Crustaceana 85 (7) 779-787
LATITUDINAL VARIATION IN THE REPRODUCTIVE CYCLE AND SIZE
OF THE NORTHERN ROCK BARNACLE SEMIBALANUS BALANOIDES (L.)
(CIRRIPEDIA, ARCHAEOBALANIDAE) IN THE BAY OF FUNDY
BY
GABRIELLE M. BOUCHARD and RONALD B. AIKEN1 )
Department of Biology, Mount Allison University, Sackville, NB, Canada E4L 1G7
ABSTRACT
The reproductive phenology and size of populations of Semibalanus balanoides were studied at
four sites along a latitudinal gradient in the Bay of Fundy. Animals were collected from May 2006
through January 2007, their basal widths measured and the first appearance of immature oocytes,
mature oocytes, brooded embryos, and sperm scored. Proceeding from south to north, there was a
significant decline in the size of the barnacles and a delay in appearance of all stages. The possible
effects of temperature and turbidity are discussed.
RÉSUMÉ
La phénologie de la reproduction et la taille des populations de Semibalanus balanoides ont été
étudiées dans quatre sites le long d’un gradient latitudinal de la baie de Fundy. Les animaux ont été
récoltés de Mai 2006 à Janvier 2007, la largeur de leur base a été mesurée et la première apparition
d’oocytes immatures, d’oocytes mûrs, d’embryons, et de sperme notée. Du sud vers le nord, il y a eu
une diminution significative de la taille des balanes et un délai dans l’apparition de tous les stades.
L’influence possible de la température et de la turbidité est discutée.
INTRODUCTION
Studies on the correlation of latitudinal gradients with various aspects of any
species’ ecology are well known. Early studies resulted in the generation of several
ecophysiological rules (e.g., Allen’s, Bergmann’s, Gloger’s, and Rapoport’s) (Hall
& Hallgrimson, 2008).
These very general rules hide the fact that the response of several species to
ecological and climatic changes associated with different latitudes are not always
consistent. Some studies show predictable changes with latitude (e.g., Crisp, 1959;
1 ) Corresponding author; e-mail: [email protected]
© Koninklijke Brill NV, Leiden, 2012
DOI:10.1163/156854012X650214
780
GABRIELLE M. BOUCHARD & RONALD B. AIKEN
Barnes & Barnes, 1965; Hummel et al., 1985; Jonsson & L’Abbé-Lund, 1993;
Nebel, 2005). Others show discontinuous latitudinal clines that can be ascribed to
variation in local conditions such as temperature (Barnes, 1958; Schmidt & Rand,
1999; Rand et al., 2002), primary productivity (Bertness et al., 1991), restriction
of gene flow (Holm & Bourget, 1994; Brown et al., 2001; Dufresne et al., 2002),
hydrodynamic conditions (Hayden & Dolan, 1976; Connolly et al., 2001; Haase,
2003), shoreline exposure (Kirby et al., 1997) and behaviour (McClain, 1985;
Nebel, 2005). Still other studies have pointed to the interactions between biotic
and abiotic factors on a species (Leonard, 2000).
The acorn (or northern rock) barnacle, Semibalanus balanoides (Linnaeus,
1767), is a common inhabitant of rocky intertidal shorelines of the western
Atlantic. Its distribution on the shore is confined to the ‘barnacle band’ (Stephenson
& Stephenson, 1972) whose lower limit within the tide range is thought to
be determined by biological interactions and the upper limit by abiotic factors
(Bertness, 1999).
Studies on clinal variation in S. balanoides have pointed to temperature as a
controlling factor. While temperature may be involved, its influence is not simple.
Barnes (1958) and Crisp (1959) showed that temperature was a factor in egg
development only above an 8 or 9°C winter isotherm and the effects of temperature
lessened as development proceeded. Davenport et al. (2005) concluded that
temperature had no effect on breeding phenology in S. balanoides. Responses to
temperature and thermal stress can depend on the genetic composition of local
populations (Schmidt & Rand, 1999; Rand et al., 2001) and restriction of gene
flow (Brown et al., 2001). Finally, some authors (Barnes & Barnes, 1958; Bertness
et al., 1991) have shown that primary production and food availability is a more
important factor than temperature in growth and reproduction.
We examined changes in the reproductive phenology and size of S. balanoides
across a latitudinal gradient in the Bay of Fundy. At one location we have studied
(Cape Enrage, NB), S. balanoides typically mates in October and November,
broods embryos in January and February and releases nauplii in February and
March (R. B. Aiken, unpubl.). In this study, we hypothesize that this cycle would
start earlier in the lower (southern) Bay of Fundy and become progressively later
at higher latitudes.
MATERIAL AND METHODS
Semibalanus balanoides were collected at four sites within the Bay of Fundy
from May 2006 to January 2007. Sampling sites were Machias Seal Island
(44.30°N 67.06°W), St. Andrew’s (45.07°N 67.05°W), Cape Enrage (45.59°N
64.79°W) and Slack’s Cove (45.75°N 64.49°W) (fig. 1).
LATITUDINAL VARIATION IN SEMIBALANUS BALANOIDES
781
Fig. 1. A map of the four collecting localities for Semibalanus balanoides (L., 1767) in the Bay of
Fundy, Canada. 1, Machias Seal Island; 2, St. Andrews; 3, Cape Enrage; 4, Slack’s Cove.
Barnacles were collected on Machias Seal Island from May through August
2006 only. The island is a migratory bird sanctuary and travel to and from the
island was too difficult for frequent collection.
Barnacles were collected from the middle intertidal region and in areas not
covered by macroalgae (see Leonard, 2000). On each collection, ten rocks that
had at least one surface covered in barnacles were arbitrarily selected, packed in
wet Ascophyllum and transported to the laboratory. Samples were preserved in
8% formalin in seawater for a minimum of two weeks, at which point they were
transferred to a 70% ethanol solution.
On one collection date at each site (except Machais Seal Island), ten water
samples were taken to determine the turbidity to which the barnacles were exposed.
Samples were taken as the approaching tide advanced over the middle intertidal
region where the barnacles were collected. Turbidity was measured using a
Novaspec® Spectrophotometer set to 350 nm. Data presented represent the mean
percent transmission of the samples taken at each site.
Basal widths of all collected barnacles were measured using Marathon® Electronic Digital Calipers. Measurements were taken from the widest point at the base
of the barnacle. A restriction on barnacles used for histological work was that they
need to have a basal width of greater than 4 mm. Barnacles with basal widths less
than 4 mm are considered to be non-reproductive (Pineda et al., 2002; R. B. Aiken,
unpubl.).
782
GABRIELLE M. BOUCHARD & RONALD B. AIKEN
Ten arbitrarily selected barnacles from each collection date were chosen for
histological examination. The selected specimen was placed in a 50% acetic acid
solution for a minimum of 24 hours to aid in the decalcification of the outer
calcareous plates. The plates and cirri were then removed and the specimen
run through a series of graded isopropanol and Histoclear® baths in order to
dehydrate the tissue (Humason, 1979). Dehydrated tissues were then embedded
in a Paraplast® and sectioned into 6-μm sections using an American Optical
820 Rotary Microtome. Sections were stained using Milligan’s trichrome stain
(Humason, 1979).
Determination of developmental stage
The sectioned and stained tissues were examined and first appearance of each
of four developmental stages was used to determine the development rate at each
site. These stages were the presence of immature eggs, mature eggs, and the
differentiation of the embryo (Caldwell, 2004). Immature oocytes were identified
as red staining cells of variable shape and size with a deeply staining cellular
vesicle (Caldwell, 2004). Mature oocytes were identified as cells larger than
immature oocytes, which stained a deeper red, contained no cellular vesicle, and
were consistently round in shape (Caldwell, 2004). The presence of brooded
embryos was identified by elongate balls of cells, showing a differentiation of cell
types within the developing embryo (Caldwell, 2004). The presence of sperm was
determined by looking for masses of blue staining tissue containing many long
tail-like structures.
Means and standard errors of basal widths of all sampled barnacles for each
of the four sites were compared using a non-parametric ANOVA (Kruskal-Wallis
test). Further analysis was performed using a Dunn’s Multiple Comparison test.
All tests were performed using the InStat statistical analysis computer program.
RESULTS
The average size of all Semibalanus balanoides collected at all four sites was
determined (fig. 2). All data are presented as the mean ± one standard error of
the mean. The average size of S. balanoides collected at Machias Seal Island
(7.19 ± 0.12 mm) and St. Andrew’s (mean = 7.45 ± 0.11 mm) were similar.
The average size of barnacles collected at Cape Enrage was smaller than at the two
most southerly sites (5.71 ± 0.07 mm). The most northerly site, Slack’s Cove, was
found to have the smallest average barnacle size of all four sites (4.59 ± 0.06 mm).
Comparison between sites showed a significant difference between the average sizes of barnacles collected at the four sites (Kruskal-Wallis non-parametric
LATITUDINAL VARIATION IN SEMIBALANUS BALANOIDES
783
Fig. 2. Mean basal widths of all Semibalanus balanoides (L., 1767) collected at each of the four
collecting sites. Numbers in parentheses are sample sizes and vertical bars represent one standard
error of the mean.
ANOVA, KW = 327.78, p < 0.001). Further analysis (Dunn’s Multiple Comparison test) revealed that significant differences were found between all sites (p <
0.01) with the exception of Machias Seal Island and St. Andrew’s (p > 0.05).
Turbidities (measured as mean percent transmission of ten replicate samples ±
1 SD) were 94.6 ± 2.2 at St. Andrew’s, 95.3 ± 1.6 at Cape Enrage and 32.6 ± 1.8
at Slack’s Cove. A one-way ANOVA showed these differences to be significantly
different (F = 3306.4, p < 0.0001). A Tukey-Kramer Multiple Comparisons test
indicated that the differences were significant (p < 0.001) between St. Andrew’s
and Slack’s Cove (q = 99.03) and between Cape Enrage and Slack’s Cove
(q = 100.15) but not between St. Andrews and Cape Enrage (q = 1.12, p > 0.05).
The first appearance of each identified stage is presented in table I. There is
clearly an effect of latitude and position in the Bay of Fundy on the phenology or
reproductive events in S. balanoides. The first appearance of several oogenic stages
can be delayed by several months depending on location. Animals from Machias
Seal Island had immature oocytes 28 and 33 days before those at St. Andrews and
Cape Enrage, respectively, and mature oocytes 78 and 90 days earlier (table I).
Brooded embryos appear 38 days earlier at St. Andrews than at Cape Enrage.
Animals from Slack’s Cove showed no development beyond immature oocyte.
784
GABRIELLE M. BOUCHARD & RONALD B. AIKEN
TABLE I
Dates of first occurrence of immature oocytes, mature oocytes, brooded embryos and sperm in
Semibalanus balanoides (L.) for all four collecting sites
Machias Seal Island
St. Andrew’s
Cape Enrage
Slack’s Cove
Immature oocytes
Mature oocytes
Brooded embryo
Sperm
27 May
22 June
29 June
23 October
18 June
19 August
31 August
Not found
Not found∗
30 November
7 January
Not found
20 July
1 September
13 August
Not found
All dates are in 2006 except for brooded embryos at Cape Enrage (2007).
∗ Collection ceased on Machias Seal Island in late August.
Not only are stages seen earlier at more southerly sites but development
proceeds more quickly. Immature oocytes were first detected 26 days earlier at
Machias Seal Island than at St. Andrews but mature oocytes were detected 78
days earlier. Similarly the lag between the appearance of identified stages at St.
Andrews compared to Cape Enrage is 7, 12 and 38 days for immature acolytes,
mature oocytes and brooded embryos, respectively.
The appearance of sperm followed a similar pattern. Sperm were detected about
42-43 days later at St. Andrews and Cape Enrage than at Machias Seal Island.
Sperm were not found in any samples from Slack’s Cove.
DISCUSSION
The results of this study clearly show a decrease in the size and a delay in
the reproductive cycle of Semibalanus balanoides with increasing latitude. It is
tempting to conclude that temperature is a controlling factor in these results.
Indeed, several authors have stated that temperature is a key factor in latitudinal
gradients (e.g., Zacharias & Roff, 2001). Other have stated, however, that food
supply may have a greater role (Barnes & Barnes, 1958). Summer sea surface
temperature readings from the Bay of Fundy show a consistent temperature over
much of the latitudinal gradient we sampled (Fox et al., 1995). The only exception
to this is Machias Seal Island, which actually had the coldest water in summer (Fox
et al., 1995).
A more important factor may be turbidity, both in determining primary productivity and in affecting the feeding ability of S. balanoides. Turbidity is well
known as an inhibitor of primary production (Cole & Coern, 1984) and (although
we could find no studies on the topic) could affect the ability of S. balanoides to
capture food by clogging the feeding apparatus (see Anderson, 1994). Thus, in
affecting both its supply of food and it ability to capture available food, turbidity
can have a dramatic effect on the energy available to S. balanoides. This energetic
LATITUDINAL VARIATION IN SEMIBALANUS BALANOIDES
785
restriction may be especially important for simultaneous hermaphrodites like S.
balanoides that must devote more energy to reproduction in that it must elaborate
two reproductive systems (Heath, 1977).
The Bay of Fundy is much more turbid at its northern end. This is because
of the prevalence of salt marshes and mudflats at that end of the Bay (Amos,
1987; Gordon, 1994). Samples taken in this study indicate that there is a significant
increase in turbidity at Slack’s Cove with other sites having similar and far lower
turbidity. Hence, turbidity could be expected to affect the animals at the upper end
of the bay to a greater extent. While our results by no means prove that turbidity is
the reason for the smaller size and much inhibited reproductive development, they
are consistent with such an hypothesis and bear further investigation.
Finally, the asynchronous reproductive cycles of S. balanoides in the Bay of
Fundy may lead to genetically distinct subpopulations. The extent to which this
phenomenon may occur would depend on the dispersal distances of larvae, which
are unknown. In any event, the Slack’s Cove population may not reproduce at all
and depend solely on recruitment from other populations.
ACKNOWLEDGEMENTS
We thank Molly McGlauflin and Katie Goodine for help in collecting the
barnacles and K. Fraser for reviewing an earlier draft of the manuscript. Funding
for this project was from Internal Grants to R.B.A. from Mount Allison University.
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First received 3 June 2010.
Final version accepted 3 March 2012.