© Mark Priest
Herbivorous coral reef fishes
• Main consumers of primary production (50 - 100% consumed daily)
• Reduce algal establishment, growth & competition with corals
• Increase availability of coral recruitment substrate
• Structure benthic communities
• Herbivory = key ecological process
- important for reef resilience
 Essential to understand species-specific roles
Acanthuridae (surgeonfishes, unicornfishes, tangs)
• Widespread, common & abundant
• Mostly herbivores (27/41 GBR species)
• Little known of ecological roles of grazer/detritivores
- foraging morphology/strategies
- interspecific grazing relationships
- diet/trophodynamic analyses
-> investigate roles of key grazing surgeonfish spp.
Common Indo-Pacific surgeonfish species
Acanthurus nigrofuscus - Brown surgeonfish
Grazer - Epilithic microalgae (crops)
Ctenochaetus striatus – Lined Bristletooth surgeonfish
Detritivore - Detritus & sediment (brushes)
 Both feed upon epilithic algal matrix
 Grazing impact on algal turfs?
Dense/Older
Objectives:
• Individual impact = bite size/volume x # of bites per day
vs.
• Population impact = mean abundance x individual impact
-> Aquaria experimental trials – bite size/volume & grazing impact
Sparse/Younger
-> Field surveys – abundance, biomass, bite rates
Methods – aquaria experimental trials
-> Algal turf biomass per bite/bite size
-> Grazing impact on algal turfs
Dense/long
Grazer
vs.
n = 9, χ = 16.2cm
Sparse/short
BEFORE & AFTER trial
Mean algal turf biomass (g C/cm-2)
Algal turf biomass per bite
mean biomassbefore – mean biomassafter
no. of bites cm-2
Bite size – measure grazing scars
Detritivore
n = 8, χ = 19.7 cm
Methods - survey sites
Heron Island, southern GBR
Six sites
- 3 windward, 3 leeward
- Deep (12m), Shallow (4m)
Methods – field surveys
Six sites
Herbivorous fish surveys
• 8 x Deep, 8 x Shallow (30 x 5m)
• surgeonfish, parrotfish, rabbitfish
• Benthic photo quadrats, rugosity
• Morning surveys, at high tide
Key surgeonfish spp. bite observations
• 5-min observations
• Mean # bites per day
Number of bites
Grazer
Detritivore
-> Sparse turf preferred
Mean biomass per bite
Grazer
Detritivore
-> Sparse turf preferred
-> Dense = more turf per bite
Aquaria trials –
removed biomass
Grazer
vs.
Detritivore
Sediment removed
Turf removed
BEFORE 
^
*
* p = < 0.001
^ p = < 0.004
^
*
AFTER 
 Detritivore removed more turf than grazer
Individual Grazing Impact
Turf removed (g.C.day-1)
-1
M a ss o f a lg a l tu rf re m o ve d p e r d a y (g C d )
1 .4
= Bite volume (g) × # of bites/day
- sparse turf data
1 .2
1 .0
0 .8
0 .6
0 .4
0 .2
0.07
0.88
0 .0
A . n ig ro fu s c u s
C . s tria tu s
Individual Grazing Impact
Turf removed (g.C.day-1)
-1
M a ss o f a lg a l tu rf re m o ve d p e r d a y (g C d )
1 .4
= Bite volume (g) × # of bites/day
- sparse turf data
1 .2
1 .0
0 .8
0 .6
0 .4
0 .2
0.07
0.88
0 .0
A . n ig ro fu s c u s
= Bite size (m2) × # of bites/day
C . s tria tu s
Area grazed (m2 day-1 )
177.6° gape
112.8° gape
(Purcell & Bellwood 1993)
-1
0.8 ± 0.03 cm2
0.9
2
0.002 ± 0.0003 cm2
Substrate area grazed per day (m d )
1.0
0.9
0.8
0.7
0.003
0.0
A. nigrofuscus
C. striatus
Fish survey data - Herbivorous fish mean biomass (g/m2)
-> Surgeonfish - dominate biomass at shallow depths
Windward
Leeward
Deep
Shallow
Deep
Shallow
Population Grazing Impact
300
-1
-1
Mass of material removed (g.C. ha .day )
Mass removed (g.C.ha-1.day-1)
200
239 g
2.7 g
= Individual impact × mean abundance
0
Mean abundance (individuals/ha)
Size class 15 - 20cm
Sparse turfs
Shallow reef slope (~4m)
A. nigrofuscus
-100
v
-
0
-200
-300
C. striatus
Abundance (individuals/ha)
Summary
• Most abundant surgeonfish species - detritivorous Ctenochaetus spp.
- previously thought to cause little damage to algal turfs
- capacity to remove significant quantities of algal turfs
• Need to quantify Ctenochaetus impact on algal turfs on the reef
 Reconsider role of Ctenochaetus spp. in algal turf dynamics
 Revaluate exclusion of Ctenochaetus from resilience monitoring
Acknowledgements
Volunteers: Julia Lawson, YY Chang, Chris Doropoulos, George Roff, Mark Priest
Heron Island Research Station Staff
Marine Spatial Ecology Lab
ARC Laureate Fellowship Funding (Peter Mumby)
Photo Credits: Mark Priest, www.hawaiifishes.com, www.guamreeflife.com
Marshell, A. Mumby, P.J. (2012) Revisiting the functional roles of the
surgeonfish Acanthurus nigrofuscus and Ctenochaetus striatus.
Online First, Coral Reefs
S u rg e o n fish sp e cie s
S u rg e o n fish sp e cie s
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T o ta l a b u n d a n ce
Most abundant surgeonfish
Leeward Shallow
100
S u rg e o n fish sp e cie s
Windward Shallow
200
200
150
150
100
50
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0
0