Discussion

A multiple­prey experiment with M. nudus cultured in aquaria indicated preferential grazing of S. longissima compared to the brown alga Agarum cribrosum, the green alga Ulva pertusa, and the red alga Neodilsea yendoana (Machiguchi et al. 1994). The feeding rates on S. longissima, S. japonica, and Saccharina angustata were high compared to those on other macrophytes (Machiguchi et al. 1994; Nabata et al. 1999). In addition, feeding experiments demonstrated that Saccharina kelp promotes gonad production of M.

nudus (Agatsuma 1999; Nabata et al. 1999). Gonad size can be increased at any time of year (e.g. Andrew 1986; Klinger et al. 1997), but it is most readily manipulated in the season when nutrients are beginning to be mobilized for gametogenesis. In the Sea of Japan and the Tsugaru Strait in southern Hokkaido, Japan, gonad development of M.

nudus proceeds from the recovering stage to the growing stage from January to May and to the maturation stage from June and August (Fuji 1960b; Agatsuma et al. 1988). The gonads increase in size during the growing stage (Fuji 1960b; Agatsuma et al. 1988) due to accumulation of nutrients in NPs (Holland and Holland 1969; Walker et al. 2005; 2007;

2013). These past studies support the premise that the observed enhancement of gonad production and associated changes in gonad qualities of M. nudus in the current study were due to feeding with S. japonica for 42 d. The low consumption of U. pinnatifida stipes would be also affected by low water temperature of < 6 °C (Agatsuma 1997). In addition, low protein content supports this finding. Temporal decrease in water temperature to < 6 °C in April is assumed to be affected by southward First Oyashio Intrusion in the Pacific waters along the northeastern coast of Japan (Mizuno 1984;

Ogawa 1989), which might decrease the kelp intake of M. nudus (Agatsuma et al. 1996;

Machiguchi et al. 1994).

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McBride et al. (2004) first examined gonad hardness of Mesocentrotus franciscanus using a creep meter and reported that gonad hardness is affected by reproductive cycle, with greater values in fall during the recovering and growing stages than in spring during the maturation stage. At the end of the culture experiment, gonad hardness of EC and EK urchins was lower than that of EB urchins, likely due to differences in gonad development between growing and recovering stages (i.e., increased numbers of gametogenic cells and predominance of NPs in EC and EK urchins). Harder recovering gonads of EB urchins is coincident with that of the recovering gonads in M. franciscanus starved for 3 months (McBride et al. 2004). For both sexes of sea urchin, gonads containing predominantly NPs are preferred by consumers (Walker and Lesser 1998; Walker et al. 2007; Phillips et al. 2009; Unuma and Walker 2009; 2010). One of the best gonad qualities is firmness (Agatsuma et al. 2004; Kato and Schroeter 1985). Moreover, gonad hardness is a major factor associated with texture. Sensory evaluation indicated a higher texture score for gonads from the kelp bed urchins than from the barren urchins, which suggests that growing gonads with less hardness relative to the recovering gonads but harder than mature gonads are preferred. Some studies indicated that lower gonad texture or firmness values were linked to high water content of the gonad (McBride et al. 2004; Pearce et al.

2004), but this relationship is not always detected (Pearce et al. 2002b; Azad et al. 2011).

Moisture content of mature and partly spawned gonads is greater than that of recovering to growing gonads, leading to differences in hardness (McBride et al. 2004). In the present study, no relationship between gonadal moisture content and hardness between recovering and growing gonads was detected.

L* values of the ovary and testis at the end of culture were similar to those of the urchins from the Eisenia kelp bed, which differed from those of urchins from the barren.

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This result indicates improvement of L* by consumption of S. japonica. Low ΔE*ab

values of ovary and testis of EC urchins compared to those of EB urchins also indicate improvement of total color (L*, a*, and b*) regardless of the lack of difference in a* and b* between the gonads from EC and SCB urchins. Sensory evaluation results revealed a significantly low score for color of gonads of urchins from the barren, which likely explains the low L* value. These results were confirmed visually. Yellow and reddish colors of sea urchin gonads are caused by carotenoids, mainly β­echinenone (Griffiths and Perrott 1976; Goodwin 1980; 1984; Matsuno and Hirao 1989). Additionally, in all urchin specimens except for EK, testes had higher a* and b* values than ovaries. This would be responsible for the sexual difference in content of the pigment in gonads (Goodwin 1984; Borisovets et al. 2002). Agatsuma et al. (2005) reported that brown colorization of M. nudus gonads occurred in individuals at > 7 years of age and also < 7 years of age with small size gonads due to low food availability. Ages of sea urchins used in the present study varied among urchin specimens but all were below 8 years of age: 2 years from the kelp bed, 2–6 years from the barren, and 2–7 years for culture. Low L*

values indicating brown color were particularly observed in the gonads of SCB urchins.

Using urchins collected from a habitat with low food availability as the starting point demonstrated improvement of gonad color by short­term feeding with S. japonica. For improvement in cage cultured urchins, choosing the subjects younger than 8 years is necessary.

The present study firstly compared the uniformity of gonad quality of cultured and wild urchins quantitatively. In the present study, coefficients of variation of gonad indices, gonad water content, and gonad color (L*, a*, b* values) for EC individuals were low compared to those of SCB urchins and almost the same as or less than those of EK urchins.

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Coefficients of variation of gonad hardness for EC urchins were also lower than those for EK and EB urchins. These results indicate that the uniformity of good quality among individuals was upgraded in cultured urchins, which should lead to increased confidence in sea urchin buyers and thus result in enhanced market value.

Komata (1964) reported that the free amino acids closely associated with sea urchin gonad taste are Glu, Ala, Gly, Met and Val. In contrast, Asp, Pro, Thr, His, Lys, Phe, Tyr, taurine, cysteine and tryptophan are not associated with gonad taste. Of the sweet tasting FAAs in the gonads of EC urchins, Ala content per 100 g of testes (339.3 mg) and ovaries (379.4 mg) was much higher than that of the other urchin specimens. Hirano et al. (1978) reported that Ala content in testes and ovaries of M. nudus fed Saccharina sp. and U.

pinnatifida for 11 months until June was 269 mg and 164 mg, respectively. In gender­

mixed gonads of adult M. nudus fed S. angustata for 8 weeks from June to August, Ala content was about 280 mg (Nabata et al. 1999). In contrast, Ala content in testes and ovaries of M. nudus fed frozen S. japonica var. ochotensis for 2 months from April to June was 42.8 mg and 38.0 mg, respectively (Inomata et al., 2016). In sex­mixed gonads of wild Diadema setosum, alanine content increased from 25.7 mg/100 g in April to 260.9 mg/100 g in June and then decreased to 27.7 mg/100 g in October (Kaneko et al. 2012).

Komata et al. (1962) examined FAA contents in gender­mixed gonads of three species of sea urchin from the wild and reported that alanine content was 91.8 mg in March, 126.5 mg in July, and 102.0 mg in August for H. pulcherrimus; 102.8 mg in August for Heliocidaris crassispina; and 126.0 mg in March and 101.1 mg in July for S. intermedius.

Alanine content of mixed­gender gonads of H. crassispina fed U. pertusa, U. pinnatifida, and Sargassum spp. for about 7 weeks from April to June was 231 mg, 261 mg, and 308 g, respectively (Osako et al. 2006). These alanine values from past studies are low

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compared to those in the gonads of M. nudus fed S. japonica for 42 d in the present presumably due to gonad developmental stages, and among species and by feed type.

Phillips et al. (2010a) indicated umami and sweet taste of wild Evechinus chloroticus testes and ovaries are correlated to Glu and Gly concentrations, respectively. However, in the present study, there were no significant differences in these FAAs among the gonads from all urchin specimens. Degree of sweetness of Gly is comparable to about 60% of that of Ala (Schutz and Pilgrim 1957). The organoleptic threshold values of Ala and Gly are 60 mg/100 mL and 130 mg/100 mL, respectively (Kirimura et al. 1969), although little difference in the taste between these free amino acids was obtained from sensory examination (Komata et al. 1962). Even so, the marked increase in Ala content of gonads from EC urchins is likely to make a major contribution to sweet taste. Of the bitter tasting FAAs associated with gonad taste, Komata (1964) reported that Arg in the gonad makes the taste undesirable. There was no significant difference in Arg content in gonad by sex among the EC, SK, and EK urchins, although Arg content in testes from EC urchins was higher than that in SCB urchins. Leu, Met, Tyr and Val contents in the gonads of EC urchins were higher than those of EK and/or EB urchins. Val is an essential FAA for a specific sea urchin gonad taste (Komata et al. 1962; Murata 2009). While, synergistic effects of increase in these other bitter tasting FAAs and increase in sweet tasting Ala on gonad taste remain unclear. In the present study, sensory evaluation of gonads was not conducted separately by urchin gender. However, the significantly higher score of sweetness of the gonads of cultured urchins compared to those from the barren was caused principally by the markedly high levels of alanine content in testes and ovaries of EC urchins. Higher bitter tasting FAA contents (excluding Arg and Lys) and higher Ala content in the gonads of EC urchins compared to those from the kelp bed might have

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contributed to the lack of significant difference in the sensory scores of overall taste between them (although bitterness was not scored in the sensory evaluation).

FAA contents in sea urchin gonads increase at the maturation stage (Lee and Haard 1982; Liyana­Pathirana et al. 2002; Osako et al. 2007). Increase in levels of most FAAs in the growing gonad of EC urchins compared to the recovering gonad from SCB urchins may be attributable to the development of germ cells. Hirano et al. (1978) examined sexual differences in FAA contents in the gonad of M. nudus in June after feeding with Saccharina sp. and U. pinnatifida. They reported a high proportion of Ile, Thr, Lys, and Arg in the ovary and of Glu, Ala, and taurine in the testis. In the present study, the same sexual differences in the contents of Thr and Ile were detected. This may suggest that sexual differences in FAA contents in the gonads vary according to food source as well as feeding season and length, which affect gonad development.

In the present study, gonad size, color, hardness, taste, and consistency among individuals of M. nudus collected from barren habitats could be improved to reach a level similar to that of animals from the E. bicyclis kelp bed by cage culture and short­term feeding with Saccharina kelps. In particular, sweet tasting alanine contents of the gonads were markedly high.

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