Abstract
Sea urchin roe is a highly valued product in the international marketplace. New Zealand’s coastal zones, have significant stocks of sea urchin (Evechinus chloroticus; local name Kina), but quality issues including colour, taste and texture limit export opportunities. The objective of this study was to investigate compounds which contribute to desirable (bright yellow to orange red) as opposed to undesirable (brown or black) roe coloration, to contribute to an understanding of Kina roe pigmentation and the establishment of a Kina roe export industry for New Zealand in the future.
The current belief is that carotenoid compounds are responsible for sea urchin roe pigmentation. Sea urchins cannot synthesise carotenoids de novo, and are therefore dependent upon dietary carotenoid intake. It is reported that dietary carotenoids may be assimilated selectively or transformed to other forms. Echinenone and β-carotene have been identified as the main carotenoids found in Kina roe and also in 10 of the 11 other sea urchin species studied globally.
In this study, carotenoids were extracted from differently pigmented Kina roe and from sections of Kina digestive tract gut wall tissue. The carotenoids were analysed by RP-HPLC. Comparison of the carotenoid profiles revealed that echinenone was the predominant carotenoid in both light and dark pigmented roe, with increases in fucoxanthin, astaxanthin and β-carotene being detected in dark brown compared to light coloured roe. The predominant carotenoids detected in Kina gut wall were fucoxanthin, violaxanthin and echinenone. The collected Kina were feeding predominantly on Macrocystis pyrifera (kelp) that contains mainly fucoxanthin, and minor amounts of β-carotene and violaxanthin.
The source of echinenone in the Kina roe is uncertain, as it is not present in kelp, which forms the major component of sea urchin diet. Results from this study suggest that echinenone is generated from β-carotene, which is present in low levels in kelp. It has been suggested in the literature that the conversion takes place exclusively in the roe, however, this study found high concentrations echinenone present in the gut wall of Kina. This study suggests that the gut wall is therefore directly involved in the metabolism of carotenoid compounds and that separate carotenoid modification events are occurring in different tissues. The results of the carotenoid analysis in this study suggest that carotenoid content may not be entirely responsible for the colour variation observed in Kina roe. It is now the belief that other non-carotenoid compounds are also affecting coloration.
To obtain information about the Kina roe proteome, Kina roe proteins were displayed by large format 2-D PAGE and analysed by an in-gel digest/mass spectrometry/informatics workflow, involving interrogation of the Echinodermata and Strongylocentrotus purpuratus genome databases. A number of proteins in the Kina roe proteome were identified from homologues in the databases, and some of these proteins appear to be unique to either light or dark pigmented Kina roe. The results of this study indicate that some proteins in Kina roe are capable of contributing to roe pigmentation by association with other compounds.
In addition, a diet trial was undertaken with Kina to evaluate the effect of the introduction of carotenoid compounds on Kina roe colour. Although the lack of availability of carotenoids in quantity limited the scope of this trial, the results indicate the potential for roe colour manipulation via the diet.
Overall, the results of this study have contributed to the knowledge of colour development in Kina and have indicated possible future research directions.