It takes two to stay afloat: interplay of morphology and physiological acclimation ensures long-term floating dispersal of the bladderwrack Fucus vesiculosus (Phaeophyceae, Fucales)

Figures & data

Table 1. Environmental conditions of surface water temperature, solar radiation and UV-index during the course of experimentation in 2014.

	June	July	August	September
Temperature (°C)
mean ± SD	14.4 ±1.3	19.6 ± 3.4	21.0 ± 1.9	18.1 ± 0.4
max	22.3	26.1	24.8	19.0
min	11.8	13.4	17.7	17.5
Solar radiation (µmol photons m^–2 s^–1)
mean ± SD	410 ± 280	400 ± 275	390 ± 280	350 ± 260
max	1640	1640	1375	1170
min	120	135	105	100
UV-index
mean ± SD	1.8 ± 0.7	2.2 ± 0.5	1.3 ± 0.5	0.8 ± 0.4
max	3.6	3.8	3.5	3.2
min	0.9	1.5	0.9	0.6

Solar radiation and water temperature were extracted from HOBO data loggers tied to the main lines in the field. UV-index data were obtained from the Seili weather station (http://seili.utu.fi/projects/saa/NOAA.php), with irradiance sensors (Davis Instruments, California, USA) detecting UV-radiation at wavelengths of 290–390 nm. Irradiance data were extracted between 11 am and 2 pm, and were transformed from lux to µmol photons m^–2 s^–1 with the conversion factor 0.085 (https://www.apogeeinstruments.com/conversion-ppfd-to-lux/).

Table 2. Variance components (%) due to the random effects of genetic individual and experimental line with their statistical significance in physiological responses (ETR_max, E_k and α_ETR) of Fucus vesiculosus at the end of the floating period.

	ETRmax			Ek			αETR
Source of variation	Var. (%)	χ^2	p	Var. (%)	χ^2	p	Var. (%)	χ^2	p
Random factors
Genetic individual	0	0.0	0.99	22.1	1.65	0.19	45	5.8	0.015
Experimental line	0	0.03	0.98	0	0.0	1	2.6	0.23	0.63

Fig. 1. Floating time (days) of different genetic individuals of Fucus vesiculosus (n = 14) tethered in their native habitat. Data show genetic individual estimates (mean ± SE, based on n = 3 thalli). Due to the loss of four thalli, the mean of the genetic individuals 1, 7, 8 and 10 are based on n = 2 thalli.

Fig. 2. Scatter plots showing the relationship between floating time and final ETRmax (a) as well as floating time and final Ek (b) of all tethered thalli recovered at the end of experimentation (n = 38, four were lost). Different symbols show the n = 38 thalli with different morphologies. Dots = receptacle and vesicle carriers; triangles = only vesicle carriers; and squares = only receptacle carriers. R² values for the fit of the polynomial regressions are displayed. Bar plot (c) shows the genetic individual estimates (mean of the n = 3 thalli ± SE, n = 14) for the final αETR.

Table 3. Results from the structural equation models.

Model	χ^2	df	p-value	AIC	RMSEA	CFI	R^2
I	2.1	1	0.14	1866	0.17	0.99	0.69
II	2.2	2	0.3	1570	0.05	0.99	0.67
III	2.1	1	0.3	1228	0.05	0.99	0.64

Model I includes the variables: initial biomass, initial number of vesicles, initial number of receptacles, growth rate (wet biomass) and ETR_max. Model II does not include initial biomass but all others: number of receptacles, growth rate (wet biomass) and ETR_max. Model III does not include initial biomass and ETR_max but it includes initial number of vesicles, initial number of receptacles and growth rate (wet biomass). Goodness of fit of the three tested models was evaluated through Root Mean Square Error of Approximation (RMSEA), Comparative Fit Index (CFI) and Akaike Information Criterion (AIC). Degrees of freedom (df) and p-value for the chi-square are provided. The R² refers to the response variable floating time.

Fig. 3. Path model III (see Table 3) showing the best fit. (a) Standardized path coefficients are provided for each one-headed arrow, and correlation coefficient for the two-headed one. The asterisk indicates significant regression weights. The curved arrow indicates significant correlation among the observed variables. (b) Multiple regression scatter plot, where the floating time is plotted against the predicted values. The latter ones are generated by a multiple regression model including the three independent variables: # of receptacles (β ± SE: –0.26 ± 0.8, p < 0.01), # of vesicles (1.21 ± 0.32, p < 0.001), and growth rate (−1.74 ± 0.84, p < 0.05), with an R² of 0.53. The grey area surrounding the regression line represents the confidence interval.

Fig. 4. Colonization of invertebrates with floating time (days). Results are from n = 24 Fucus vesiculosus thalli.