**8. Pisces (Fish)**

Many fish accumulate carotenoids in their integuments and gonads. On the other hand, Salmonidae fish peculiarly accumulate astaxanthin (**8**) in muscle. Except for catfish, carotenoids in the integuments of fish exist in an esterified form. 

Astaxanthin (**8**) is widely distributed in both marine and fresh water fish. Cyprinidae fish, which inhabit fresh water, can synthesize (3S,3ȝS)-astaxanthin (**8a**) from zeaxanthin (**70**) by oxidative metabolic conversion (Figure 13). On the other hand, Perciformes and Salmonidae fish cannot synthesize astaxanthin from other carotenoids [6,7,36]. Therefore, astaxanthin present in these fish originates from dietary crustacean zooplankton. Astaxanthin in these marine fish comprises three optical isomers. Perciformes and Salmoidae fish can convert astaxanthin to  zeaxanthin [36,37]. Therefore, zeaxanthin in these fish also exists as three optical isomers [38]. 

**Figure 13.** Metabolism of zeaxanthin in Cyprinidae and astaxanthin in Salmonidae

fish. 

Tunaxanthin (**71**) is widely distributed in fish belonging to Perciformes. The bright yellow color in the fins and skin of marine fish is due to the presence of tunaxanthin. Feeding experiments involving red sea bream and yellow tail revealed that tunaxanthin (**71**) was metabolized from astaxanthin (**8**) 

via zeaxanthin, as shown in Figure 14 [7,36]. Carotenoids with a 3-oxo-Ή-end group such as 

<sup>Ή</sup>,<sup>Ή</sup>-carotene-3,3<sup>ȝ</sup>-dione (**72**) [37] are key intermediates in this metabolic conversion. 

> **Figure 14.** Metabolism of astaxanthin in Perciformes fish.

Unique apocarotenoids, micropteroxanthins (**<sup>73</sup>**–**<sup>76</sup>**), were reported from the integuments of the black bass *Micropterus salmoides* [39]*.* They were assumed to be corresponding oxidative cleavage products of tunaxanthin, lutein, and alloxanthin. 

Since 2000, there are a few reports on new structures of carotenoids from fish (Figure 15). Carotenoids with a 3,6-dihydroxy-Ή-end group, salmoxanthin (**77**), deepoxysalmoxanthin (**78**) 

(from the salmon *Oncorhynchus keta*) [40], and gobiusxanthin (**79**) (from the freshwater goby 

*Rhinogobius brunneus*) [41], were isolated. A series of carotenoids with a 7,8-dihydroand/or 

7,8,7<sup>ȝ</sup>,8<sup>ȝ</sup>-tetrahydro polyene chain were isolated from the integuments and eggs of the Japanese common catfish *Silurus asotus* [42]. Recently, new carotenoids, 7<sup>ȝ</sup>,8<sup>ȝ</sup>,9<sup>ȝ</sup>,10<sup>ȝ</sup>tetrahydro-Ά-cryptoxanthin (**80**), 

<sup>7</sup><sup>ȝ</sup>,8<sup>ȝ</sup>-dihydrodiatoxanthin (**81**), and (3*S*,6*S*,6<sup>ȝ</sup>*S*)-Ή-cryptoxanthin (**82**), were isolated from the integuments and gonads of the Japanese common catfish as minor carotenoids [43]. 

> **Figure 15.** New carotenoids from fish.

*Mar. Drugs* **2011**, *9*, 278–293 
