Diet overlap as a primary factor influencing territorial aggression of the dusky damselfish, Stegastes adustus Amanda Rosenblum Department of Biological Sciences California State University, Long Beach

 Small, grayish, laterally compressed fish  Territories < 2 m 2 (Bartels, 1984)  Reef crest near surge zone, < 10 m deep (Cleveland, 1999)  Farm an algal garden  Highly territorial  Aggression usually drives out single invaders, but not groups (Foster, 1985 & 1987)

 Role in community structure ◦ Area of reduced macroalgae growth ◦ Algal garden provides nutrition and shelter for benthis invertebrates ◦ Differential distribution of fish inside/outside territories ◦ Not a keystone species: interactions with Diadema antillarum

 Alternative to model-bottle experiments, serial territories ◦ Only made counts of chases (Thresher, 1976) ◦ Disruption of natural cues (Losey, 1982)  Take into account full diet  Hypothesis: Aggression toward territorial invaders will increase with increasing dietary overlap Model-bottle experiment More overlap Less overlap

Study Site Modified from Bing Maps, © Microsoft Corporation, 2012

 One to two meters deep  Forty-nine mature fish from edge territories  No egg guarding males  Five minute assessment ◦ Territory observed ◦ Resident size recorded  Ten minute data collection ◦ Invader species and behavior S. adustus ignoring Chromis spp. and blenny S. adustus

 Stegastes adustus  Microspathadon chrysurus  Scaridae  Acanthuridae  Abudefduf saxatilis  S. leucostictus  Blennioidei  Haemulidae  Holocentridae  Tetraodontidae  Carangidae  Lutjanidae  Serranidae  Labridae Algae Zoobenthos Nekton Zooplankton Thalassoma bifasciatium

SpeciesFACLB0  IP Dusky1-A, 1-C Chromis spp.3-0 Yellowtail S L Blennioidei1-0 In the water F: fin flare A: approach C: chase L: long chase B: bite 0: Ignore

SpeciesFACLB0  IP Dusky1-A, 1-C2132 Chromis spp Yellowtail S L Blennioidei In the lab Aggression index = (A+C+L+B) / (IP)

SpeciesAlgaeBenthosPlanktonNektonDetritusOther Dusky Sgt. major Yellowtail Labrid Serranid From Raymond, 1965 & 1967

 Most data not normally distributed  Kruskal Wallis one-way ANOVA ◦ Small/large resident  Mann-Whitney U-Test ◦ Small/large invader, small/medium Scarid  Two-sample T-Test ◦ Small/large yellowtail damselfish

 Assess significance of multiple factors on AI  Predictors of AI: ◦ Invader size ◦ Primary diet of invader ◦ Diet overlap ◦ Percent zoobenthos in invader diet ◦ Percent algae in invader diet

Approach Chase Long chase Bite Ignore Invader species or family Decreasing diet overlap

Aggression index of small and large dusky damselfish elicited by small and large invaders.  AI SR = AI LR ◦ Kruskal-Wallis H = 1.19, ◦ P =  AI SI > AI LI ◦ Mann-Whitney W 203 = ◦ P =

 Kruskal-Wallis H 3 = P < 0.001

General linear model: F 3,208 = 21.11, P < 0.001, R 2 = 23.26%

General linear model: F 13,198 = 9.08, P < 0.001, R 2 = 37.23%

General linear model: Resident size + invader size + diet overlap Resident size: F 1, 197 = 4.09, P = Invader size: F 1, 197 = 0.01, P = Diet overlap: F 12, 197 = 8.18, P < R 2 = 38.32% Normality plot of model

 Plants and algae: GLM F 8, 203 = P < R 2 = 32.78%  Zoobenthos: GLM F 13, 198 = 9.37 P < R 2 = 38.09%

1.Residents showed most aggression toward conspecifics ◦ Osório et al, Resident size not significant alone, possibly significant in model including diet overlap ◦ Cleveland, Size of invader not significant ◦ Ebersole (1977): Diet overlap and consumption provide better model than diet overlap alone ◦ Size and diet already related: Small species tend toward herbivory, large species tend to eat nekton S. adustus

4.Diet overlap better predictor than invader size, resident size, primary diet 5.Adding more predictors to diet overlap does not improve model much S. adustus  Basses ◦ Aggressive acts not successful  Conspecifics ◦ Aggression highly individualized Indigo hamlet

S. adustus  AI as alternate to serial territories eg Thresher, 1976  Importance of complete diet as predictor of aggression  Protection of food (algal) resources paramount Algae on Bonaire reef

S. adustus  Role of S. adustus in community structure ( Robertson, 1996; Williams, 1980) ◦ Not just herbivores!  Keystone species and the Diadema dieoff Diadema antillarum Galapagos gregory attacking slate-pencil urchin

S. adustus  Dr. Christine Whitcraft and Dr. Gwen Goodmanlowe  Students of Biology 454/554  Bonnie Ahr, for assistance with data collection  Southern California Tuna Club, for funding  Staff of Little Cayman Research Center CCMI