STRUCTURAL REINFORCEMENT AND BIOCHEMICAL PRIMING: STRATEGIC RESOURCE ALLOCATION IN STARVED GREEN SEA URCHINS Strongylocentrotus droebachiensis FED WITH Saccharina latissima

Introduction

In echinoculture, commercial success is often measured by gonad yield and gross biomass. However, these parameters may overlook the critical physiological transitions occurring during nutritional recovery. Green sea urchins (Strongylocentrotus droebachiensis) frequently face long periods of food scarcity in the wild, which leads to major metabolic shifts. This study explores the "quality of recovery" by investigating how starved individuals allocate resources from a macroalgal diet (Saccharina latissima). The hypothesis was tested that sea urchins prioritize structural stabilization and biochemical "priming" over immediate biomass accumulation.

Materials and Methods

Green sea urchins harvested by hand from Tromsø, Norway, were shipped to Bodø, Norway, and maintained under controlled conditions (7±1 °C) in the laboratory. The animals were divided into two groups: one subjected to long-term starvation and the other fed to satiation with S. latissima. Morphological indices, including fresh weight (FW), test diameter, and gonad index (GI), were recorded (n=10). Biochemical analyses of minerals using ICP-OES focused on the tests (shells) and a combined visceral fraction (intestines and coelomic fluid). "True protein" content was determined by summing proteinogenic amino acid residues using HPLC, excluding the non-proteinogenic amino acid taurine to ensure analytical precision. Statistical differences were assessed using independent samples t-tests or Wilcoxon rank-sum tests at a 5% significance level (P = 0.05) in the statistical software package R (v 4.5.1).

Results and Discussion

This study revealed a sophisticated hierarchy in resource allocation. While the starved group demonstrated remarkable structural resilience by maintaining basic body dimensions without significant skeletal degradation, the kelp-fed group exhibited a profound "biochemical reset".

The fed urchins showed a highly significant increase in the Gonad Index (10.75 ± 2.33% vs. 5.70 ± 2.68%; P < 0.001; Figure 1A). Interestingly, although the total fresh weight did not differ significantly between groups, the internal quality of the tissues underwent a dramatic transformation. In the tests, ash content increased significantly in fed individuals (82.21 ± 3.87 g vs. 79.04 ± 2.70 g; P < 0.05), indicating a strategic investment in skeletal mineralization (Figure 1B). This was accompanied by higher concentrations of matrix-stabilizing amino acids such as glycine and arginine.

In the visceral fraction, a "priming" effect was observed. While the increase in total protein content (141.18 ± 27.50 mg g^-1 DW vs. 118.23 ± 23.66 mg g^-1 DW) narrowly missed the significance threshold (P = 0.061), specific essential amino acids like phenylalanine, leucine, and methionine increased significantly. This suggests that before significant gross growth occurs, the organism optimizes its metabolic machinery and amino acid pools to support future physiological demands.

Conclusion

Our findings demonstrated that green sea urchins employ a strategic rebuilding process during nutritional recovery. Investment is prioritized toward reinforcing the structural integrity of the test and priming the biochemical capacity of the viscera. For the aquaculture industry, these results suggest that traditional morphological indices should be supplemented with biochemical profiling to fully evaluate the efficacy of "roe enhancement" diets and the physiological health of the animals.

Figure 1

Figure 1: Divergent physiological and structural responses of Strongylocentrotus droebachiensis to feeding regime. (A) Gonad Index (%)significantly increased in fed individuals, which illustrates the rapid restoration of energy reserves and reproductive potential upon feeding on Saccharina latissima. (B) Ash content (g) of the sea urchin tests significantly increased in the kelp-fed group that reflects a strategic investment of dietary minerals into the skeletal reinforcement. This highlights that nutritional recovery prioritizes internal structural integrity and biochemical priming over immediate increases in gross body dimensions. Data are means and SDs of 10 individual animals per treatment (n = 10). The asterisk above the boxes represents a statistically significant difference (Student's t-test) between "starved" and "fed" animals. Note: * P ≤ 0.05, ** P ≤ 0.01, *** P ≤ 0.001.

Acknowledgment

This work is part of the project "Ocean Green: Maximizing Economic Value through Restored Kelp Forests and Sustainable Fisheries" (grant 346690), which is funded by The Research Council of Norway.