Global and specific trajectometry indexes
In total, 16 trajectometry indexes were evaluated in A. palmata larvae: 10 global and 6 specific. Irrespective of concentration, Sargassum leachates had negative effects on 8 global and 4 specific trajectometry indexes. Sargassum leachates affected larval performance: larvae in contact with leachates moved slower (hypoactive) and swam shorter distances, decreasing dispersion capacity. Additionally, larvae in contact with leachates became positively geotactic, while larvae in control and stain treatments became negatively geotactic.
Global trajectometry indexes
The displacement-length of larvae in contact with leachates (25%: 75.3 ± 11.0 mm, 50%: 65.3 ± 7.1 mm, and 100%: 74.9 ± 8.3 mm) was 2.5 times lower than larvae in both the control (190.4 ± 16.8 mm) and stain (188.6 ± 20.9 mm) treatments (KW: H(4,244) = 59.04, P < 0.001; Fig. 2A). In contrast, the displacement-time of larvae exposed to leachates (25%: 369.0 ± 23.6 s, 50%: 355.7 ± 22.8 s, and 100%: 355.1 ± 21.0 s) was greater than larvae without leachates (control: 235.1± 16.4 s, and stain: 246.5 ± 20.3 s; ANOVA: F(4,244) = 9.80, P < 0.001; Fig. 2B). Larvae exposed to leachates were three or more times slower with regard to their displacement-speed (25%: 0.21 ± 0.02 mm/s, 50%: 0.19 ± 0.02 mm/s, and 100%: 0.21 ± 0.02 mm/s) compared with larvae in the absence of leachates (control: 0.80 ± 0.05 mm/s, and stain: 0.75 ± 0.06 mm/s; H(4,244) = 144.24, P < 0.001; Fig. 2C).
Global indexes evaluated through five treatments (control, stain, and 25%, 50% and 100% leachates) in Acropora palmata larvae: (A) Mean displacement-length, (B) mean displacement-time, and (C) mean displacement-speed per larva. Letters above the bars indicate significant differences according to post hoc analyses. Error bars represent standard error. See text for more details.
The number of displacements (25% = 5.6 ± 0.5, 50% = 5.1 ± 0.5, and 100% = 4.9 ± 0.5 vs. control = 9.0 ± 0.6, and stain = 8.5 ± 0.7; H(4,244) = 37.70, P < 0.001; Fig. 3A) and the number of times of inactivity (25% = 1.7 ± 0.1, 50% = 1.7 ± 0.1, and 100% = 1.6 ± 0.1 vs. control = 3.3 ± 0.2, and stain = 3.1 ± 0.2; H(4,244) = 68.88, P < 0.001; Fig. 3B) of larvae exposed to leachates were almost half that of with larvae in the absence of leachates. The time of an inactivity interval per larva exposed to leachates (25%: 236.8 ± 23.8 s, 50%: 246.2 ± 23.2 s, and 100%: 255.1 ± 20.5 s) was at least 100 s slower than larva without leachates (control: 364.9 ± 16.4 s, and stain: 353.5 ± 20.3 s; H(4,244) = 32.34, P < 0.001; Fig. 3C).
Global behavior indexes of coral larvae evaluated in five treatments (control, stain, and 25%, 50% and 100% leachates): (A) Mean number of displacements, (B) mean number of times of inactivity, and (C) mean time of an inactivity interval per larva. Letters above the bars indicate significant differences according to post hoc analyses. Error bars represent standard error.
Spiral movement is characterized as a rotation movement on a larva’s own axis either clockwise or counterclockwise; however, this movement was observed with little or without displacement in the tank. Spiral movement responded differentially in two of the four trajectometry indexes evaluated among treatments (Fig. 4). The percentage of larvae with spiral displacement in 25% (82 ± 6%) was double or more than in the control (12 ± 4%), stain (30 ± 8%), and 100% exposure (30 ± 11%) treatments, but 50% leachates (58 ± 7%) was similar to the stain and 25% leachates (H(4,107) = 25.76, P < 0.001; Fig. 4A) treatments. The spiral displacement-time per larva in leachate treatments (25%: 336.4 ± 22.7 s, 50%: 291.9 ± 27.8 s, and 100%: 296.0 ± 30.9 s) was three times greater than in the stain treatment (86.7 ± 14.9 s), while the control (152.0 ± 80.4 s) was similar to the stain and leachate treatments (H(4,107) = 32.06, P < 0.001; Fig. 4C). In contrast, the spiral displacement-length (8.4–34.3 ± 3.1–7.3 mm; H(4,107) = 12.29, P > 0.05; Fig. 4B) and the spiral displacement-speed (0.10–0.20 ± 0.02–0.05 mm/s; H(4,107) = 5.46, P > 0.05; Fig. 4D) per larvae did not different among treatments.
The four trajectometry indexes used to evaluate spiral movement in Acropora palmata larvae exposed to five treatments (control, stain, and 25%, 50% and 100% leachates). (A) Percentage of larvae with spiral displacement. (B) Mean spiral displacement-length per larva. (C) Mean spiral displacement-time per larva. (D) Mean spiral displacement-speed per larva. Letters above the bars indicate significant differences according to post hoc analyses; ns = not significant. Error bars represent standard error.
Specific trajectometry indexes
Displacement-length of larvae at the surface in leachate exposure treatments (25%: 36.5 ± 13.1 mm, 50%: 18.6 ± 5.1 mm, and 100%: 19.4 ± 4.9 mm) was at least three times shorter than larvae without leachates (control: 111.1 ± 15.0 mm, and stain: 127.7 ± 19.5 mm; H(4,89) = 36.77, P < 0.001; Fig. 5A). Displacement-time per larva did not differ among treatments (110.0–206.2 ± 13.7–48.1 s; H(4,89) = 5.07, P > 0.05; Fig. 5B). Displacement-speed per larva exposed to leachates (25%: 0.34 ± 0.08 mm/s, 50%: 0.10 ± 0.02 mm/s, and 100%: 0.17 ± 0.04 mm/s) was at least three times slower than larvae without leachates (control: 1.12 ± 0.10 mm/s, and stain: 1.29 ± 0.11 mm/s; F(4,89) = 57.51, P < 0.001; Fig. 5C).
Specific trajectometry indexes per larva of Acropora palmata swimming at the surface (A–C) and at the bottom (D–F) of the tank exposed to five treatments (control, stain, and 25%, 50% and 100% leachates) (A,D) Mean displacement-length per larva. (B,E) Mean displacement-time per larva. (C,F) Mean displacement-speed per larva. Letters above the bars indicate significant differences according to post hoc analyses; ns = not significant. Error bars represent standard error.
Displacement-length per larva at the bottom of the tank did not differ among treatments (24.5–35.8 ± 5.0–7.3 mm; H(4,168) = 3.73, P > 0.05; Fig. 5D). Displacement-time per larva exposed to leachates (25%: 276.5 ± 27.9 s, 50%: 260.8 ± 25.1 s, and 100%: 208.4 ± 24.3 s) was three times higher than larva in both the control (78.9 ± 10.3 s) and stain (86.0 ± 15.4 s; H(4,168) = 57.69, P < 0.001; Fig. 5E) treatments. Conversely, displacement-speed per larva exposed to leachates (25%: 0.16 ± 0.03 mm/s, 50%: 0.14 ± 0.02 mm/s, and 100%: 0.14 ± 0.03 mm/s) was at least two times slower than larvae in both the control (0.54 ± 0.09 mm/s) and stain (0.40 ± 0.06 mm/s; F(4,168) = 8.17, P < 0.001; Fig. 5F) treatments.
Frequency of behavioral displacements
Overall, A. palmata larvae displayed 25 different types of behavioral displacements, but not all of them occurred in all treatments. The frequency with which each type of behavioral displacement occurred per treatment was assigned to one of three ranks: low (0.00–0.33), medium (0.34–0.66) and high (0.67–∞) (Fig. 6 and Suppl. Table 1). Larvae either swam continuously or in spiral movements, but there were also moments of inactivity (called ‘motionless’). Larvae started and ended displacements in a spiral motion, any motion other than a spiral, or remained motionless (Fig. 6 and Suppl. Table 1). 18 types of larval behavioral displacements were observed in the control treatment (G = 0.9, df = 2, P > 0.05) at different levels of frequency (5 high, 8 medium, and 5 low); 22 types (5 high, 5 medium, and 12 low) in the stain treatment (G = 4.1, df = 2, P > 0.05); 22 types (2 high, 4 medium, and 16 low) in the 25% leachates treatment (G = 14.9, df = 2, P < 0.001); 24 types (2 high, 2 medium, and 20 low) in the 50% leachates treatment (G = 25.5, df = 2, P < 0.001); and 23 types (5 medium, and 18 low) in the 100% leachates treatment (G = 7.7, df = 1, P < 0.001). These results indicate that most of the different types of behavioral displacements observed in larvae exposed to leachates occurred less frequently (belonged to the low rank), while five types of behavioral displacements in the control and the stain treatments occurred quite frequently (belonged to the high rank). Spiral movement was related to 11 of the 25 types of behavioral displacements; most of them belonged to the low frequency rank. Four different spiral behavioral displacements occurred in the control treatment, eight in the stain treatment, nine in the 25% leachates treatment, nine in the 50% leachates treatment, and 10 in the 100% leachates treatment (Fig. 6 and Suppl. Table 1).
Schematic representation in the tank of the frequency of behavioral displacements of Acropora palmata larvae exposed to five treatments (control, stain, and 25%, 50% and 100% leachates). The frequency of each type of displacement per treatment was assigned to one of three ranks: high (0.67–∞), medium (0.34–0.66), and low (0.00–0.33). S = seawater surface; B = bottom of the tank. See Material and Methods for more details about the description of the beginning and the end of each displacement.
Post-observations
During post-observations, all larvae in both the control and the stain treatments were active swimmers, mainly exploring the surface at a high velocity. In contrast, larvae exposed to leachates moved slowly and eventually became motionless, depending on the leachate concentration. Most larvae in contact with leachates became motionless at the end of 20 min: 80% of larvae in the 25% treatment, and 100% of larvae in 50% and 100% leachates.
Control
Throughout the 20 min of observation, all of the larvae were active (swimming quickly) compared to the larvae in the presence of leachates. Larvae mainly swam on the surface and bottom, but they also had the capacity to swim upward, downward, and side to side.
Stain
The activity of the larvae in contact with the food colorant was similar to larvae in the control. During the 20 min of observation, all larvae moved rapidly, primarily at the surface and at the bottom; however, they also moved up, down, and side to side.
25% leachates
During the first 5 min, almost 80% of the larvae were active (moving up and down or from side to side), but they were slower in comparison to the control and stain treatments. Within 5 to 10 min, larval activity decreased (i.e., hypoactive) or stopped (i.e., motionless), and larvae floated (on the surface or in the middle of the water column) or sunk to the bottom. Within 15 min, 80% of the larvae were motionless, mostly on the bottom, but also at the surface or in the middle of water column. In the last 5 min (i.e., 20 min), larvae were again on the bottom, in the middle of the water column or at the surface. 80% of the larvae were motionless and 20% of the larvae displayed extremely low mobility.
50% leachates
During the first 10 min, mobility was low in 60% of the larvae. They were located either close to the bottom or in the middle of water column. During the last 10 min, all larvae were motionless, lying on the bottom or suspended in the middle of the water column, but not at the surface.
100% leachates
During the first minute, all larvae reduced their activity and at the end of 5 min, 70% of larvae were motionless and 30% hypoactive. These larvae moved up and down or erratically. After 5 min, all larvae lost mobility (i.e., completely stopped), and were located at the surface or on the bottom.
Source: Ecology - nature.com
