Calanus finmarchicus egg production and female size
Observations of egg production rates (EPRs) for female Calanus finmarchicus were compared for different regions of the North Atlantic
(Fig. 1c). The regions were diverse in size and sampling frequency, ranging
from a fixed time series station in the Lower St Lawrence Estuary, off
Rimouski (RIM), where nearly 200 experiments were carried out between May and
December from 1994 to 2006, to a large-scale survey in the northern
Norwegian Sea (NNWS), where about 50 experiments were carried out between
April and June from 2002 to 2004. For this compilation the stations
were grouped mostly along geographic lines, with only limited attention being
paid to oceanographic features. There is some overlap between regions,
however, where stations were sometimes kept together when they were sampled
on the same cruise. Furthermore, some stations
other than RIM were occupied more than once during different years and/or in
different seasons, although not shown in Fig. 1c. Some of the data included here have appeared in published
papers, and the citations are included. Previously unpublished data were also
provided by C. Broms, E. Gaard, A. Gislason, E. Head and S.
Jónasdóttir. Data have been submitted to PANGEA (Data Publisher for
Earth & Environmental Science) as averages by area.
Egg production in C. finmarchicus occurs in spawning bouts, which
are of relatively short duration and may occur once or more per day (Marshall
and Orr, 1972; Hirche, 1996). While there is evidence for diel spawning
periodicity in the sea (Runge, 1987; Runge and de Lafontaine, 1996), females
incubated in dishes for the first 24 h after capture do not always show a
consistent night-time release of eggs, as they did for Calanus pacificus (Runge and Plourde, 1996; Head et al., 2013). Because of the
potential for diel egg-laying behaviour, the vast majority of egg production
experiments have been carried out by incubating freshly caught females for
24 h. It has been shown that female Calanus that are kept and fed
in vitro and then transferred to an incubation chamber lay the same
number of eggs over the next 24 h whether or not they are fed (Plourde and
Runge, 1993; Laabir et al., 1995). Thus, it has been assumed that average egg
production rates of freshly caught females are the same during the 24 h
following capture as they would have been in situ (Runge and Roff, 2000).
In this study we include only results from such 24 h incubation experiments,
and we term the eggs laid during these 24 h periods “clutches”, even
though they may originate from more than one spawning bout, and we refer to the number of
eggs laid by one female during a 24 h period as the clutch size (CS). In
most experiments 20–30 females were incubated individually in separate
chambers, and the proportion of females that laid eggs over 24 h is referred
to as the “spawning frequency” (SF), which is here expressed as a
percentage per day. EPRs reported here were calculated
by individual contributing investigators either simply as the sum of all of
the eggs produced in an experiment divided by the number of females incubated
and the average incubation time (generally 1 day) or as the average of the
EPRs calculated for each experimental female individually, which takes
account of differences in incubation times for individual females. For the
WGBB (West Greenland–Baffin Bay) most experiments were carried out using prolonged incubation periods
(e.g. 36–48 h), often with relatively few females (∼ 10). For several
of the analyses carried out here it was necessary to include the results of
these prolonged incubations.
As batches of eggs are released into the water column in situ, they may hatch
and develop or they may be consumed by local predators, including female
C. finmarchicus themselves, which are sometimes the most abundant
potential predators (Basedow and Tande, 2006). To avoid cannibalism,
incubations are generally set up so as to minimize contact between the
females and the eggs they are laying. This has been done by the investigators
contributing to this work using one of five techniques. In Method A females
are incubated individually in 45–50 mL of seawater in 6–10 cm diameter
petri dishes. The eggs sink rapidly to the bottom surface, where they are
unlikely to be caught up in the females' feeding currents. Method B involves
incubating females individually in similar but smaller “multiwell”
chambers, which have a volume capacity of 10–15 mL. In Method C females are
placed individually (or in groups of two or three) in cylinders, fitted with mesh
screens on the bottom, which are suspended in beakers of 400–600 mL capacity
(Gislason, 2005). The eggs sink through the mesh and are thus separated from
the females. Method D represents a modification of Method C, in that there
is flow of seawater through the chamber (White and Roman, 1992). Finally, in
Method E, individual (or groups of two or three) females are incubated in bottles
or beakers (up to 1 L capacity), without screening (Jónasdóttir et
al., 2005). For Method E the vessels are kept upright and it is assumed that
the eggs will sink out and become unavailable to the females relatively
rapidly.
There have been relatively few comparisons of these different experimental
methods. Cabal et al. (1997) found that female C. finmarchicus from
the Labrador Sea incubated individually in 50 mL petri dishes (Method A) or
80 mL bottles (Method E) produced similar numbers of eggs after 3 days,
although only three experiments were done, and over the first 24 h, CSs were
larger for Method A. They also found that over 24 to 72 h periods, groups of
females in screened cylinders within large volume chambers (Method C) gave
higher egg production rates than those in chambers without screens (Method E)
did. Runge and Roff (2000) reported that egg laying in dishes (Method A)
yielded similar egg production rates to the egg laying of groups of 10–15
females incubated in 1.5 L screened beakers (Method C). However, the beaker
egg production estimates declined dramatically relative to dish estimates in
rough weather, presumably due to increased mixing in beakers and therefore
higher loss due to cannibalism. More recently, Plourde and Joly (2008) found
that suspending a mesh screen within petri dishes 2 mm above the bottom made
no difference to the number of eggs produced by female C. finmarchicus over 24 h, although it did increase the number of eggs
recovered from Metridia longa females, which could be seen swimming
actively and sweeping the bottom with their mouthparts in the unscreened
dishes. In the northeast Atlantic, at Ocean Weather Station M (included in
our southern Norwegian Sea (SNWS) region), B. Niehoff (personal
communication, 2013) found that females incubated for 24 h in multiwells
(Method B) had similar CSs to those incubated according to Method C. None of
these studies compared all methods and the fact that the NW Atlantic groups
have used Method A, while the central and NE Atlantic groups have used mainly
Methods C, D or E introduces a question as to whether methodological
differences might have contributed to the differences found among the CSs and
EPRs in the different regions. Such an analysis is not possible based on the
data currently available, however, and the topic will not be considered
further in this work, although it merits further attention.
Another point on which investigators differed is how they dealt with small
clutches. For the Georges Bank (GB), Rimouski station (RIM) and Scotian Shelf
(SS) regions and for the Labrador Sea (LS) data, provided by R. Campbell,
clutches of < 6 eggs were routinely not included in the data sets on CSs,
since they were regarded as being the result of interrupted spawning events.
These small clutches were apparently very rare (J. Runge, personal
communication, 2013), and indeed for the LS data reported by Head et
al. (2013) clutches of < 6 eggs accounted for only 32 of the 1324 clutches
observed, i.e. 2 %. For regions farther east, however, the proportions of
clutches of < 6 eggs were generally larger: between 13 % (SNWS) and
33 % (northern Norwegian Sea, NNWS). Because of this difference in data
reporting, CSs of < 6 eggs were excluded from the calculations of average
CSs for all regions. Small clutches were, however, included by all
investigators in their calculations of EPRs.
Previous studies of egg production have shown a significant link between
clutch size and female size (Runge and Plourde, 1996; Campbell and Head,
2000; Jónasdóttir et al., 2005; Runge et al., 2006), and most of the
data sets provided for this work included measurements of the prosome lengths
for each individually incubated female for each egg production experiment,
along with each corresponding individual clutch size (Melle et al.,
2014). One
exception to this was in the SNWS region (data from Ocean Weather Station M),
for which average female prosome lengths were determined for groups of
females that had not been used in experiments but that had been collected on
the same day. In addition, there were no measurements of prosome lengths for
some data from the region “Between Scotland and Iceland” (BIS) and the SNWS
and NNWS regions. Furthermore, prosome lengths were not measured for all
clutch sizes enumerated at RIM.
Egg production rates for the experiments carried out within a given region
were averaged seasonally. The rationale for the grouping of months into
seasons within each region was based partly on observations of seasonal
cycles of temperature and chlorophyll concentration, partly on what could be
ascertained from the literature about the timing of the appearance of
females at the surface after overwintering, and partly on the availability
of data. The spring months cover the period when water temperatures are
increasing, when the spring bloom is starting or is in progress, when
diatoms dominate the female diet, and when the overwintered (G0) generation
of females is abundant in the surface layers. Spring is the time when
community egg production rates, although maybe not individual rates, are
expected to be highest. In summer, temperatures are higher and the bloom may
still be in progress, but the female diet may be more varied and some
females of the new year's generation may be present. In autumn and winter
relatively few females are in the near-surface layers and phytoplankton
levels are generally low.
Observations of in situ temperature and chlorophyll concentration were made
at nearly all experimental stations. The original aim had been to use in situ
temperatures from 5 m and chlorophyll concentrations integrated to 30 m in
this study. Not all data were provided in this form, however. For example, in
some data sets temperature data were surface values or 0–10 or 0–20 m
averages, and chlorophyll concentrations were sometimes values integrated to
50 m. The data were standardized to a comparable format by assuming that
surface, 0–10 or 0–20 m average temperatures were the same as 5 m
temperatures and that the chlorophyll concentrations were uniform throughout
the 0–50 m depth range. These assumptions are likely to be most appropriate
in spring and winter, when mixed layers are relatively deep.