Introduction In aquaculture, marine specimens from lower trophic levels have the potential to serve as alternative food source for farmed fish and invertebrates, replacing traditional meal and fish oils. Marine amphipods are a natural food source for many marine fish and thus became of increasing interest for feeding fish in aquaculture (Alberts-Hubatsch et al. 2019). In the amphipod Gammarus locusta, diet can heavily impact the nutritional composition such as fatty acid profiles. This low-trophic species also produces other compounds valuable for feeding in aquaculture such as pigments, i.e. astaxanthin. This study investigated the impact of feed type on the pigment profile and content of G. locusta in relation to changes in feed type. Material and Methods Freshly hatched juvenile Gammarus locusta were extracted from a steady culture and raised on Fucus spp. for two weeks to produce a baseline in feeding status. Four treatments in quadruplicates were designed to reflect successive changes in feeding between a marine control diet, consisting mainly of dried Fucus serratus, and a terrestrial diet consisting of dried leaves of the carrot Daucus carota. The specimens in the first treatment (A) were exclusively fed with dried algae for 28 days, the second treatment (21/7) was fed with algae for 21 days, followed by 7 days of feeding with carrot leaves, the third treatment (14/14) were fed algae for 14 days, followed by carrot leaves for 14 days and the fourth treatment (C) was exclusively fed with carrot leaves. Feeding was carried out in excess twice a week, remaining feed was removed from the containers and replaced with fresh feed. At the end of the experiment three individuals of each replicate were pooled and stored at - 80°C for pigment analysis. Pooled samples as well as triplicate samples of the diets were then analyzed on a Hitachi La Chrome Elite™ RP-HPLC with diode array detection at 475 nm and astaxanthin as standard. Results We found varying contents of carotenoids in the four treatments with highest amounts of carotenoids in gammarids fed the carrot leaf diet (1.04 ± 0.12 mg/kg dry weight) and lowest content in the Fucus diet (0.47 ± 0.13). In total, six major different carotenoids were detected in the four treatments (Table 1). A shift in pigment profile in correlation to the diet could be observed, with the algae treatment resulting in five major carotenoids shifting to only three major pigments in the carrot treatment. No differences were observed in the astaxanthin content between the treatments (p ≥ 0.05, F(3, 17) = 0.8996; p = 0.4659), but distinct differences were observed in lutein content (p ≤ 0.05, F(3, 17) = 7.562; p = 0.003), with the gammarids fed carrot leaves showing highest levels of lutein (0.73 ± 0.10) and specimens from the Fucus diet having lowest amounts. Discussion The pigment profile of Gammarus locusta was strongly affected by the pigment profile of the feed type. The pigment profile completely changed within 28 days, with intermediate profiles reflecting both diets. Astaxanthin content was surprisingly highest in the intermediate treatment, whereas pure Fucus as well as pure carrot leaves diet did not show differences. The most apparent difference was observed in lutein content with the highest content in the carrot leave treatment and lowest in pure Fucus. Since no cantaxanthin was detected in any samples, we suggest the biosynthetic pathway through zeaxanthin (Gaillard et al. 2004). Both feeds contain similar amounts of β-carotin, which - as a precursor in the biosynthetic pathway - may result in similar amounts of astaxanthin in the gammarid. Even though astaxanthin content did not change with the diet, amount of lutein was six-fold higher than in the control diet. Even though the astaxanthin content did not change between these diets, it is available in biologically significant amounts. In relation to dry mass, the astaxanthin content in our study varied from 20 to 100 mg/kg dry weight. Aquaculture species already benefit from an astaxanthin content as low as 50 mg/kg in their diet (Olsen & Baker, 2006; Wade et al. 2017). The effect of lutein for aquaculture species has been hardly studied, but it is already known, that is has positive effects in the coloration of yellow croaker (Yi et al., 2015. However, effect of lutein on the metabolism of other aquaculture species needs to be investigated. Besides their valuable nutritional composition, marine gammarids can harbour pigment profiles that can benefit aquaculture species References Alberts-Hubatsch, H., Slater, M. J., Beermann, J. (2019): Effect of Diet on Growth, Survival and Fatty Acid Profile of Marine Amphipods – Implications for Utilisation as a Feed Ingredient for Sustainable Aquaculture. Aquaculture Environment Interactions, 11, pp. 481-491 Gaillard, M. et al. (2004): Carotenoids of two freshwater amphipod species (Gammarus pulex and G. roeseli) and their common acanthocephalan parasite Polymorphus minutus, Comparative Biochemistry and Physiology, Part B, 139, pp. 129–136. Olsen, R. E. and Baker, R. T. M. (2006): Lutein does not influence flesh astaxanthin pigmentation in the Atlantic salmon (Salmo salar L.), Aquaculture, 258, pp. 558–564. Wade, N. M., Gabaudan, J. and Glencross, B. D. (2017): A review of carotenoid utilisation and function in crustacean aquaculture, Reviews in Aquaculture, 9(2), pp. 141–156. Yi, X, Li, J., Xu, W., Zhang, W., Mai, K. (2016) Effects of dietary lutein/canthaxanthin ratio on the growth and pigmentation of large yellow croaker Larimichthys croceus, Aquaculture Nutrition, 22, pp. 683-690