JPR Advance Access originally published online on June 17, 2004
Journal of Plankton Research 2004 26(8):979-980; doi:10.1093/plankt/fbh112
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Plankton Research Vol. 26 No. 8 © Oxford University Press 2004; all rights reserved
LETTER |
Reply to Horizons Article ‘Some ideas about the role of lipids in the life cycle of Calanus finmarchicus’ Irigoien (2004): I
IHF, Universität Hamburg, Olbersweg 24, 22767 Hamburg, Germany
* Corresponding Author: rob.campbell{at}uni-hamburg.de
Received April 22, 2004; accepted in principle April 22, 2004; accepted for publication June 10, 2004; published online June 17, 2004
The biotic and abiotic factors that modulate the life history of C. finmarchicus are not well described, and Xabier Irigoien (Irigoien, 2004
) suggests some interesting hypotheses about what role lipids might play. He does, however, make some questionable assumptions about the role of lipids during overwintering. First, he states that ‘lipid storage determines the diapause depth’. Recent modelling of the effect of lipids on buoyancy depth by myself (Campbell and Dower, 2003) suggests that something approximating neutral buoyancy is only attainable in a very narrow window of lipid compositions. Moreover, since lipids are more compressible than seawater, buoyancy due to lipids is unstable, and a quasi-neutral buoyancy (very low sinking/ascent rates) can only be attained if the animals are able to diagnose fairly exactly the depth where they are neutrally buoyant.
Second, he states that ‘only a small percentage (
5%) of the stored lipids are consumed during overwintering’. Many studies of reasonably distinct populations and with good sampling frequency have shown a considerable decrease in dry weight or organic matter (presumably lipid) well in advance of the end of the overwintering period [e.g. (Marshall and Orr, 1958; Comita et al., 1966; Tande, 1982; Hopkins et al., 1984)]. As well, the lowest respiration rates measured and modelled by Ingvarsdóttir et al. (Ingvarsdóttir et al., 1999) imply a loss of 12% of carbon content (again, probably mostly lipid) over a four month period (which is conservative, 17.5% would be lost over a 6 month period). The above mentioned modelling results (Campbell and Dower, 2003) show that the buoyancy properties of an individual are very sensitive to lipid content, and considerable changes in buoyancy properties can be expected with only a 1–2% change in lipid contents (which is well within the uncertainty of lipid measurements, for that matter).
All of these considerations (mine and Irigoien’s) assume that these animals cannot control their buoyancy. In some preliminary experiments, I have observed that the calanoid Neocalanus plumchrus altered its buoyancy, and was able to achieve approximately neutral buoyancy quickly (<1 h) following manipulation of seawater density (by altering the salinity), or when the density of both seawater and the copepods was manipulated by increasing or decreasing hydrostatic pressure.
We know that there is considerable plasticity in the life history characteristics of C. finmarchicus. In areas with multiple generations, a portion of the g1 generation overwinter while the rest do not; several stages are known to overwinter (CIV–CVI), and the overwintering depth varies geographically. It is perhaps not unlikely that there is considerable variation in individual behaviour underlying that plasticity, and the current conceptual models tend to ignore it. If these animals are more than ‘dumb particles’, and are actively able to select their overwintering depth, then lipids are perhaps less important in controlling that portion of the life history. That would confer selective advantages as well. Rather than needing some ‘right’ amount of lipids, an individual could lay down ‘maximal’ lipid stores, which could be expected to increase its survival probability during overwintering, and the number and/or size of eggs that are produced following emergence in spring. In this case, the depth distribution might be expected to be related to some other physical or biotic factors (e.g. the depth of the convective mixed layer, or the distribution of predators).
REFERENCES
Campbell, R. W. and Dower, J. F. (2003) Role of lipids in the maintenance of neutral buoyancy by zooplankton. Mar. Ecol. Prog. Ser., 263, 93–99.
Comita, G. W., Marshall, S. M. and Orr, A. P. (1966) On the biology of Calanus finmarchicus. XIII. Seasonal change in weight, calorific value and organic matter. J. Mar. Biol. Ass. UK, 46, 1–17.[Medline]
Hopkins, C. C. E., Tande, K. S., Gronvik, S. and Sargent, J. R. (1984) Ecological investigations of the zooplankton community of Balsfjorden, northern Norway: an analysis of growth and overwintering tactics in relation to niche and environment in Metridia longa (Lubbock), Calanus finmarchicus (Gunnerus), Thysanoessa inermis (Kroyer) and T. raschi (M. Sars). J. Exp. Mar. Biol. Ecol., 82, 77–99.[CrossRef]
Ingvarsdóttir, A., Houlihan, D. F., Heath, M. R. and Hay, S. J. (1999) Seasonal changes in respiration rates of copepodite stage V Calanus finmarchicus (Gunnerus). Fish. Oceanogr., 8, Suppl. 1, 73–83.
Irigoien, X. (2004) Some ideas about the role of lipids in the life cycle of Calanus finmarchicus. J. Plankton Res., 26, 259–263.
Marshall, S. M. and Orr, A. P. (1958) On the biology of Calanus finmarchicus X. Seasonal changes in oxygen consumption. J. Mar. Biol. Ass. UK, 37, 459–472.[Medline]
Tande, K. S. (1982) Ecological investigations on the zooplankton community of Balsfjorden, northern Norway: generation cycles, and variations in body weight and body content of carbon and nitrogen related to overwintering and reproduction in the copepod Calanus finmarchicus (Gunnerus). J. Exp. Mar. Biol. Ecol., 62, 129–142.[CrossRef]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||