Environmental challenge trials induce a biofluorescent response in the green sea urchin Strongylocentrotus droebachiensis

Abstract

Stress in sea urchins leads to high mortality and economic losses in both the environment and aquaculture. The green sea urchin Strongylocentrotus droebachiensis has been documented emitting complex biofluorescence, yet how this responds to external stressors is unknown. Adult sea urchins (n = 210) were divided between control (n = 30) and experimental groups (n = 180), using three transport variables: out of water, in water at elevated temperatures, (warm-water) and in water at seawater temperature (cold-water). Hyperspectral imaging of external fluorescence and fluorospectrometric analysis on coelomic fluid was measured at five intervals (hour 0,3,6,9,12). External green emissions (∼580 nm) responded to all treatments, peaking at h9. External red emissions (∼680-730 nm) in the cold-water remained low until an h9 peak. The warm water increased emissions at each interval, peaking at h9. The out of water gradually increased, with the highest at h12. The coelomic fluid fluorescence (∼680 nm) was low to nonexistent except in warm-water, whose elevated levels suggest that fluorescent emissions are a measurable byproduct of internal adaptation(s) to stress. Early detection of fluorescent emissions (broken spines, lesions) may prevent economic losses. The observed link between fluorescence and the applied stressors provides a baseline for developing non-invasive technology for improving echinoderm welfare.

Fig. 1

Examples of green (~ 560–600 nm) and red (~ 660–750 nm) fluorescence emissions produced by the green sea urchin Strongylocentrous droebachiensis during the environmental challenge trials. The green fluorescence is produced from broken spines and lesions (OW_12_08, CONTROL_07) while red fluorescence is from red exudate (HW_12_05, OW_3_10) produced by exudates from pores surrounding the anus.

Fig. 2

External full spectral range fluorescence emissions (~ 550–800 nm) with the standard deviation (± SD) produced by the green sea urchin Strongylocentrous droebachiensis in response to exposure to three environmental variables (cold water, warm water, out of water) as sampled per a 12-hour period. The control group at time zero is denoted with the blue line. The sea urchins within each group increased their fluorescence emissions but varied in their intensity according to sampling time.

Fig. 3

Box plots documenting the three external fluorescence emissions (full spectral range, green, and red) produced by the green sea urchin Strongylocentrous droebachiensis in response to exposure to three environmental variables (cold water, warm water, out of water) as sampled every 3 h per a 12-hour period (in orange). The control group (n = 30; in blue) from time 0 is added as a reference for each variable group. The sea urchins within each group increased their area under the curve but varied in the intensity of spectra produced by individual sea urchins according to sampling time.

Fig. 4

Fluorescence photon counts per nm of green sea urchin Strongylocentrous droebachiensis coelomic fluid randomly sampled from animals (n = 30) upon arrival at the research facilities (A). Fluorescence emittance of the coelomic fluid was overwhelmingly low in the control animals as can be seen by the clustering along the median in the box plots (B).

Fig. 5

Fluorescence photon counts per nm of coelomic fluid sampled from the green sea urchin Strongylocentrous droebachiensis in response to exposure to three external variables (cold water, warm water, out of water) as sampled per 3-hour interval within a 12-hour sampling window. The fluorescence levels in the cold-water replicates remained consistently low (L), while fluorescence levels in the replicates exposed to warm water showed high responses at hour 6,9, and 12 (M). The replicates kept out of water showed an increase in fluorescence at hour 6, while samples from hour 9 and 12 h remained low (R).

Fig. 6

Box plots of the fluorescence count in the spectral range 650–750 nm of coelomic fluid from cold water (CW), warm water (WW), and out of water shipping (OW). The CW and OW analysis detected fewer outliers and greater clustering along the median in comparison to the warm water samples.

Fig. 7

Green sea urchins Strongylocentrous droebachiensis were collected from the Kvalsund strait of Northern Norway.

Fig. 8

Annotation methodology in the hyperspectral images to analyze the external fluorescence emissions produced by the green sea urchin Strongylocentrous droebachiensis in response to exposure to three environmental variables (cold water, warm water, out of water). The manual annotation area and then region of interest is noted in blue. The region of interest was detected by reducing the manual annotation by 20%.