**1. Introduction**

Since the first structural elucidation of Ά-carotene by Kuhn and Karrer in 1928– 1930, about 750 naturally occurring carotenoids had been reported as of 2004 [1]. Improvements of analytical instruments such as NMR, MS, HPLC, *etc.*, have made it possible to perform the structural elucidation of very minor carotenoids in nature [2–4]. 

Marine animals contain various carotenoids that show structural diversity [3–9]. Among the 750 reported carotenoids found in nature, more than 250 are of marine origin. In particular, allenic carotenoids, except for neoxanthin and its derivatives, and all acetylenic carotenoids originate from marine algae and animals [1]. 

In general, animals do not synthesize carotenoids *de novo*, and so those found in animals are either directly accumulated from food or partly modified through metabolic reactions [5–9], as shown in Figure 1. The major metabolic conversions of carotenoids found in animals are oxidation, reduction, translation of double bonds, oxidative cleavage of double bonds, and cleavage of epoxy bonds. 

Up until 2001, marine animal carotenoids were reviewed by Liaaen-Jensen [5,6], Matsuno [7,8], and Matsuno and Hirao [9]. Since then, there have been no reviews of carotenoids in marine animals. The present review describes progress in the field of carotenoids in marine animals over the last decade. 

## **2. Porifera (Marine Sponges)**

Characteristic carotenoids in marine sponges are shown in Figure 2. Many marine sponges are brilliantly colored due to the presence of carotenoids. Sponges are filter feeders and are frequently associated with symbionts such as microalgae or bacteria [6]. The characteristic carotenoids in sponges are aryl carotenoids such as isorenieratene (**1**), renieratene (**2**), and renierapurpurin (**3**) [6,7]. More than twenty aryl carotenoids have been reported in sponges [1]. Except for sea sponges, aryl carotenoids are found only in green sulfur bacteria [1,6]. Therefore, aryl carotenoids in sponges are assumed to originate from symbiotic bacteria [6,7]. Novel carotenoid sulfates having an acetylenic group, termed bastaxanthins (**4**), were isolated from the sea sponge *Ianthella basta* [1]. Recently, a new acetylenic carotenoid (**5**) was isolated from the marine sponge *Prianos osiros* [10]. Based on the structural similarity, bastaxanthins and compound **5** were assumed to be metabolites of fucoxanthin originating from microalgae. 

**Figure 2.** Characteristic carotenoids in marine sponges. 

## **3. Coelenterata (Sea Anemones)**

Astaxanthin, which originates from dietary zooplankton, was found in some jelly fish. Peridinin, pyrrhoxanthin, and diadinoxanthin were found in some corals [11]. They originate from symbiotic dinoflagellates. Unique nor carotenoids, 2-norastaxnthin (**6**) and actinioerythrin (**7**), have been reported in the sea anemones *Actinia equina* and *Tealia felina* [1] (Figure 3). 

> **Figure 3.** Characteristic carotenoids in sea anemones.

## **4. Mollusca (Mollusks)**

Many chitons are herbivorous and feed on attached algae. Major carotenoids found in chitons are lutein, zeaxanthin, fucoxanthin, and their metabolites [12]. 

Abalone, *Haliotis discus discus*, and turban shell, *Turbo cornutus*, feed on brown and red algae. Carotenoids found in these shells are Ά-carotene, ΅-carotene, zeaxanthin, lutein, and fucoxanthin [11]. 

On the other hand, many sea snails are carnivores. The triton *Charonia sauliae* feeds on starfish. Therefore, astaxanthin (**8**), 7,8-didehydroastaxanthin (**9**), and 7,8,7<sup>ȝ</sup>,8<sup>ȝ</sup>-tetradehydroastaxanthin (**10**), characteristic carotenoids found in starfish, were isolated as major carotenoids in triton. Astaxanthin (**8**), originating from dietary microcrustaceans, was found to be a major carotenoid in the whelk *Buccinum bayani*. Alternatively, *Drupella fragum* preys upon corals. Thus, peridinin and diadinoxanthin are present as major carotenoids in this sea snail [11]. Carotenoids in sea snails well reflect their diet. 

Canthaxanthin (**11**), (3*S*)-adonirubin (**12a**), and (3*S*,3<sup>ȝ</sup>*S*)-astaxanthin (**8a**) were found to be major carotenoids in the spindle shell *Fushinus perplexus* [13]. Furthermore, a series of carotenoids with a 

4-hydroxy-5,6-dihydro-Ά-end group and/or 3,4-dihydroxy-5,6-dihydro-Ά-end (**13**– **15**) were isolated from *Fushinus perplexus* [13] (Figure 4). They were assumed to correspond to reduction metabolites of canthaxanthin (**11**), (3*S*)-adonirubin (**12a**), and (3*S*,3<sup>ȝ</sup>*S*)-astaxanthin (**8a**).

Sea slugs and sea hares also belong to Gastropoda. They are herbivorous and feed on brown and red algae. Several apocarotenoids have been reported in sea slugs and sea hares [1]. A series of 

<sup>8</sup><sup>ȝ</sup>-apocarotenal and 8<sup>ȝ</sup>-apocarotenols derived from Ά-carotene, lutein, and zeaxanthin were found in the sea hare *Aplysia kurodai* [14]. They are oxidative cleavage products of the polyene chain at C-8 in C40 skeletal carotenoids [14]. 

Bivalves (oyster, clam, scallop, mussel, ark shell, *etc.*) contain various carotenoids that show structural diversity [3,6]. Bivalves accumulate carotenoids obtained from their dietary microalgae and modify them through metabolic reactions. Many of the carotenoids present in bivalves are metabolites of fucoxanthin, diatoxanthin, diadinoxanthin, and alloxanthin [3,6], which originate from microalgae. 

> **Figure 4.** Characteristic carotenoids in sea snails.

Oxidative metabolites of diatoxanthin (**16**) and alloxanthin (**17**), such as pectenol (**18**), pectenolone (**19**), 4-hydroxyalloxanthin (**20**), and 4-ketoalloxanthin (**21**), are distributed in scallops and ark shells [3,6,7]. 8ȝ-Apoalloxanthinal (**22**), which is an oxidative cleavage product of alloxanthin, was also found in bivalves [15] (Figure 5).

A novel 3,6-epoxy derivative of diadinoxanthin (**23**), named cycloidadinoxanthin (**24**), was also isolated from the oyster [16] (Figure 5). 

**Figure 5.** Metabolites of diatoxanthin, alloxanthin, and diadinoxanthin in bivalves.

Fucoxanthin (**25**) and its metabolites fucoxanthinol (**26**) and halocynthiaxanthin (**27**) were found to be widely distributed in oysters and clams [3,6,7]. 

Mytiloxanthin (**28**), which has a unique enol hydroxy group at C-8ȝ in the polyene chain and a 

3<sup>ȝ</sup>-hydroxy-6<sup>ȝ</sup>-oxo-Ύ-end group, is a characteristic carotenoid in marine mussels and oysters [6,7]. Furthermore, three mytiloxanthin analogues containing an allenic end group (**29**), a 3,6-epoxy-end group (**30**), and a 3,4-dihydroxy-Ά-end group (**31**) were isolated from the oyster [16,17]. Compound **<sup>29</sup>**, termed allenic mytiloxanthin, was assumed to be a metabolic intermediate from fucoxanthinol to mytiloxanthin. 

Some edible clams have a bright orange or red color due to the presence of carotenoids. Fucoxanthin 3-ester (**32**) and fucoxanthinol 3-ester (**33**) were found to be major carotenoids in *Mactra chinensis* [18], *Ruditapes philippinarum*, and *Meretrix petechialis* [19]*.* Amarouciaxanthin A (**34**) and its ester were also identified as major carotenoids in *Paphia amabills* and *Paphia amabillis* [20]. 

Other metabolites of fucoxanthin, crasssostreaxanthin A (**35**) and crassostreaxanthin B (**36**), were isolated from the Japanese oyster *Crassostrea gigas* [21]. Tode *et al.* demonstrated that crassostreaxanthin B could be converted from halocynthiaxanthin by bio-mimetic chemical reactions [22,23]. Further studies of carotenoids in marine animals revealed that crassostreaxanthin A, crassostreaxanthin B, and their 3-acetates were widely distributed in marine  bivalves [16,17]. Moreover, two crassostreaxanthin A analogues, **37** and **38**, were isolated from the oyster as minor components [16,17]. Metabolic pathways of fucoxanthin in bivalves are shown in Figure 6. 

**Figure 6.** Metabolic pathways of fucoxanthin in bivalves. 

Bivalves also feed on dinoflagellates. Peridinin (**39**), a characteristic carotenoid in dinoflagellates with a C37-skeletal structure, and its metabolites (**<sup>40</sup>**–**43**) were also found in some bivalves. Recently, four new C37-skeletal carotenoids (**<sup>44</sup>**–**47**) were isolated from *Crassostrea gigas* [16,17], *Paphia amabillis* [20], and *Corbicula japonica*  [24,25]. The metabolic pathways of peridinin in bivalves are shown in Figure 7. As well as fucoxanthin, the major metabolic conversions of peridinin in bivalves are hydrolysis of acetyl group, conversion of the allenic bond to an acetylenic bond, and hydrolysis cleavage of the epoxy ring, as shown in Figure 7. 

> **Figure 7.** Metabolic pathways of peridinin in bivalves.

There are many reports on carotenoids in marine shellfish [6,7]. However, there are few 

reports on the carotenoids of shellfish inhabiting brackish or fresh water [24,25]. Four new 

carotenoids, corbiculaxanthin (**48**), corbiculaxanthin 3ȝ-acetate (**49**), 6- epiheteroxanthin (**50**), and 

<sup>7</sup><sup>ȝ</sup>,8<sup>ȝ</sup>-didehydrodeepoxyneoxanthin (**51**), were isolated from the brackish clam *Corbicula japonica* and freshwater clam *Corbicula sandai* (Figure 8) [24,25]. 7<sup>ȝ</sup>,8<sup>ȝ</sup>-Didehydrodeepoxyneoxanthin (**51**) has an interesting structure, with both allenic and acetylenic moieties. 

> **Figure 8.** New carotenoids in corbicula clams.

Carotenoids found in bivalves provide a key to the food chain as well as metabolic pathways. 

Astaxanthin and its esters were found to be major carotenoids in species of octopus and cuttlefish. Their astaxanthins consisted of three optical isomers and originated from dietary zooplankton [26]. 
