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Evaluating the use of ram and suction during prey capture by cichlid fishes

Peter C. Wainwright^*, Lara A. Ferry-Graham, Thomas B. Waltzek, Andrew M. Carroll, C. Darrin Hulsey and Justin R. Grubich

Section of Evolution and Ecology, University of California, 1 Shields Avenue, Davis, CA 95616, USA

View larger version (17K): [in a new window]   Fig. 1. The ram–suction space illustrating the potential combinations of predator movement towards the prey (ram distance) and suction-induced prey movement towards the predator (suction distance) used to close the distance between aquatic predator and prey. This study addresses the occupation of this feeding strategy space by seven species of cichlid fishes.

View larger version (27K): [in a new window]   Fig. 2. Diagrams of the seven study species, illustrating overall patterns of body form. (A) Petenia splendida, (B) Cichla ocellaris, (C) Astronotus ocellatus, (D) Crenicichla geayi, (E) Cichlasoma minckleyi, (F) Heros severus and (G) Cyprichromis leptosoma. See Materials and methods for a discussion of the feeding biology of each species.

View larger version (140K): [in a new window]   Fig. 3. Sample sequences from prey capture sequences captured on video. The four frames analyzed in each sequence are illustrated: onset of slow mouth opening, onset of fast mouth opening, peak gape distance and time of prey capture. Times shown (ms) were calculated relative to the onset of slow mouth opening. The background is a grid of 1cm squares.

View larger version (17K): [in a new window]   Fig. 4. Mean and standard errors for the seven cichlid species in the ram–suction space. Note that species varied more in ram distance than in suction distance and that the latter never exceeded 0.5cm. All variables are calculated from the onset of fast mouth opening (see text for details). Species are as follows: () Petenia splendida; () Cichla ocellaris; () Astronotus ocellatus; () Crenicichla geayi; () Cichlasoma minckleyi; () Heros severus; () Cyprichromis leptosoma. Filled symbols indicate strikes on brine shrimp and open symbols indicate strikes on guppies.

View larger version (21K): [in a new window]   Fig. 5. The effect of prey type on ram distance. Species that feed on larger, more active prey in the wild (i.e. Petenia splendida, Cichla ocellaris, Crenicichla geayi) showed the greatest increase in ram distance when feeding on guppies, while predators of zooplankton and small insects (i.e. Cichlasoma minckleyi, Heros severus, Cyprichromis leptosoma) showed no prey effect or even a reversal of the trend.

View larger version (16K): [in a new window]   Fig. 6. Relationships between (A) ram and (B) suction distance and mouth diameter (gape at tcapture) when feeding on guppies. Values are means for each species in the study. The regression between ram distance (r2=0.72, P=0.016) and gape distance is significant, but that between suction distance and gape distance is not (r2=0.06, P=0.65). Species are as follows: () Petenia splendida; () Cichla ocellaris; () Astronotus ocellatus; () Crenicichla geayi; () Cichlasoma minckleyi; () Heros severus; (e) Cyprichromis leptosoma.

View larger version (10K): [in a new window]   Fig. 7. Scaling of ram distance with fish standard length (SL). The relationship between ram distance and SL is not significant (r2=0.04, P=0.68). The relationship between suction distance and SL (not shown) was also not significant (r2=0.006; P=0.77). Species are as follows: () Petenia splendida; () Cichla ocellaris; () Astronotus ocellatus; () Crenicichla geayi; () Cichlasoma minckleyi; () Heros severus; (e) Cyprichromis leptosoma.

View larger version (23K): [in a new window]   Fig. 8. Samples of the complete ram and suction distance data for three cichlid species: Petenia splendida, Cichla ocellaris and Cyprichromis leptosoma. Different symbols refer to the different individuals in each plot. Filled symbols represent strikes on brine shrimp prey, and open symbols represent strikes on guppy prey. Note that the far right-hand region of the potential ram–suction space is not occupied by any species and that the peak values of suction distance never exceeded 1.0cm.

View larger version (16K): [in a new window]   Fig. 9. Relationship between mean ram distance and mean attack velocity in seven cichlid species feeding on guppies and brine shrimp. Points plotted are the means for each species. Attack velocity was calculated as ram distance divided by time of capture (tcapture; measured from fast mouth opening). Although species differ in tcapture, a significant effect of ram distance on attack velocity for guppy prey (r2=0.76, P=0.011) is caused by variation in ram distance (for brine shrimp, P>0.05). There is no relationship between ram distance and tcapture (P=0.28). Species are as follows: () Petenia splendida; () Cichla ocellaris; () Astronotus ocellatus; () Crenicichla geayi; () Cichlasoma minckleyi; (dtrif;) Heros severus; () Cyprichromis leptosoma. Filled symbols indicate strikes on brine shrimp and open symbols indicate strikes on guppies.

View larger version (20K): [in a new window]   Fig. 10. The relationship between the ram–suction space and the ram–suction index, RSI (Norton and Brainerd, 1993). RSI values are isoclines in this space. Paired values of ram and suction distance along each line give a common value of the RSI. Thus, RSI does not reflect the absolute amounts of ram distance or suction distance, and a given value of the RSI can be formed by any value of ram distance or suction distance paired with the appropriate partner value. These relationships indicate that the RSI will not perform well as a measure of suction- or ram-feeding performance.