15 JUNE 1999
COMMENTS 13:
The point on MDS was recently switching to problems of low water oxygenation. What I would like to stress is the complexity of the O2 problem. Indeed one can consider today that, in addition to its well known function in the mitochondria, the oxygen also plays the role of a neuromodulator in some neutral networks. The homeostasis of the milieu interieur, in terms of oxygenation status is then probably much more important than previously thought.
From what we understand with the molting story in ADULT lobsters, the conclusion is that a remarkably precise partial pressure of oxygen (Po2) in the arterial blood could be required for a proper molting (through its action on the stomatogastric nervous system). In the European lobster, this arterial Po2 is 0.1 - 0.2 kPa (1 mmHg or Torr) at 15 C. As everybody is not familiar with these Po2 and kPa, to fully appreciate it, you must first remember that in a normoxic water at this temperature (water Po2 = 21 kPa, water O2 concentration about 10 mg/l), the normal arterial Po2 in a resting adult crustacea, fed or unfed, is only ranging from 1 to 4 kPa. Remember also that in our arterial blood of humans the arterial Po2 is 10-14 kPa.
As a blood Po2 at 0.1-0.2 kPa during ecdysis is incredibly low, one must accept the possibility that even small lowering of water Po2 (and not water O2 concentration, as the driving force is the pressure gradient) in the close vicinity of the animal can have dramatic effects (keep also in mind that one must think in terms of inspired water and not bulk water).
We all know that larvae are molting on the bottom of the tanks which are not always absolutely clean, that the O2-consumption of crustacea increases before moulting and that the question on unstirred layers (around a small animal which is no more efficiently ventilating) is obviously quite complicate...
Dr Jean-Charles Massabuau
UMR 5805, Laboratoire d'Ecophysiologie et Ecotoxicologie des Systemes Aquatiques
Universite Bordeaux I and CNRS
Place du Dr Peyneau
33 120 Arcachon
France
Tel: +33 (0)5 56 22 39 25
Fax: +33 (0)5 56 22 39 26
E-mail: massabuau@Inpc.u-bordeau.fr
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COMMENTS 14:
Thank you for the hypotheses regarding the shape that our larvae sometimes take before dying of moult death syndrome. I should say that our larvae can still be alive, and swimming around the tank while taking on these shapes. It does seem as if some parts of the body are swelling faster than the rest of the body, and it does indeed seem as if the body then gets stuck while the larva is attempting to escape from its old exoskeleton. I should also mention that not all moult death larvae take on this funny "flower shape', but it is common during MDS. The 'level' of MDS can vary considerably- some larvae can die during the initial swelling (usually the funny shaped ones), while some larvae can die following the removal of the majority of the old
exoskeleton encasing the cephalic shield, but before the complete escape from the old exoskeleton encasing the periopods, and the antennules (for these latter larvae, it almost appears as if they have insufficient energy to kick free of their old exoskeleton, and thus the new one hardens and the two exoskeletons stick together). The periopods appear all tangled in the latter larvae.
The water salinity of the main seawater supply for the centre has been
monitored during some, but not all, trials resulting in MDS, and it appears that most of the readings were normal (when compared to salinity of the successful trials). Interestingly enough, however, during one larval rearing trial we did have a period of very heavy rain, which resulted in a significant drop in salinty in the water supply. A large number of larvae from this trial did suffer from a moulting problem, however with this problem, the larvae survive the moult, but their old moult remained attached to some part of their body (mainly their periopods). This old exoskeleton could be removed and the larvae survived, although many had slightly deformed periopods (this experiment had to be terminated soon after the moult, although they probably would have had more problems at the next moult because of this deformity). Because of this result, we are very interested in how the osmoregulatory ability of the larvae changes during the moult, and we are hoping to investigate this further at the next spawning season.
Lee Perry
Department of Zoology
Queensland University
Brisbane, 4072, Qld, Australia
lperry@zoology.uq.edu.au
or
Bribie Island Aquaculture Research Centre
P.O. Box 2066
Bribie lsland, Woorim
4507 Qld, Australia
perryl@dpi.qld.gov.au
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COMMENTS 15:
1. What causes MDS?
1.1 Energy Imbalance
As stated by a number of CRUST-L members there are several different factors which have been linked to MDS - poor nutrition, high temperature and low DO. These are all environmental stressors to which crustaceans respond by eliciting a stress response. The response may be acute, as in the case of low DO and elevated temperature, or chronic, as in the case of poor nutrition. Elaboration of a stress response requires energy - in Michel's discussion on energy balance he didn't include stress responses as one of the vital functions for which energy is required (Perhaps energy expenditure on stress should be given a different category in energy balance equations).
Regardless of whether or not energy for stress responses should be included in discussions on energetics, it is clear that when an animal elicits a stress response there will be a diversion of metabolic energy from other vital functions - growth, host defense, reproduction etc - into the stress reactions - increased catabolism, altered osmoregulation and ion regulation, etc. Thus, one explanation of MDS is that the animal simply has insufficient energy to achieve a successful shedding of the shell.
1.2 Lack of essential nutrients
David pointed out that careful selection of the type of Artemia can
eliminate MDS in crustacean larval rearing. Eric emphasised the importance of dietary lipids and commented that improved digestibility of poly-unsaturated fatty acids will improve stress resistance. We have consistently observed MDS in the freshwater crayfish Cherax tenuimanus when rearing conditions were such that there was a lack of natural feeds in the rearing system.
Thus, adequate nutrition is clearly important in the prevention of MDS. Perhaps the nutritional abnormality in crustaceans exhibiting MDS relates to the lack of a vital precursor for the synthesis of one or more of the stress hormones. Alternatively there may be a lack of a co-factor for a key enzyme involved in one or more of the processes occurring at ecdysis.
2. Is hormone imbalance a contributing factor to MDS?
It is certainly likely that hormone imbalance is a contributing factor to
MDS but the key question is which hormone(s) are being secreted, or are acting, abnormally. The focus appears to have been on moulting hormone. However, as Lee pointed out in her original discussion, there is some evidence (Christiansen et al. 1984) that moulting hormone secretion is normal in crab larvae with MDS. Perhaps the emphasis should shift to those hormones involved in controlling stress responses in crustaceans.
3. How can MDS be prevented?
The discussions on high temperature and low DO suggest that these factors can cause MDS so careful attention to rearing conditions is important as a preventive measure. However, it would appear that the main causative factor is inadequate nutrition, possibly of a micronutrient, an essential lipid or perhaps an imbalance in mineral intake. David has found that prevention can be achieved by altering the type of Artemia and other workers have found dietary lipids to be important. What appears to be needed is a better way of evaluating dietary deficiencies in crustaceans exhibiting MDS. If an improved diagnostic procedure could be developed this could be used in dietary studies aimed at correcting the dietary deficiency. Could measurement of crustacean hyperglycemic hormone, dopamine or noradrenaline or perhaps some parameter related to immunocompetence be a fruitful line of investigation?
Louis Evans
Aquatic Science Research Unit
Curtin University of Technology
Kent Street
PERTH WA 6102
AUSTRALIA
Tel. 08 9266 4400
Fax: 08 9266 4422
E-mail: tevanslh@cc.curtin.edu.au
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COMMENTS 16:
I have visited many research facilities and seen trays/ tanks of larvae
with the minimum of aeration or water flow and often wondered about DO levels, and if water temperatures are up over 15c, to get accurate DO readings are sometimes difficult.
I believe there could be stratification of the water column causing
effectively dead spots of low DO or uneven temperatures.
What I have seen is I believe a bad practice of placing long air stones
laid across the laminar water flow rather then inline with it, what you
get in shallow systems is a turning back of the water column increasing the contaminates on the surface water that interferes with the surface tension gas exchange.
What I have done is to use an electronic thermometer reading to within 0.1c and measure the temperature around the tank. I have found up to 0.75c difference from one area to another.
This thermocline can slow down the gas exchange.
Sometimes it is worth looking at increasing the water velocity through the tank even for just a few minutes every hour; this can be done by a siphon drain effect but with an air break built in to stop at a certain tank depth.
This then changes (increases) the water exchange rate/water velocity from your preset/ existing inflows/ outflows of your tank. This may then help flush out and increase the DO levels.
The other feature I recommend is to use an airlift system to pump water
around the tank and aerate.
John Seccombe
Aquahort Ltd
Fax: 64 9 5366362
Tel: 64 9 5366201
Auckland, New Zealand
Australian toll Free Tel: 1800 146784
Fax: 1800 145668
E-mail: jseccombe@clear.net.nz
Web address http://www.angelfire.com/az/aquahort/
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COMMENTS 17:
I strongly agree with the information provided by Louis (below).
Energetics must play a role in MDS and contributing factors such as stress, poor nutrition and rearing conditions especially low DO seem likely.
I have also observed MDS in a freshwater crayfish, redclaw Cherax
quadricarinatus, under artificial conditions where there was a lack of
natural feeds in the rearing system, where all other parameters were
considered optimal.
It is interesting that there has been success in preventing MDS by altering the type of Artemia used in the larval rearing (which I understand are no longer available) with other workers citing lipids as a possible causative factor. Teshima (1997) states that the characteristic symptom of dietary deficiency in phospholipids is MDS and is common in early ontogeny. Jenn (1989, cited from Teshima 1997) shows that the effect of a phospholipid (phosphatidylcholine) on growth was enhanced by a protein source which closely mimics the amino acid profile of the species under culture. Perhaps, (I presume unboosted) Artemia nauplii have a level of phospholipid
which is just adequate providing the amino acid profile of the Artemia is close to the profile of the species under culture. David Mann may have fluked the right combinations of both in his success. The biochemistry will be telling.
Phospholipids have also been shown to influence (increase) the release of lipids from the hepatopancreas, including cholesterol (Teshima 1997). Stress (steroid) hormones are synthesised from cholesterol which crustaceans are unable to synthesise de novo. So, under stressful conditions (high temperatures, low DO) accompanied by lack of dietary phospholipids and/or cholesterol and/or a favourable amino acid profile, perhaps the synthesis of stress hormones from cholesterol is impaired, and the animals are unable to compensate metabolically, they utilise excess energy (consuming large
amounts of DO across a double barrier of cuticle) attempting to compensate against the stress, and, unfortunately, die.
Teshima, S. 1997. Phospholipids and Sterols. In D'Ambro, Conklin and Akiyama (eds); Advances in World Aquaculture, Vol. 6. Crustacean Nutrition. World Aquaculture Society, Louisana, USA.
Ian Ruscoe
Queensland Department of Primary Industries
Freshwater Fisheries and Aquaculture Center
Walkamin Q4872
Australia
Tel: (07) 40 929 914 / 40 929 929
Fax: (07) 40 933 903
E-mail: ruscoei@dpi.qld.gov.au
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COMMENTS 18:
I was very interested to see that Dr. Evans suggested that perhaps a factor such as a specific micronutrient could be responsible for causing MDS. I think that I have mentioned before that nutrition of our Moreton Bay bug larvae has always been considered as one possible cause of MDS, and this theory is still being investigated. If it is a nutritional factor affecting moulting of our bug larvae, the nutrition is, however, adequate enough so that the morphology of the 3rd instars, the growth characteristics of the 3rd instars and the intermoult period of larvae dying of MDS are the same as for those larvae which moult normally. So far we have investigated inadequate nutrition in terms of starvation and we have found that in the starvation treatments the intermoult period is significantly longer, and
the larvae are smaller than their fed counterparts. I was thinking that
perhaps a specific micronutrient, essential to the moulting process of the later instars, but not visibly affecting the larvae during the earlier instars, could be absent from the diet, with the absence of this factor causing problems at moulting time in these later instars?
Lee Perry
lperry@zoology.uq.edu.au
or
perryl@dpi.qld.gov.au