EFFECTS OF COMPETITION, PREDATION BY THAIS LAPILLUS, AND OTHER FACTORS ON NATURAL POPULATIONS OF THE BARNACLE BALANUS BALANOIDES

INTRODUCTION

• Joseph H. Connell's study investigates recruitment and mortality in a natural population of Balanus balanoides over 2 2/3 years (1952-1955).

• The study focuses on biological interactions like intraspecific competition and predation by Thais lapillus.

• Associated animals were studied, but food and parasites were not.

• The study area was small to minimize variability from wave action, salinity, and temperature.

• Weather and tide records provided information on physical factors.

• Barnacles are advantageous for study because individual survival can be accurately determined by mapping positions and conducting regular censuses.

• This method is as accurate as laboratory populations and superior to methods using age at death or differences between successive generations.

• Field experimentation is facilitated by the small size and dense concentrations of barnacles, and their intertidal location.

• Thanks C. M. Yonge and the Zoology Department of the University of Glasgow, the staff of the Marine Station, Millport, T. B. Bagenal, Charles Elton and the Bureau of Animal Population, Oxford, and E. W. Fager.

METHODS

• The study area was located on Farland Point, Isle of Cumbrae, in the Firth of Clyde, Scotland, facing south.

• The shoreline consists of ledges of Old Red sandstone, dipping westward, creating parallel ravines oriented at right angles to the shoreline.

• The east wall of each ravine has a slope of about 30 degrees to the horizontal, while the west wall is almost vertical, varying in height from 2 to 6 ft.

• Loose pieces of bostonite rock occurred in the ravines.

• Detailed studies were made in three areas within a 50-ft stretch of this shore.

• Area 1 was on the vertical west wall of a ravine, with study plots at four levels where barnacles were mapped in small squares (approximately 25 cm2 each) marked by drilled pits at the corners.

• Some squares were caged with stainless steel wire netting to protect barnacles from predators.

• Wave action was moderate, but the alga Alaria esculenta indicates fairly strong wave exposure, while Ascophyllum nodosum occurred in sheltered parts of the ravines.

  • Fucus spiralis and Fucus vesiculosus were dominant algae in Areas 1 and 2, and Gigartina stellata in Area 3.

• Two flat stones of bostonite, about 4 inches in diameter, were fastened at the "middle" level (2.1 feet above mid-tide level) and numbered 7 and 8; these could be removed to follow the spring settlement pattern.

• Area 2 consisted of two pairs of removable stones at different levels (1 ft above and below mid-tide level) in a ravine, one facing upward and the other downward.

• Area 3 consisted of four boulders, each about 3 ft in diameter. Squares were mapped on the sloping tops of these boulders.

• The three areas spanned almost the entire intertidal distribution of Balanus balanoides.

• Stones and cages were fastened with stainless steel screws in plastic tubes drilled into the rock. Cages were made from stainless steel wire netting (60% open area).

• "Covers" (netting stretched out from a cage) were used to create a similar physical environment but allow access to predators.

• Surface area for population densities was the rock surface projected onto a plane surface.

• Population density of the youngest age group was calculated on the "available area" for that group.

• Mapping was initially done on graph paper, but later a glass slide was used to mark barnacle positions with glass-marking ink.

• For the first six months after settlement, counts were made on a small portion of each area, using yellow water color paint.

• In the first census (November 1952), every individual was mapped, and the settlement of that year was distinguished from the rest by its smaller size and shell appearance.

• Above mid-tide level, barnacles aged 6 months had thin shells with uneroded parietes, while older groups had thick, rounded, and eroded shells.

• Growth in barnacles occurs along the basal and lateral edges of each wall plate while erosion of the top edge is not replaced.

• Copies of the original data are available at the Bureau of Animal Population, Department of Zoological Field Studies, University of Oxford, Oxford, England.

SETTLEMENT

Methods

• To determine rate of attachment and mortality, individual cyprids were followed by mapping new settlements daily on detachable stones.

• Every other low tide, stones were examined under a dissecting microscope.

• Changes in attachment, losses, and metamorphoses were marked on an enlarged map of the area.

• Stones were returned before the tide rose and kept outside on a window ledge to mimic natural conditions.

• Mortality from the examinations was estimated to be small (5%).

• Each cyprid as assigned a grid number, metamorphosis and death were recorded.

• Fate of about 8,000 individuals was followed over three seasons.

• Temporary clinging may have been mistaken as mortality.

• Temporary clinging occurred rarely, usually due to the position or side-lying cyprids.

• Loss of cyprids between examinations seemed minimal.

• Cyprids removed in dishes of seawater could metamorphose and reattach in the laboratory.

Pattern of settlement

• Settlement patterns involved an initial period of low, increasing attachment rates, followed by a period of high attachment rate, and finally a return to a low rate.

• This could be attributed to individual variations in nauplii development speed following a sudden massive liberation of larvae.

• Settlement rate could also be proportional to numbers already attached.

• The gradual decrease in settlement rate as the maximum density is approached is explained not by gradual diminution in plankton numbers.

• Settlement rate decreased on Stone 1 in 1953 long before numbers of planktonic larvae had decreased.

• Cyprids were available in plankton in great numbers, cessation of settlement was not due to an inadequate supply of larvae.

Behavior of attaching cyprids

• First cyprids attached in hollows and concave portions of the surface.

• Preference for grooves and depressions has been described in detail.

• Loss of an individual followed by a new attachment on the same spot, usually in the next few days, showed a bare space is the only requirement for a cyprid to attach.

Variations in settlement

Variation in rate of settlement on closely adjacent areas

• Daily counts were compared separately, as a "matched pair" of values analyzing by the Wilcoxon matched-pairs test.

• Significant local variation occurred in three of the six comparisons.

• Variations occurred in both 1954 and 1955 but on different pairs of stones in each year.

• The most likely cause of is a difference in surface contour between the two areas.

• The surface irregularities may have resulted in the squares of the pair being of slightly different surface area.

Differences in settlement between shore levels

• An analysis of variance was made of this data, between and within levels, and a significantly greater difference p = .05 was found between the levels.

• Effect of surface texture may be ruled out.

• Explanation for delay in settlement on large bare surfaces is larvae of this species and of other sessile marine animals are stimulated to settle by the presence of individuals of their own species.

Variations in settlement within one season

• Three maxima in intensity of settlement at Liverpool, spaced 16 and 12 days apart, two at times of spring tides, one at neaps.

• Pyefinch (1948b), sampling from a pier during the 1947 season sampling from a pier during the 1947 season .at Millport, found that the proportion of total larvae represented by each larval stage varied in a regular manner.

• Each stage showed three maxima, spaced 15 and 18 days apart.

• Successive liberations of nauplii from the parent stock can cause variation.

• Crisp & Davies (1955) have shown that the same individual of Elminius modestus may produce several successive broods in one· season.

Variation in the sizes of cyprids settling early and late in the settlement season

• Cyprids measurements of newly metamorphosed individuals confirm the earlier cyprids are much larger.

• The measurements of the individuals which attached in the first week were actually made during the second week; even so they were much smaller than most of the ones settling that week.

• The cyprid measurements were grouped into 110 mm classes and the average sizes of the cyprids and the barnacles which had metamorphosed from them were computed.

• Barnes (1953a) found that measurements of cyprids of Balanus balanoides taken from the plankton showed two modal sizes.

• Stage I nauplii removed from adults aged 1 yr collected at a low intertidal level were slightly smaller than those from adults of the same age at a high level.

Annual variations

• Cyprids usually first appeared in early April, although settlement time varied.

• Barnes (1956) found that the phytoplankton bloom which normally develops in early March failed to do so in 1954.

MORTALITY DURING SETTLEMENT

Differences in mortality between the early and late settlers

• Barnacles which attached in the early weeks appeared to survive better than those attaching in the later weeks.

• This is particularly clearly shown by the good survival of the barnacles of the second week in all three years; the slope of the curve indicates the relative mortality rate.

Mortality and weather

• Precise date of death of a cyprid was usually harder to decide than that of a metamorphosed barnacle, so that it would have been difficult to determine any relationship between the mortality of cyprids and daily changes in the weather.

• The effect of gales on metamorphosed barnacles was also shown in another analysis.

Mortality associated with other animals, including intraspecies relationships

• Broken barnacles were observed associated with regular grooves, which appeared to be radula marks, on the surface.

• Those spat which settled in pits or among older barnacles survived.

• An indirect effect of Patella on Balanus has been suggested by Southward (1956).

Factors Determining the Population Density 4t The End of the Settlement Season

• The density at the end of the settlement season is the difference between the numbers which attached and those which died during the season.

• In some instances, it was obvious that a low settlement density was the result of a limited supply of larvae from the plankton.

• Bousfield (1955) found that Balanus balanoides colonized only the lower portions of a large estuary in eastern Canada because the planktonic larvae remained near the surface where they would be carried seaward.

MoRTALITY FROM CROWDING

Factors Determining Crowding

• Crowding will refer to the process in which barnacles grow while in contact with each other.

• The degree of erowding is thus determined by the rate of growth, population density and average size.

• Growth is more rapid when there is more water movement, such as on wave-beaten points or in tidal currents

• Moore noted that barnacles packed closely in a groove grew more slowly than adjacent isolated individuals.

• At the same age, barnacles at the higher densities were larger than those growing at low density.

Physical Effects of Crowding

• At low levels, or at higher ones where water movement was very rapid, barnacles grew very rapidly; at moderate population densities the barnacles had no room to expand laterally, so grew upward, developing into cylindrical or trumpet shapes.

• This elongated condition is shown on stone 2 in Fig. 7, where it is compared with an area of lower density on stone 1 where most of the barnacles were shaped normally.

Mortality Associated with Crowding

• During the first year of life the survival curves often showed a decrease in the relative mortality rate in the winter (Figs. 9, 10, 16, 17), at the time of decrease in the growth rate.

• However, the later survival of the barnacles of this week was poorer than that of any of the other weeks; except on Stone 1, they were all dead within six months.

• For those areas at the same level which had higher population densities at the end of settlement, and had thus experienced greater crowding, the mortality during the winter was also higher.

• From all these studies it appears that while the density observed at the end of the settlement season may occasionally be severely limited by the supply of planktonic larvae at Millport it was usually determined by occurrences on the shore, there being a vast oversupply of larvae.

MORTALITY AT HIGH SHORE LEVELS

The Upper Limit of Balanus Balanoides Distribution

• Near HWN the upper limit of distribution of Balanus balanoides merged into the bottom of the narrow Chthamalus stellatus zone.

• Little crowding occurred at this level since the population density was low and growth slow.

• At this level on the shore, where some individuals survived for at least four years, one year class outnumbered the rest.

An Unusual Occurrence of High Mortality in the Upper Shore Levels

• Sometime between the censuses of mid-March and early June, 1955, great mortality occurred at and above the upper level of Area 1, as shown in Figs. 12 and 16.

• Measurements on Area 1 indicated that the height of the upper level was 8.6 ft above Ordinance Datum, and that the vertical amplitude of the waves was 2.5 times that recorded by the tide gauge.

• The records from March 1954 to May 1955 were examined and all high tides which did not reach 8.6 ft were noted.

PREDATION BY THAIS LAPILLUS

On Balanus Balanoides

Population Density of Thais

• At each visit to the study area, counts were made of the numbers of Thais on certain areas, such as the boulders of Area 3.

• At each level more Thais occurred on the seaward side.

• The density at the highest level was markedly less than that lower down, but there was little difference between the other levels, except that the seaward side of the lowest level supported a much higher density than the others.

• The winter has the lowest densities.

Movements During Feeding

• Pattern was a series of short movement followed longer one.

• Whelks did most of they're feeding on sedentary periods.

• The average density of Thais at various levels on Area 1 is given in Table 14; each level was divided into seaward and landward halves.

Rate of Feeding of Thais

• In order to do this, whelks were enclosed in cages with barnacles which were counted periodically.

• An adjacent empty cage served as a control.

• The physical conditions were similar in the experimental and control cages.

Effects of Predation by Thais on The Balanus Population

• Cages were set up at three heights to get rid of preadtors.

Factors Determining the Population Density 4t The End of the Settlement Season

*Light Set 1958.

Discussion and Conclusions

• Rate of activity, as represented by cirral beat, was studied by Southwar
  The intertidal zone of Balanus balanoides is thus bounded by physical factors at the top and biotic ones at the bottom, with fluctuations in mortality caused by these factors being greatest at the limits and decreasing toward the mid tide region. Thus, that the extrinsic and intrinsic causes of mortality in barnacles cannot be separated.