Growth patterns, chemical composition and oxygen consumption in early juvenileHyas araneus (Decapoda: Majidae) reared in the laboratory

Abstract

Early (instar I and II) juveniles of the spider crabHyas araneus were reared under constant conditions (12 °C, 32‰S) in the laboratory, and their growth, biochemical composition, and respiration were studied. Every second day, dry weight (W), ash-free dry weight (AFW), and contents of ash, organic and inorganic carbon (C), nitrogen (N), hydrogen (H), protein, chitin, lipid, and carbohydrates were measured, as well as oxygen consumption. Changes in the absolute amounts of W. AFW, and C, N, and H during the moulting cycle are described with various regression equations as functions of age within a given instar. These patterns of growth differ in part from those that have been observed during previous studies in larval stages of the same and some other decapod species, possibly indicating different growth strategies in larvae and juveniles. There were clear periodic changes in ash (% of W) and inorganic C (as % of total C), with initially very low and then steeply increasing values in postmoult, a maximum in intermoult, and decreasing figures during the premoult phase of each moulting cycle. Similar patterns were observed in the chitin fraction, reaching a maximum of 16% of W (31% of AFW). Ash, inorganic C, and chitin represent the major components of the exoskeleton and hence, changes in their amounts are associated with the formation and loss of cuticle material. Consequently, a high percentage of mineral matter was lost with the exuvia (76% of the late premoult [LPM] ash content, 74% of inorganic C), but relatively small fractions of LPM organic matter (15% of AFW, 11% of organic C, 5–6% of N and H). These cyclic changes in the cuticle caused an inverse pattern of variation in the percentage values (% of W) of AFW, organic C, N, H, and biochemical constituents other than chitin. When these measures of living biomass were related to, exclusively, the organic body fraction (AFM), much less variation was found during individual moulting cycles, with values of about 43–52% in organic C, 9–10% in N, 6–9% H, 31–49% of AFW in protein, 3–10% in lipid, and <1% in carbohydrates. All these constituents showed, on the average, a decreasing tendency during the first two crab instars, whereas N remained fairly constant. It cannot be explained at present, what other elements and biochemical compounds, respectively, might replace these decreasing components of AFW. Decreasing tendencies during juvenile growth were observed also in the organic C/N and in the lipid/protein weight ratios, both indicating that the proportion of lipid decreased at a higher rate than that of protein. Changes were observed also in the composition of inorganic matter, with significantly lower inorganic C in early postmoult (2–4% of ash) than in later stages of the moult cycle (about 9%). This reflected probably an increase in the degree of calcification, i.e. in the calcium carbonate content of the exoskeleton. As a fraction of total C, inorganic C reached maximum values of 17 and 20% in the crab I and II instars, respectively. The energy content of juvenile spider crabs was estimated independently from organic C and biochemical constituents, with a significant correlation between these values. However, the former estimates of energy were, on the average, significantly lower than the latter (slope of the regression ≠1). Since organic C should be a reliable integrator of organic substances, but the sum of protein, lipid, chitin, and carbohydrates amounted to only 60–91% of AFW, it is concluded that the observed discrepancy between these two estimates of energy was caused by energy from biochemical constituents that had not been determined in our analyses. Thus, energy values obtained from these biochemical fractions alone may underestimate the actual amount of organic matter and energy. Respiration per individual in juvenile spider crabs was higher than that in larval stages of the same species (previous studies), but their W-specific values of oxygen consumption (QO₂) were lower than in conspecific larvae (0.6–2μg O₂·[mg W]⁻¹). QO₂ showed a consistent periodic pattern in relation to the moult cycle: maximum values in early postmoult, followed by a rapid decrease, and constant values in the intermoult and premoult phases. This variation is interpreted as an effect mainly of cyclic changes in the amounts of cuticle materials which are metabolically inactive. From growth and respiration values (both expressed in units of organic C), net growth efficiency, K₂, values may be calculated. In contrast to previous findings in larval stages, K₂ showed an increasing trend during growth of the first two juvenile instars ofH. araneus.