The role of body surface area in quantity discrimination in angelfish (Pterophyllum scalare)

Abstract

Although some fish species have been shown to be able to discriminate between two groups (shoals) of conspecifics differing in the number of members, most studies have not controlled for continuous variables that covary with number. Previously, using angelfish (Pterophyllum scalare) we started the systematic analysis of the potential influence of such continuous variables, and found that they play different roles in shoal discrimination depending on whether large (≥ 4 fish) or small (<4 fish) shoals were contrasted. Here, we examine the potential role of the overall body surface area of stimulus fish in shoal preference, a prominent variable not yet examined in angelfish. We report that both when numerically large (5 versus 10 fish) and when small (2 versus 3 fish) shoals were contrasted, angelfish were unable to discriminate the numerically different shoals as long as the surface area of the contrasted shoals was equated. Thus, we conclude that body surface may be an important continuous variable in shoal discrimination. This conclusion was further supported by the analysis of preference when shoals of the same numerical size but different body surface area were contrasted. We found subjects to spend significantly more time close to the shoals with the greater overall surface area. Last, we conducted an experiment in which we simultaneously controlled a set of continuous variables, including overall surface area, and found angelfish to use the number of shoal members as a cue only in large shoal contrasts but not in small shoal contrasts. This result suggests the potential existence of different processing systems for large and small numbers in fish.

Figure 1. One of the experimental angelfish (Pterophyllum scalare).

Figure 2. The experimental apparatus.

(A) Diagram of the experimental apparatus showing the central test tank and the two stimulus tanks at each end of the test tank. Removable opaque white dividers were used to separate a 10-cm compartment close to the test tank, where the stimulus shoals were presented to the test fish. Opaque white barriers (B) were used to visually isolate the two stimulus tanks (containing the stimulus shoals) from the test tank. These barriers were removed when preference tests commenced. The time test fish spent within 10 cm of the stimulus shoals (preference zones) was recorded. (B) The test tank and the stimulus compartments. Diagram showing the test tank and the two stimulus compartments at each side. To simultaneously control for several continuous variables, the stimulus compartments were divided into 10 identical sectors by transparent Plexiglas partitions and each fish of the stimulus shoals was individually placed into each of the adjacent sectors. An example of 5 vs. 10 fish contrast is shown (Exp. 1). When shoals of 2 vs. 3 fish were contrasted (Exp. 2), fish were placed into the adjacent central sectors.

Figure 3. Results of Experiment 1.

Proportion of time (preference index, Mean ± SEM) test fish spent in close proximity to the stimulus shoals. Values above 0.5 indicate a preference for the more numerous shoal of stimulus fish or a preference for the stimulus shoal with the greater overall surface area when the stimulus shoal is of the same numerical size. In the X-axis legend the numbers indicate the number of members of the contrasted shoals. We also illustrate the size of the body surface area: the large font size means that the body surface area of the stimulus fish was large and the smaller font size indicates that the surface area was small. Note that in case of the second condition (second bar) the total surface area of the two contrasted shoals was made similar by increasing the surface area of the individual stimulus fish in the less numerous shoal and decreasing it in the more numerous shoal. Numbers in between bars | | (fourth condition) represent stimulus shoals confined in small sectors of a transparent compartment which was designed to minimize differences in several continuous variables that may have covaried with the sizes of these shoals (see text for details). Significant departure from the null hypothesis of no shoal preference is indicated by asterisks: * P<0.05; ** P<0.01; *** P<0.001.

Figure 4. Results of Experiment 2.

Proportion of time (preference index, Mean ± SEM) test fish spent in close proximity to the stimulus shoals. Values above 0.5 indicate a preference for the more numerous shoal of stimulus fish or a preference for the stimulus shoal with the greater overall surface area when the stimulus shoal is of the same numerical size. In the X-axis legend the numbers indicate the number of members of the contrasted shoals. We also illustrate the size of the body surface area: the large font size means that the body surface area of the stimulus fish was large and the smaller font size indicates that the surface area was small. Note that in case of the second condition (second bar) the total surface area of the two contrasted shoals was made similar by increasing the surface area of the individual stimulus fish in the less numerous shoal and decreasing it in the more numerous shoal. Numbers in between bars | | (fourth condition) represent stimulus shoals confined in small sectors of a transparent compartment which was designed to minimize differences in several continuous variables that may have covaried with the sizes of these shoals (see text for details). Significant departure from the null hypothesis of no shoal preference is indicated by asterisks: *** P<0.001. Note that the difference between figures 3 and 4 is that in the latter the number of fish in the contrasted shoals is small (2 vs. 3).