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Reviews
P. R. Walne. "Experimental rearing of the larvae of Ostrea edulis L. in the
laboratory". Min. Agric. Fish and Food, Fish. Invest., Ser. 2, 20 (9): 1-23,
4 figs, and 2 pi. London, 1956.
The author describes a technique of rearing the larvae of Ostrea edulis in
vessels of 1 litre capacity without change of sea water. With this technique he
carried through a great number of experiments in a glass house of the Conway
laboratory. It is clear that a far greater number of experiments, involving
variation of several factors, can be carried out in a given amount of time by
using 1 litre vessels, than was ever possible in using the few big concrete tanks
of the Conway station in which so much practical experience on the rearing
of oyster larvae has been gained in previous years.
Everyone interested in shellfish biology and shellfish farming will be very
grateful that the results obtained have been made available in printed form, for
too often such valuable information, especially when difficulties hard to understand are encountered, are put to sleep in the files of a laboratory. And is it
not primarily the failures which sooner or later appear to increase our knowledge and insight, rather than immediate success?
Of the approximately 200 feeding experiments made, many have been
failures for no apparent reason. Contrary to the former experiments in big
tanks, the type and quantity of food used was under strict control. It could
be concluded that the suitability as food varies widely from one species of
nanoplankton to the other. Relatively few appeared to be very good larval
food if used in pure culture (e. g., Pyramimonas grossii Parke (Chlorophyceae),
Chromu/ina pleiades Parke, and Isochrysis galbana Parke (Chrysophyceae). No
growth could be obtained with many a nanoplankton form, including thick
walled Chlorococcales, and some forms (e. g., Prymnesium parvum Carter)
appeared to be definitely toxic to oyster larvae.
Sometimes even the most successful food gave some failures. As successful
and unsuccessful series of experiments often occurred at the same time, or
very close together, WALNE is inclined to think that the fault lies with the
larvae, rather than with the quality of the food or of the sea water in which
the larvae were being reared. He believes that the condition of the parent
oysters may greatly influence the vigour of the larvae produced, especially
through variation of the amount of food reserve in the larvae when they are
liberated. This may be true, but before assuming that this is really the most
important reason for failures in the rearing of larvae we should very much
like to have figures on the condition index or on the chemical composition of
both adult oysters and samples of the newly liberated larvae. It is certainly
not impossible that a correlation between condition and vigour might be
found in this way. That would provide a very promising lead, indeed, for, one
then could try to control and increase the condition of the parent oysters kept
in the laboratory in anticipation of the emission of larvae, by providing them
with an ideal diet, included a given amount of flagellates to be used by the
incubated larvae in more advanced stages of development. Under field conditions such larvae do already ingest tiny food particles according to the reviewer's
personal experience.
Experiments on the removal of flagellates showed that at 20°C. a larva
sweeps about 0-45 ml. clean of flagellates in 24 hours at a density of about
45 cells per mm 3 . Larger larvae did not seem to remove more flagellates than
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467
small larvae. It is not yet clear how under field conditions larval development
may proceed very well at much lower concentrations of flagellates than those
considered as optimal for the rearing under laboratory conditions. However,
there is as yet no proof that the species of flagellates which have been tested
in the laboratory form any significant part of the natural nanoplankton. As
WALNE rightly remarks, there may be other species which are superior to those
tested, but which do not thrive in the culture media used thus far. It could
also be that a mixed diet as used in the field leads to better results than the
feeding with a pure culture of one species only; but few experiments only have
been carried out with carefully balanced diets consisting of more species.
WALNE is also worried about the fact that very often his larvae failed to
swim continuously. Prolonged resting on the bottom of the jar often preceeds
failures. He supposes that the metabolism is insufficiently rapid to permit
continuous swimming, and that in the field they may in part rely on turbulence
and the length of the water column to keep them afloat. The reviewer observed
repeatedly that the larvae do often take a rest, falling down slowly, velum
expanded, but resume swimming after having dropped over some 50 cm. In
nature this will rarely bring them in contact with the bottom, but in the small
jars such intermittent resting seems hardly possible without touching the
bottom, where they may meet trouble.
It is interesting to note that salinity toleration is quite considerable. Good
results were obtained from 26 to 35 0 / 00 . but it is not recorded whether the
parent oysters came from water with a high or a low average salinity, and with
greater or smaller daily ranges in salinity.
P. Korringa
P. R. Walne. "The biology and distribution of the slipper limpet Crepidula
fornicata in Essex rivers." Min. Agric, Fish and Food, Fish. Invest.,
Ser. 2, 20 (6): 1-50, 33 figs. London, 1956.
The investigations discussed in this paper were carried out to increase the
knowledge on the biology of an important pest of the east coast oyster beds
of England, a pest which is steadily extending its range, and is now found on
nearly all the important oyster grounds on the English coast.
Conditions appealing to oysters correspond so very closely to those under
which Crepidula thrives that it is virtually impossible to select oyster grounds
which are ecologically unsuitable to the slipper limpet. The abundance of
Crepidula even forms a very good guide to the hydrographic suitability of an
area for oyster farming.
WALNE'S studies elucidate many aspects of Crepidula'% biology, but do not
bring much essential news on the true character of the damage inflicted to
oyster farming, which can be summarized as: accumulation of mud, competition
for food and space — very hard to put down in figures — and an increased
laboriousness in oyster farming.
The conclusion that resistance to air frost is similar in Mytilus edulis and
Crepidula fornicata, and is apparently correlated with the time taken for the
tissues to freeze, should be considered with caution, as data obtained with sublittoral west coast mussels have been used. The reviewer's experience is that
mussels living in the intertidal zone are much hardier to air frost than Crepidula
living on beds exposed at low tide, though shell thickness may be about equal.