Reef Fish Reproductive Biology

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Biology of Reef Fishes.

• Reproduction & mating systems
• Larval biology & settlement
• Recruitment & population dynamics

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Reef Fish Reproduction

• Fishes have simple gonads, and no external reproductive organs. The gonad is a simple strip of tissue inside the body.
• High fecundity - large numbers of very small eggs are produced – 100’s to 1000’s per female per spawning event.
• Fishes breed mainly in the warm months October - February
• Spawning usually takes place early morning & evening.
• Many species aggregate in groups to spawn at reef passes and points of high current flow
• Eggs are fertilised externally, and there is no parental care in most taxa after spawning.
• Exceptions: Triggerfishes (Balistidae) and Damselfishes (Pomacentridae) lay eggs in benthic nests, which are guarded by the adults.
• Cardinalfishes (Apogonidae) are male mouth-brooders *

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Typical Reef Fish Spawning Locations

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Reef Fish Spawning Aggregation

(Acanthuridae – Surgeonfishes)

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Odonus niger

Fishes with Benthic Nests – Triggerfish

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Fishes with Benthic Nests - Damselfish

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Mouth-Brooding Cardinalfishes (Apogonidae)

Sex Change & Mating systems

• Most reef fishes can change sex during their lives.
• Protogynous hermaphrodites: female → male
• Protandrous hermaphrodites: male → female
• Sex change is an evolutionary adaptation designed to maximise the life-time fecundity of the fish.
• When competition for mates is low, it is best to be male when small and change to a female later.
• However, when fishes spawn in groups, competition is high, and in this case it is best to be female when small and a dominant male when large.
• Some groups (eg. Gobies) can change back again - this is called bi-directional sex change. *

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Sex Change & Mating systems

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Sex Change & Mating systems

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No ♂- ♂competition

Protandrous hermaphrodites

Active ♂- ♂ competition

Protogynous hermaphrodites

Amphiprion - Anemonefishes

Labridae - Wrasses

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Scaridae - Parrot-fishes - partially protogynous

Chlorurus sordidus T.P. →

↓ Chlorurus sordidus I.P.

Juvenile

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Anthiinae – Anthiases - protogynous

female

Pseudanthias squamipinnis

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Serranidae – Rock Cods - Protogynous

Cephalopholis cyanostigma →

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Gobiidae – Gobies – bi-directional

Gobiodon okinawae

Larval Biology

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• After hatching, reef fishes enter an obligate larval stage which usually lasts for 1 - 4 weeks.
• They inhabit the pelagic environment during this time, which is the deep ocean water surrounding the reefs.
• They are well adapted to a pelagic life, with transparent colouration, and large spines to deter predators. The larvae of most species look very different from the adults they will become.
• Due to predation and starvation, > 90% of larvae perish during the pelagic phase.
• Surviving larvae may move some distance from their natal reef, due to currents and directional swimming.
• However, recent research has shown that some larvae also have the ability to stay near the natal reef, in gyres and current systems. *

Larval Biology

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The Pelagic Environment

The Reef Environment

Larval Fishes at hatching

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Single larval fish with yolk-sac

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Stages of Larval Development

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Naso Surgeonfish

Modelling of larval dispersal

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• Advances in computing power allow the realistic modelling of larval dispersal on the GBR, allowing for oceanographic features and larval swimming behaviour.
• The models predict that most larvae will be advected away from their natal reef.
• However, the degree of self-replenishment varies considerably from reef to reef *.

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Modelling of larval dispersal – Coral Trout

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Modelling of larval dispersal – Example Coral Trout

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• Dr David Willliamson and colleagues have studied Bar-cheek coral trout (Plectropomus maculatus) at the Keppel Islands (SE QLD) for over 15 years.
• Important fishery species. Key Question – do Green Zones (MPA’s) really work?
• Do they contain more fish than Blue Zones (areas open to fishing), and do the Green Zones export larvae to the Blue Zones?
• The team used barium tagging of otoliths and genetic parentage analysis to identify fate of larvae, as well as extensive modelling and field surveys.
• (show quicktime movie /img/Middle Is_P.mac_broad scale.mpg) *

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Study Sites – Keppel Islands

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Effectiveness of MPA’s

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Otolith Structure

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Modelling of Coral Trout larval dispersal

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Coral Trout - Larval dispersal

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Green to Blue - 83%

Green to Green - 14%

Coral Trout – Larval export

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Green Zones

• 28% reef area
• Export 83% of larvae

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Blue Zones

• Received 57% green larvae

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• MPA’s Do Work!

Coral Trout - MPA effectiveness Palms & Whitsundays

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1983-84 – All Blue

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1987 – Rezoning

Some Blue –> Green

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2000 – all resurveyed

Settlement

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• At the end of the pelagic phase, larval fishes metamorphose into a pre-settlement stage, which is similar to the adult form.
• Pre-settlement larvae are active swimmers, and can gather into groups in the open water, and orientate toward reefs.
• Settlement always occurs at night, and often around the new moon when there is little light for nocturnal predators.
• Most reef fish larvae are very specific in their choice of settlement sites, at a number of spatial scales:
• Reef locations across the continental shelf
• Habitat zones within reefs (eg. front, crest, lagoon, etc)
• Microhabitats within zones (eg. plate corals, rubble, algae, etc) *

Pre-settlement larval Acanthurid (surgeonfish)

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Pre-settlement larval Lutjanid (snapper)

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Cross-shelf settlement sites

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Inshore fringing Reefs

Mid-Shelf Reefs

Outer-Shelf Reefs

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Micro-habitat settlement sites

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eg. Staghorn Corals

Micro-habitat settlement sites

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Reef - sand border

Reef fish juveniles at settlement

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Chaetodontid (Butterflyfish)

Labrid (Wrasse)

The Reef Fish Life Cycle

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Recruitment

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• Recruitment = the input of juvenile fishes to the observed reef-based population.
• On the GBR, most settlement and recruitment occurs between October and February.
• Within these “summer” months, recruitment can be very patchy in space and time.
• For a given species at a given reef, there will usually be one large “pulse” of recruitment each summer.
• During the pulse, more than half the recruits for the whole year may arrive on the reef over only 2-3 nights. *

Recruitment Dynamics

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Population Dynamics

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• What determines the size of a population of fishes on a coral reef?
• Immigration and recruitment can increase population size, while mortality and emigration can cause decreases.
• Immigration → time of year, fish species, habitat use, etc.
• Recruitment → larval survival, currents, predation at settlement
• Mortality → Predators, disease
• Emigration → Competition, maturation, habitat selection
• In the 1980’s and early 90’s, recruitment was thought to be the most significant factor. Recent research emphasises plurality.
• All these factors affect population size from time to time, depending on the reef and the fish species. So there is no easy formula to estimate temporal variation in numbers of reef fishes. *

Population Dynamics - example

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• Shima (Ecology 2001) studied the effects of habitat availability, larval supply, and post settlement mortality on the population size of the 6 bar wrasse in French Polynesia.
• Larvae are brought into the lagoon over the reef crest, and prefer to settle in Stegastes damselfish farms.

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6-bar wrasse: Thalassoma hardwicke

Population Dynamics - example

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• Shima (Ecology 2001) studied the effects of habitat availability, larval supply, and post settlement mortality on the population size of the 6 bar wrasse in French Polynesia.
• Larvae are brought into the lagoon over the reef crest, and prefer to settle in Stegastes damselfish farms.

• Larval settlement declines with distance from the crest, but quality of damselfish territory does as well!
• Manipulative experiments and ANCOVA were used to separate these effects. Both were significant
• Post-settlement mortality was also manipulated and shown to be density-dependant – fish died quicker if they were in dense groups.
• Thus, a suite of interacting factors was affecting recruitment of this species to the lagoon – larval density, settlement habitat, and post-settlement mortality.
• Obviously, another set of factors would influence the final density of adult fishes.

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6-bar wrasse example

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6-bar wrasse example

Dynamics of open and closed populations

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• An OPEN population is one where local adult reproductive output (eggs, larvae) is de-coupled from local recruitment.
• Marine populations are generally open because most larvae are moved away from their natal site and eventually settle elsewhere.
• A CLOSED population is one where local reproductive output IS coupled to local recruitment (eg. Goanna’s on Lizard Island). Most terrestrial vertebrates have closed populations.
• Open populations are characterised by greater temporal variation in population size, and a higher frequency of local extinctions and re-colonisations.
• Clearly, the scale at which the population is considered has important implications (100m versus 10km versus 100km) *

Reef Fishes - open populations?

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• Traditionally, reef fish were considered to have open populations: larvae disperse from the natal reef during the pelagic stage.
• Recent research has suggested that some species may be partially self-recruiting – larvae somehow stay in the vicinity of the natal reef and settle back there after metamorphosis.
• Jones et al 1996 tagged the ear bones of 10 million damselfish eggs at Lizard island, and found 15 tagged returning larvae. They estimated 15-60% of larvae may be returning.
• Genetic studies, modelling, and patterns of recruitment to isolated open ocean reefs, also suggest a degree of self-recruitment.
• Given the wide range of larval durations and swimming capabilities amongst reef fishes, it is likely that the proportion of self recruitment will vary between species and reefs. *

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Modelling self recruitment

Status of reefs in the Cairns section according to estimated proportion of larval self-recruitment.

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Blue < 10%

Green = 10-20%

Red > 20%

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