A comprehensive evaluation of the nutritional value of semiterrestrial isopods, Ligia exotica: a potential new aquatic feed?
Authors: Xu Lele , Li Yongqin , Wang Liyun , Jiang Xiang , Sun Yulin , Zhao Haiyong , Mi Haifeng , Liu Yao , Chen Daohai 
Published: April 2020

BackgroundThe semiterrestrial isopod, Ligia exotica represents one of the oldest documented species introductions of marine organisms and is known as an intermediate form between marine and strictly terrestrial isopods. They are considered to make a significant contribution to nutrient cycling and ecosystem services in the near shore environment and in helping to maintain the biodiversity of the coastal zone. The special biological role and its environmental plasticity contribute to its potential value for academic research. In order to explore the practical value for food & feed of Ligia, this study focused on growth rate under laboratory rearing conditions and detailed analysis of the overall nutrient content of the species in comparison to two other aquatic food media (krill and fish meal). ResultsEvaluation of the growth rate of juveniles demonstrated a weight gain rate of 13,026.76% to70 days after hatching (DAH) and a total specific growth rate of 6.97%, which suggests it is a relatively fast-growing species of the Ligiidae family. Compared with Antarctic krill meal and commercial white fishmeal, Ligia meal is closer to krill in amino acid content and contained 43.45% (wet weight) crude protein. Its essential amino acids content was the lowest (35.21% in total amino acids) but the proportion of flavor amino acids was 49.37%, higher than that of krill meal (38.98%) and fish meal (39.80%). In particular, the content of taurine (9.45 g/100 g) was much higher than that of the other two meals. Amino acid score (AAS) and Chemical score (CS) show that the most restricted amino acids of isopod meal are methionine and cysteine, which are less than half of those of krill meal and fish meal. The extremely unbalanced amino acid composition may affect the absorption and utilization by consumers. In terms of fatty acids, isopod meal contains 8.99% (wet weight) crude fat and more saturated fatty acids (33.66%). The total polyunsaturated fatty acids (PUFA) are only 23.61%, lower than 40.50% of krill meal and 47.27% of fish meal. The Σ n-3 PUFA content is even lower with only 12.12%. A total of 12 vitamins were examined. The VK1, VE, VB2, VB3, VB5 content of isopod meal were significantly higher than those of krill meal and fish meal. Similarly, most of the 11 mineral elements are the highest in isopod meal including the ubiquitous elements of calcium and potassium, and the trace elements ferrum, chromium, and selenium. ConclusionsLigia offers potential as an alternative natural food source especially in aquaculture given the growth rate under culture and the overall nutrient content (as demonstrated by the preliminary trials on cuttlefish) but Ligia collected in most of the field would be deemed unfit for human consumption because of the relatively low nutritional value and heavy metal content exceeding the provided standard. At the same time, Ligia isopods offer some potential to become a crustacean model animal for commercial aquaculture crustaceans for research on physiological and other aspects because of their position in ecological food chain and unique semi-terrestrial characteristics that allow them to be cultured out of seawater. Further study is warranted to elucidate its biological characteristics.