• Cryptic coloration and color change (e.g., gray treefrog)
• Rapid change of habitat (land—air—water) in frogs
• Aposematic (warning) coloration in many spp. confers presence of toxins.
• Ünken reflex in salamanders and frogs exposes contrast-colored ventral surface to predators
• Poison-dart frogs w/ derived skin toxins (alkaloids) that are lethal

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