ANGER and DAWIRS: ELEMENTAL COMPOSITION OF HYAS ARANEUS 



often crawls on the bottom, presumably will test 

 suitable benthic habitats, before it goes through 

 metamorphosis at a selected site. It appears that 

 a rigid exoskeleton is advantageous as a pro- 

 tective means in the evolution of benthic 

 crustaceans, whereas in pelagic species and 

 stages it is probably disadvantageous for 

 energetic reasons (increasing swimming energy). 



The preliminary determinations of feeding 

 rates on Artemia sp. nauplii yielded almost the 

 same values (in C per day) as observed by Anger 

 and Nair (1979) using Polydora ciliata larvae as 

 prey. This confirms that the amounts in the 

 above data are correct, but further studies con- 

 sidering the whole molt cycle in each larval stage 

 will be necessary for comparisons with the 

 growth measurements of the present investiga- 

 tion. The feeding rates observed in the Z-l of H. 

 araneus as well as larval DW are similar to those 

 found by Mootz and Epifanio (1974) in the Z-4 

 stage of Menippe mercenaria. 



Measuring the loss of organic matter and 

 energy during starvation bears the same 

 technical problems as measuring growth. FW, 

 apart from its inaccuracy, is practically constant 

 even during long-term starvation. This masking 

 of changes in organic constituents is again 

 caused by changes in water content. We assume 

 that the underlying mechanism is some kind of 

 starvation edema. Due to degradation of amino 

 acids, the osmotic pressure in the hemolymph 

 must decrease, and consequently water may pas- 

 sively enter body tissues. The water lost in the 

 hemolymph might be replaced by seawater. This 

 assumption would explain the observed net in- 

 crease in body volume, water, and ash contents of 

 starving larvae (Ikeda 1971, 1974; Mayzaud 

 1976). Instrusion of inorganic salts replacing 

 degraded organic ions presumably is responsible 

 not only for increasing ash portions, but also for 

 the low degree of loss in DW. The latter observa- 

 tion means that DW can also be used in energetic 

 studies of starving zooplankton to a limited 

 degree, because it does not reflect actual losses in 

 organic matter and energy. 



Losses in C, N, H, and individual J's followed 

 an exponential pattern with a weak curvature. 

 The maximum possible losses until death 

 amounted to ca. 40 to 60% of initial values, de- 

 pending on the parameter and larval stage under 

 consideration. These observations correspond to 

 those by Anger and Nair (1979) and Dawirs 

 (unpubl. data) on starved larvae of H. araneus 

 and Carcinus maenas, respectively. Ikeda(1974) 



reported reductions in biomass of other zoo- 

 plankton down to 20%. Our figures also become 

 higher if chitin is excluded from these calcula- 

 tions. 



Anger and Dawirs (1981) found that feeding 

 after initial starvation in Z-l larvae of H. 

 araneus is successful only if a certain time (point- 

 of-no-return, PNR) is not exceeded. Comparing 

 this time span with the above biomass data, the 

 actual limits of starvation resistance are already 

 reached when 25 to 30% of organic matter 

 (C,N,H) or 30 to 35% of individual energy are lost. 

 Beyond this PNR another ca. 10% loss in all these 

 parameters is possible, before the larva dies, re- 

 gardless of eventual food availability. Fifty per- 

 cent of the larvae already reach this limit 

 (PNR50) when only ca. 20% of the organic sub- 

 stance or ca. 25% of energy is lost. The PNR 

 values for the other stages have not yet been 

 determined. 



Another finding reported by Anger and 

 Dawirs (1981) is that relatively short initial feed- 

 ing periods suffice for zoeae of H. araneus to suc- 

 cessfully reach the next stage (Z-2 or megalopa), 

 regardless of further food availability. Convert- 

 ing these time spans to biomass data, a sur- 

 prising agreement in both zoeal stages is found: 

 50% of the larvae reach this "point-of-reserve- 

 saturation" (PRS50), when they have gained ca. 

 70% N, 90% C and H each, and ca. 95% energy (re- 

 lated to early postmolt levels). If food is contin- 

 ually available, considerable further accumula- 

 tion of organic matter and energy will take place 

 (see above), but this additional reserve will not be 

 needed before the next stage is reached. If no 

 prey is available during this period (presumably 

 premolt) the next stage will be significantly pro- 

 longed, thus revealing a certain dependence on 

 reserves accumulated during the preceding 

 zoeal stage. Anger and Dawirs (1981) suggested 

 that sterols (precursors of the molting hormone, 

 ecdysterone) may play a crucial role in this 

 phenomenon. 



It is doubtful that energetics alone can explain 

 the early appearance of the PNR, since the actual 

 losses in organic body substance are rather low 

 at that time. We assume that an irreversible 

 damage in some hormonal or enzymatic system 

 is involved in ecdysis. 



The weight-specific metabolic rate is a major 

 factor deciding the maximal survival time under 

 starvation (Ikeda 1974). It is far lower in starved 

 as compared with fed zooplankton (see, e.g., 

 Ikeda 1977 and earlier papers cited therein; 



431 



