Astaxanthin derivatives of the general formula I wherein R is in each case a group --NH--CH(R.sup.1)--COOR.sup.2, --OR.sup.3 or --(Y).sub.n--Z and R.sup.1, R.sup.2, R.sup.3, Y, Z and n are significances given in detail in the description, are novel compounds with improved stability during extrusion at the elevated temperatures as required in feed manufacture and during the storage of the manufactured feed and which accordingly are useful as pigmenting carotinoids for feed for aquatic animals. The derivatives are produced by reacting astaxanthin with the pertinent acid RCOOH as such or as its acid chloride RCOCl or acid anhydride (RCO).sub.2O, or, in the cases where R signifies a group --NH--CH(R.sup.1)--COOR.sup.2, with the appropriate N-carbonyl-amino acid ester of the formula OCNCH(R.sup.1)COOR.sup.2. In particular, astaxanthin disuccinate and astaxanthin dimethyldisuccinate are disclosed. The invention also concerns a formulation containing such an astaxanthin derivative as the pigmenting carotenoid for use in a feed for aquatic animals, a process for producing a such a formulation by dissolving the astaxanthin derivative in a plant or vegetable oil or fat, or in an organic solvent, or in a mixture of both a plant or vegetable oil or fat and an organic solvent, emulsifying the solution with an aqueous solution of a protective colloid, at least partially removing the solvent and water to afford a concentrated emulsion, and spray-drying the concentrated emulsion to finally produce a formulation suitable for incorporation in a feed for aquatic animals, and a feed for aquatic animals containing such a pigmenting carotenoid.