*Received: 16 January 2014; in revised form: 19 February 2014 / Accepted: 3 March 2014 / Published: 10 March 2014*

**Abstract:** Sea angels, *Clione limacina* and *Paedoclione doliiformis*, are small, floating sea slugs belonging to Gastropoda, and their gonads are a bright orange-red color. Sea angels feed exclusively on a small herbivorous sea snail, *Limacina helicina*. Carotenoids in 

*C. limacina*, *P. doliiformis*, and *L. helicina* were investigated for comparative biochemical points of view. Ά-Carotene, zeaxanthin, and diatoxanthin were found to be major carotenoids in *L. helicina*. *L. helicina* accumulated dietary algal carotenoids without modification. On the other hand, ketocarotenoids, such as pectenolone, 7,8-didehydroastaxanthin, and adonixanthin were identified as major carotenoids in the sea angels *C. limacina* and *P. doliiformis*. Sea angels oxidatively metabolize dietary carotenoids and accumulate them in their gonads. Carotenoids in the gonads of sea angels might protect against oxidative stress and enhance reproduction. 

**Keywords:** carotenoids; sea angels; food chain; metabolism 

## **1. Introduction**

*Clione limacina* is a small, floating sea slug (0.5~3 cm body length) belonging to the family Clionidae, which is a group of pelagic marine gastropods. *Paedoclione doliiformis* is a very small, floating sea slug (<0.5 cm body length) that also belongs to the family Clionidae. Their shells are lost during development and their body is gelatinous and transparent. On the other hand, their gonads and viscera are a bright orange-red color. They float by flapping their "wings". Their floating styles resemble angels and so they are called "sea angels" [1]. From spring to autumn, sea angels live at a depth of 200 m in the Sea of Okhotsk. In winter, they migrate to the coast of north Hokkaido with drift ice. The sea angels, *C. limacina* and *P. doliiformis*, are carnivorous and feed exclusively on *Limacina helicina*, which is a small, swimming predatory sea snail belonging to the family Limacinidae (Gastropoda) which feed on micro algae such as diatoms and dinoflagellates [2]. Chum salmon, *Oncorhynchus keta*, is one of the major predators of sea angels in the Okhotsk Sea of north Hokkaido [3,4]. 

Marine animals, especially marine invertebrates, contain various carotenoids, showing structural diversity [5–8]. New carotenoids are still being discovered in marine animals [9]. 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 [6–8]. The major metabolic conversions of carotenoids found in marine animals are oxidation, reduction, the translation of double bonds, oxidative cleavage of double bonds, and cleavage of epoxy bonds. Therefore, structural diversity is found in carotenoids of marine animals [6–8]. 

We have studied carotenoids in several marine animals from chemical and comparative biochemical points of view [8–10]. We have been interested in the orange-red pigments, which were assumed to be carotenoids, of sea angels. Thus, we studied the carotenoids of the sea angels *C. limacina* and 

*P. doliiformis*. Furthermore, carotenoids in the small snail *L. helicina* and chum salmon *O. keta* were studied from the perspective of the food chain (Figure 1). In the present paper, we describe the carotenoids of these marine animals from the viewpoints of comparative biochemistry and the food chain.

**Figure 1.** Food chains from phytoplankton to salmon via sea angels in the Okhotsk Sea of north Hokkaido. 

## **2. Results**

Structural formulae of carotenoids identified from the sea angels *C. limacina* and *P. doliiformis* and the small herbivorous sea snail *L. helicina* are shown in Figure 2. 

**Figure 2.** Structure of carotenoids found in *C. limacina*, *P. doliiformis*, and *L. helicina*. 
