Energetics of Chiton pelliserpentis (Quoy & Gaimard, 1835) (Mollusca: Polyplacophora) and the importance of mucus in its energy budget

J. Exp. Mar. Biol. Ecol., 1986, Vol. 101, pp. 119-141 Elsevier

119

JEM 00736

Energetics of Chiton pelliserpentis (Quoy & Gaimard, 1835) (Mollusca: Polyplacophora) and the importance of mucus in its energy budget Peter Zoology

(Received

Department,

12 March

University

1985; revision

L. Horn* of Canterbury,

received

Christchurch,

New Zealund

8 May 1986; accepted

29 May 1986)

Abstract: Energy budgets are reported for high- and low-shore groups of the chiton Chiton pelliserpentis (Quoy & Gaimard, 1835) from a sheltered shore, Kaikoura Peninsula, New Zealand. Annual energy flow through the high-shore group (532 kJ m _ a) was about half of the low-shore flow (113 1 kJ m 2). Most components of the budget were measured; i.e. growth production, reproductive production, production of mucus, respiration, defaecation, excretion and consumption. Previous studies of molluscan energetics have usually ignored mucus and excretion, and calculated consumption by summing all other terms. However, mucus appears to be a major component of molluscan production. The largest single component of the budgets of both chiton groups was production of mucus which accounted for 74 and 66% of assimilated energy on the high and low shores, respectively, whereas respiration (generally assumed to be the largest component of energy expenditure by animals) accounted for only 21 and 29% of assimilation, respectively. No significant differences in utilisation of energy between the two groups were apparent despite considerable differences in population structure. High-shore chitons exhibited adaptations enabling them to maintain consumption in the face of reduced available feeding time, reduce metabolic costs in the face of exposure to greater environmental temperatures, and reduce losses of energy via egestion, and hence maintain a maximal rate of growth. Key words: Energetics;

Polyplacophora;

Chiton pelliserpentis;

mucus;

adaptations

INTRODUCTION

Many studies of molluscan

energetics have been reported

previously

(e.g. Hughes,

1970,

1971a, b; Paine, 1971; Workman, 1983). The primary product of such studies is the energy budget and the basic equation for the components of an energy budget is as follows: C=P,+P,+P,+R+F+U, where C is the energy content of the food consumed; P, is the energy content of somatic tissue added to the population by growth, recruitment and migration; P, is the energy content of the gametes liberated during spawning; P, is the energy content of produced * Present address: Fisheries Box 2034, Napier, New Zealand. OO22_O981/86/$O3.50

Management

@ 1986 Elsevier

Science

Division,

Publishers

Ministry

of Agriculture

B.V. (Biomedical

Division)

and

Fisheries,

P.O.

P.L. HORN

I20

mucus; R is the energy lost as a result of respiration; F is the energy content of the faeces; and U is energy lost as urine, dissolved organic matter, and other exudates. The energetics of the major gastropod grazers of various marine communities have been studied (e.g. Paine, 1971; Wright & Hartnoll, 1981). However, chitons have received little attention in this regard mainly because they are major grazers in only a few ecosystems. In New Zealand, however, the chiton fauna is numerous and diverse, and one species, Chiton (Sypharochitun) pelliserpentis (Quoy & Gaimard, 1835), is common on many rocky shores. The present study aimed to construct comprehensive energy budgets for C. peIIi~e~enti~by measuring most components directly, and hence, investigate the impo~~ce of mucus in the energetics of this chiton, Separate budgets were constructed for high- and low-shore groups of C. pelli~e~e~t~ as these groups exhibit different physiological adaptations to environment (Horn, 1982, 1985) and different age-structures and size-frequency distributions (Boyle, 1970). Hence, energy budget differences may be anticipated. The study area was Mudstone Bay, Kaikoura Peninsula, New Zealand (173”41’10”E : 42”25’3O”S), a shore sheltered from strong wave action, where C. pelliserpentis occurred throughout the intertidal zone. “Low-shore” chitons were distributed between low water neap and extreme low water spring in a boulder-strewn channel. “High-shore” chitons were between mean sea level and high water neap and occupied small crevices and depressions in gently sloping platforms.

METHODS DETERMINATION

OF ACTIVITY

PATTERNS

To calculate consumption, a knowledge of activity patterns is required. Activities by chitons were categorised using the simple criteria of “moving” or “not moving”, and “feeding” or “not feeding”. The two chiton groups were observed during January, April, July and October 1982, during daylight and darkness, and when the animals were immersed, emersed on wet rock, and on dry rock, There were thus six time-state categories. For each category, at least 20 animals were observed for a minute (i.e. 20 min of observations) and the propo~ions of time spent moving and/or feeding were noted. Feeding was detected using a microphonic sensor (Boyden & Zeldis, 1979) held for 1 min on the rock surface less than 50 mm from each chiton. GENERAL

Chitons from the study areas already described were used to obtain data on population structure, mortality, feeding rates and activity. For all other aspects of the study, chitons were taken from shores in Mudstone Bay experiencing similar physical conditions to the specified study areas, but not immediately adjacent to them. This prevented any unnat~~ disturbance of population structure.

ENERGETICS

In laboratory after rinsing content

experiments,

dry weights of somatic tissue and gonad were determined

in fresh water and drying

of tissue was determined

weights were obtained of animal and plant calorimeter.

Material

at 75 “C to constant

MEASURING

after heating in a muflle furnace

containing

ash

at 500 ‘C for 4 h. All

more than 70% ash was mixed with an equal weight and a correction for endothermy was were obtained from the Kaikoura

station, which is 1.2 km from Mudstone

RATES

weight. Percentage

using a Mettler balance accurate to + 0.1 mg. Calorific content matter was determined using a Parr No. 1411 combustion

of benzoic acid to ensure complete combustion, applied (Paine, 1966). Meteorological data meteorological

121

OF A CHITON

Bay and 100 m above sea level.

OF CONSUMPTION

Rates of consumption of microflora by grazing molluscs have proved difficult to measure (e.g. Southward, 1964; Wright & Hartnoll, 1981). In this study, an estimate of consumption was obtained by measuring the area of substratum cleared by each radular rasp, the radular rasping rate, and the energy available per unit area of substratum. To measure area cleared per rasp, microalgae were cultured on Perspex plates by holding them for a week in flowing sea water in an aquarium. (This method assumed that chitons would clear a similar area per rasp on Perspex as they would on rock.) Ten chitons of different sizes were allowed to graze overnight on the cultured algae plates (one chiton per plate). The plates were then rinsed with tap water, dried, and the sizes of the cleared areas were measured using a stereoscopic microscope fitted with a micrometer eyepiece. The cleared areas were nearly elliptical, so the equation, A = 7cab/4,where a and b are the longest and shortest axes of the cleared areas, was used to calculate the area A. Each side of the radula creates a cleared area, so area A was doubled to give the area cleared per rasp. At least 30 cleared areas per individual were used to calculate a mean area cleared, and this area was related to animal length. Consecutively cleared areas usually overlapped. The mean proportion of overlap (approximated by a circular area and measured using a micrometer eyepiece) was calculated from 115 individual rasps. The mean radular rasping rate (based on three 30 s periods

of continuous

rasping)

was determined in the field using the microphonic sensor described previously (Boyden & Zeldis, 1979). At least 12 animals of different sizes were used on each occasion. Size-related radular rasping rates were obtained for the high-shore group at water temperatures of 12, 14 and 17 ‘C, and for the low-shore group at temperatures of 9, 12, 14, 17 and 18 o C. These values covered the annual range of sea-water temperatures experienced by chitons. The food energy on the substratum available to the chitons was estimated each month by collecting a minimum of six rocks from both low- and high-shore habitats. These were dried and a known area (from 210-300 cm’) of their surface was scraped lightly with a blunt knife. Scrapings were taken from the upper and lower surfaces of all rocks. The

122

amount

P.L. HORN

of’organic

matter in the scraped

value was converted calorimetry

material

was determined

to organic matter per unit area of substratum.

to determine

the energy content

by ashing, and this An attempt

of the organic matter was unsuccessful

to use due

to the very small proportion of organic matter relative to the amount of benzoic acid that had to be added to ensure combustion. Therefore, it was assumed that marine diatoms

and detritus

made up the major

component

of the organic

portion

of the

scrapings, and that this organic material had a calorific value of 21 J . mg- ’ ash-free. This value is probably a satisfactory compromise between diatoms with at most 23 J . mg- ’ (Paine & Vadas, 1969; Platt & Irwin, 1973) and detritus with at least 18 J . mg ’ (the value of cellulose, Southwood, 1978). Total consumption was calculated taking into account monthly changes in both water temperature (and hence, in radular rasping rate) and population structure.

MEASUREMENT

OF GROWTH

PRODUCTION

Production of a population due to growth is made up of two components: the change in non-gonadal biomass of the population resulting from growth and recruitment during the time in question (AB), and non-gonadal biomass lost as a result of mortality and migration (E) (Petrusewicz & Macfadyen, 1970). In this study, AB was measured once over a time interval of 1 yr. In January 1982, the body lengths of chitons were measured in 33 randomly placed 1 m2 quadrats at the high-shore site and in 6 quadrats at the low-shore site. This process was repeated in January 1983, this time sampling 50 high-shore and 15 low-shore quadrats. Regression lines relating body length to dry weight of somatic tissue for both groups were used to convert these quadrat measurements into chiton biomass. As the shell-valves of chitons also contain organic matter in the form of protein and chitin (Hyman, 1967) this was estimated by dissolving the inorganic components of the valves from 30 chitons of known body length in 10% HCl for 48 h and weighing the dried residue (Hughes, 1970). Shell-valve organic matter was assumed to have an energy content of 23.9 kJ . g- ’ (Paine, 1971). Total energy content of chiton somatic tissue, including shell organics, per square metre was calculated and the difference between the values obtained in 1982 and 1983 was taken as AB. Determination of growth curves for both chiton groups was a prerequisite for estimation of the elimination (E) components. Growth-check lines were visible externally and on polished cross-sections of the chiton shell-valves, and were shown to be laid down annually by monitoring individuals for a year. Annual growth-checks have been validated for at least one other chiton species (Baxter &Jones, 1978). The lengths and inferred ages of 132 chitons from each group were determined in December 1981. Mean lengths of each year-class were calculated and fitted to the von Bertalanffy growth equation, 1, = L,

- (L,

- Lo)eeK’,

ENERGETIC!3

where 1,is the estimated theoretical

maximum

OF A CHITON

body length at age t, L,

123

and L, are constants

length and length at zero age respectively,

related to the rate of approach

representing

the

and K is a constant

to length L,.

To calculate E for each group the body lengths of 300 randomly chosen chitons were measured in January 1982 and January 1983 and plotted on separate length-frequency histograms.

Using the growth equation,

all chitons

in the histograms

were placed in an

age-class (e.g. all chitons between 20 and 25.6 mm in length were designated age-class 2, all those between 25.6 and 31 mm as age-class 3, and so on). The change in area on the histograms between age-class x in 1982 and age-class x + 1 in 1983 gave an estimate of the net annual change in biomass attributable to mortality and migration for chitons of that age. By averaging over all age-classes, a mean annual mortality-migration factor was obtained MEASUREMENT

for both chiton groups. OF REPRODUCTIVE

PRODUCTION

C. pelfiserpentis usually spawns once a year (Johns, 1960). Therefore, if the calorific value of gonads after spawning is deducted from that before spawning, annual P, for each chiton group can be determined. To describe the reproductive cycle of C. pelliserpentis, the gonads of at least 35 chitons were examined each month, and the mean percentage of shell-free dry body weight made up by gonad (the gonad index) was calculated for males and females. The calorific value of male and female gonad was measured at four stages of the cycle (pre-spawning, January; spawning, March; spent, May-August; developing, October-December) to establish whether these values fluctuated seasonally. Reproductive production lost through mortality was estimated by taking the mean annual gonad index, multiplying by the mean biomass of the group, then multiplying by the previously obtained mortality proportion. MEASUREMENT

Respiration

OF RESPIRATION

in air was measured

RATES

using compensating

respirometers

(Southwood,

1978), while respiration in water was calculated using the Winkler technique (Strickland & Parsons, 1972). Full details are given in Horn (1985). Aquatic respiration was measured at five temperatures (9, 11, 13.5, 15.5, 18 “C) and respiration in air was measured at six temperatures (5, 9, 13.5, 17, 21.5, 27 “C) which covered the range experienced by most of the animals in the field. Total oxygen consumed by each chiton group was calculated taking into account activity patterns and monthly changes in population structure and temperature. The weight : respiration regression line used in any particular month was that obtained at the experimental temperature closest to the mean monthly air or water temperature (e.g. in May the mean air temperature was 12.3 “C so the aerial respiration curve obtained at 13 “C was used). Metabolic energy output in J . ml ’ 0, at STP was calculated by multiplying oxygen consumed during respiration by an oxycalorific coefficient of 20. RQ values were

124

P.L. HORN

measured

for four chitons

indicating

a coefficient

MEASUREMENT

in air at 13 “C and gave a mean value of 0.81

of about 20 J. ml-

OF DEFAECATION

' 0, (Southwood,

=

O.Ol),

RATES

It was not possible to collect the entire faecal production cycle in the field. However,

(SD

1978).

chitons

of an individual

would feed in the laboratory

over a tidal

and the faeces could

be collected (Horn, 1984). At 2-mth intervals, at least 80 chitons were taken from the shore and divided into groups comprising four to nine individuals of about the same size. They were allowed to adhere to a rock recently removed from their habitat (one group per rock), and each rock was placed in a 2-l plastic container with a hole covered by a 0.25 mm mesh gauze in its base. The containers were covered with 1.5 mm mesh gauze and placed on racks in a tidal aquarium (modified from Ottaway, 1975) at a height where the chitons experienced typical periods of immersion and emersion as determined from field observations. The gauze-covered hole in the container base ensured that water could flow in and out, but that faeces were not lost. Each trial was run for 2 days; faeces were collected at each simulated low tide and dried to constant weight. Body lengths of all experimental animals were obtained, and the mean length of animals in each container was regressed against the mean dry weight of faeces produced per animal per day. Energy and ash contents of faeces were determined for each 2-month sample to establish whether these values fluctuated seasonally.

MEASURING

THE PRODUCTION

To measure

production

RATE OF MUCUS

of mucus,

nine

chitons

of different

sizes were placed

individually on pre-weighed glass plates, and the distance they moved was recorded. The plates were then rinsed with distilled water, air dried and reweighed. No measurable weight gain (> 0.1 mg) was recorded in eight of the nine trials. However, it was noted that chitons removed from the substratum after being stationary for several hours had secreted a thin film of mucus. Therefore, 14 chitons of different sizes were placed on pre-weighed microscope slides and their movement was restricted so they soon became attached firmly. After 8 h, the animals were removed and residual mucus adhering to each chiton’s foot was scraped off lightly with a smooth glass rod and transferred to the glass slide. The slides were then rinsed, dried and weighed. Increase in weight was assumed to represent dried mucus. As insufficient dried mucus was collected for bomb calorimetry, a calorifrc value of 23.97 kJ . g- ’ from Calow (1974) was used in calculations. MEASUREMENT

OF EXUDATES

To measure nitrogen excretion rates, chitons were collected in April, July and December from the shore just before tidal immersion and placed immediately into 230-ml plastic jars containing 200 ml of aerated sea water. Chitons were removed from

ENERGETICS OF A CHITON

125

the jars after 6 h, and their dry weights were determined. Duplicate samples (0.2 ml) of water from each jar were taken to determine quantities of ammonia nitrogen and urea nitrogen by microdiffusion (Conway, 1947). Results obtained from analyses of seawater controls run simultaneously were subtracted from the experimental values. It was assumed that chitons release dissolved organic matter only when immersed (i.e. about 6 h in each tidal cycle for high-shore chitons, and 10 h for low-shore chitons). Hence the results obtained for low-shore chitons were multiplied by 10/6 when calculating energy budgets. Energy values used were those given by Elliott & Davison (1975) (24.86 J . mg- 1 ammonia nitrogen, 23.06 J. mg- ’ urea nitrogen). This approach, while quantifying two nitrogenous compounds, may have left some components unidentified (e.g. dissolved organic carbon and phosphorous). However, it seems unlikely that the energy budget of a chiton should differ so radically from that of the marine gastropod ZZyuna~~aobsoletu as to make dissolved organic matter anything more than a small part of the total budget. Total excretion by I. obsoleta accounted for only 1.5% of assimilated energy (Edwards & Welsh, 1982).

RESULTS ACTIVITY

PATTERNS

Activity pattern data showed that high-shore chitons fed at night only, usually when immersed, although some fed when emersed on wet rock (Table I). On average, highshore chitons fed for 4.8 h each day, although feeding time could be up to an hour longer in winter and an hour less in summer due to seasonal changes in night length. Each day, high-shore chitons moved during z 50% of their 11.5 h immersion time, and 13 % of their 12.5 h emersion time. Most chitons moving during daylight or on emersion were found to be not feeding, but simply returning to their home sites. Low-shore chitons fed for about 5.6 h each day and the length of this feeding period did not vary much seasonally as approximately equal numbers of animals fed in daylight and darkness (Table II). Most feeding occurs during immersion. Low-shore chitons move during z 80% of their 19 h immersion and 33 % of their 5 h emersion time. During the 3 months for which complete data are available, high-shore chitons fed during 79.6% (SD = 0.83) of the time they were moving while immersed, whereas animals from the low shore spent a significantly shorter portion (33.2%, SD = 2.41) of their active immersion period feeding (t-test for independent samples, P < 0.001) (Steel & Torrie, 1980). It appears, therefore, that high-shore chitons exhibit a behavioural adaptation which enables them to maintain an adequate level of consumption in the face of a shorter period when feeding can occur.

P.L. HORN

126

TABLE I Activity patterns

for high-shore chitons showing the proportions (as a percentage that were moving (M) and feeding (F): nd, no data.

of a sample of n chitons)

Daylight Situation Immersed

Emersed on wet rock

On dry rock

Darkness

Month

n

M

F

n

M

F

January April July October

20 25 28 21

5 4

0

20 21 nd 26

95 90

76

93

77

January April July October

25 25 25 25

0

28 25 nd 25

38 38

I 8

40

January April July October

25 25 25 25

0

25 25 25 25

0 0 0 0

0 0 0

0 0 0

0 0 0

80

TABLE II Activity patterns

for low-shore chitons showing the proportions (as a percentage that were moving (M) and feeding (F): nd, no data.

of a sample on n chitons)

Daylight Situation

Darkness

Month

n

M

F

n

M

F

January April July October

30 20 35 29

80 80 71 16

27 25 26 23

27 20 nd 22

78 85

30 25

II

21

Emersed on wet rock

January April July October

38 37 34 39

41 42 32 28

8 11 6 8

30 25 21 30

40 48 48 53

10 12 14 13

On dry rock

January April July October

50 49 44 50

0 0 2 0

35 33 33 20

0 3 0 0

0 0 0 0

Immersed

CONSUMPTION

Chiton pefliserpentis were observed to graze primarily on microflora. However, an examination of faeces and field observations of grazing animals showed they fed also on macroalgae. Although observations indicated that < 5 yO of feeding time is spent in that activity, measured consumption would have been underestimated as more energy per rasp would be obtained from macroalgae relative to “bare” rock.

ENERGETIC3

OF A CHITON

127

In both chiton groups, radular rasping rate increased with increasing temperature, and decreased with increase in body size. Comparisons of high- and low-shore rasping rates showed no significant differences at similar temperatures between groups (ANCOVA, all P > 0.05), hence data obtained on both shore levels were combined. The relationship between rasping rate (R,rasps per min), body length (L, mm) and temperature (T, “C) was best expressed by the multiple regression equation:

R = - 0.224L + (1.795T - 2.394). Area cleared per rasp (A, mm’) was related linearly to chiton body length (L,mm) by the following equation :

A = 0.0088L - 0.044, r = 0.977. This equation has been corrected to account for a mean overlap of consecutively cleared areas of 30% (n = 115, SD = 7.2%). Biomass of ash-free organic matter on the high and low shores showed no seasonal trends. Mean values ( f SD) were 7.30 f 2.80 g * mm- 2 on the high shore and 7.92 + 2.07 g. mme2 on the low shore. Assuming a caloritic value of 21 J . g ~ ’ organic matter, then 0.153 J. mmm2 and 0.166 J. mmP2 were present on the high and low shores, respectively. Calculated monthly consumption was about three times higher in February (68 and 212 kJ . mP2 on the high and low shores, respectively) than in August (22 and 76 kJ . m- 2, respectively). This variation was due mainly to differences in water temperature, and hence, in radular rasping rate. Total annual measured consumption (C,) by the high-shore chitons was calculated to be 471 kJ . m- 2, and for the low-shore group, 1521 kJ .rnA2. GROWTH

PRODUCTION

Length-frequency histograms are presented for both chiton groups (Fig. 1). The size structure of the low-shore group was strongly biased in favour of smaller animals and mortality appeared to be high (as indicated by the marked decrease in frequency with increasing body length). The virtual absence of a first year cohort in January 1983 indicates that recruitment in 1982 was only about 15y0 of that which occurred in 1981. High-shore chitons were generally larger than those on the low shore (Fig. 1). Of 300 chitons measured in January 1982, 15 were in the year 0 size range and 55 were in the year 1 size range. In January 1983, there were 33 and 81 chitons in the year 1 and 2 size ranges, respectively. The increase in frequency in these two cohorts over 1 yr suggests that this group is maintained by immigration from the low shore, rather than by recruitment on the high shore. Growth of both chiton groups was almost identical. The constants of the von Bertalanffy equation for low-shore chitons were L, = 48.7 mm, L, = - 0.8 1 mm and

128

P.L. HORN

0.197, and for high-shore chitons they were L, = 49.45 mm, L, = 0.45 mm and K = 0.245. measured onus growth increments of five low-shore individ~~s monitored in a tidal aquarium, and six homing high-shore chitons monitored on the shore, agreed well with the growth curves, and hence, support the method used to age the chitons.

K =

0. 25 20 15 10 5 30 20 Body length (mm1

40

50

Fig. 1. Leach-frequency histograms for the high-snore group in January 1982 (A) and January 1983 (B), and the low-shore group in January 1982 (C) and January 1983 (D): for each histogram, n = 300.

The correlation of peaks in the length-frequency histograms calculated a year apart (Fig. 1) also supports this ageing technique. The calorific value of somatic tissue of male and female chitons from both groups varied throughout the year (Fig. 2). On the high shore, highest calorific values of somatic tissue were recorded at the times of peak gonad index, and lowest values immediately after sealing. Conversely, on the low shore, somatic tissue calorific value and gonad

ENERGETIC% OF A CHITON

,’ ,’

,’ ,’

\

129

130

P. L. HORN

index appeared to be inversely related. The calorific value of immature animals (those for which sex could not be determined) from both shore levels was less than that of mature chitons and showed no seasonal trends. Hence a mean value of 17.8 kJ. g- ’ ash-free (n = 13, SD = 0.45) was used in calculations of P, for all chitons < 1 yr old. Relationships between chiton body length and three weight variables (shell-free dry somatic tissue weight, dry shell weight, dry weight of shell organic matter) are listed in Table III. Data obtained from the quadrat surveys of chiton density are listed in Table IV. The estimate of annual mortality and emigration for the low-shore group was 34.3%. For the high-shore group, the estimated mortality-immigration factor was 22.9%. The composition of P, for both groups is shown in Table V.

TABLE III Calculated relationships between chiton body length (L, mm) and three weight variables: shell-free dry somatic tissue weight ( W, g), dry weight of shell-valves (S, g), and dry weight of shell-valve organic matter (P, g). Chiton High Low High High

group

Equation

shore shore and low and low

55 55 105 30

log, W = 2.46 log, L - 9.5 1 log, W = 2.38 log, L - 9.25 log, s = 3.43 log, L - 11.91 log, P = 0.077 L - 6.234

0.975 0.981 0.982 0.972

TABLE IV Chiton

density, somatic tissue weight, and shell organic in January 1982 and January 1983: standard

Chiton

group

1982 1983

Low shore Low shore

1982 1983

Somatic tissue weight (g m - ‘)

Shell organics

(9.0) (8.9)

3.102 (3.486) 3.439 (3.694)

0.213 0.237

34.8 (19.1) 26.3 (17.3)

5.058 (2.581) 4.231 (2.596)

0.377 0.306

Density (no. .rn-a)

Year

High shore High shore

weight, per square metre for both chiton groups deviations are given in parentheses.

9.1 8.7

(g.mm2)

TABLE V Energy

of growth

production

(P,= AB + E) for both chiton groups: all units are kJ m ~’ yr ’ E

AB

Chiton

group

High shore Low shore

Somatic tissue

Shell organics

Somatic tissue

Shell organics

2.99 - 6.98

0.32 - 1.12

13.71 39.82

1.56 4.5 1

p, 18.6 36.2

ENERGETICS REPRODUCTIVE

131

OF A CHITON

PRODUCTION

The sex ratios of the two chiton groups were not significantly different from 1 : 1 (X’test, P > 0.05). Of 864 low-shore chitons, 53.5 y0 were males, whereas 47.7 y0 of the 792 high-shore chitons were males. These percentages were used in the estimation of reproductive production. The annual cycle of gonad indices showed that both sexes spawned simultaneously, and that spawning was complete by early May (Fig. 3). High-shore males released 6.5 y0 of their dry body weight as gametes, and females 5.9%. Corresponding percentages for low-shore chitons were 7.9% and 5.0%. Caloritic values of whole gonadal tissue at four distinct periods of the reproductive cycle are shown in Table VI. Ovaries exhibited a far greater calorific increase with maturation than testes. Reproductive production, made up of spawned gametes and gametes lost through mortality, is listed in Table VII.

TABLE VI

Energy values of gonads (kJ . g- t ash-free) for January (immediate pre-spawning), March (mid-spawning), May to August (resting), and October to December (development): each value is the mean offour replicates, with standard deviations in parentheses. Chiton

group

Period

Male

Female

High shore

January March May-August October-December

24.66 (2.51) 24.58 (0.61) 24.45 (1.43) 23.67 (2.62)

26.79 26.58 24.34 25.62

(1.73) (0.80) (1.83) (0.95)

Low shore

January March May-August October-December

24.67 23.54 24.51 23.05

26.19 26.60 24.10 24.35

(1.91) (1.97) (0.68) (3.22)

(0.55) (1.06) (0.97) (1.80)

TABLE VII

Reproductive

Chiton group

production

(kJ mm2), showing the contributions through mortality. Contribution

made by spawned

gametes

and gametes lost

Female

Male

Totals

High shore

Spawned Mortality Total P,

gametes loss

2.5 0.3

2.2 0.3

4.7 0.6 5.3

Low shore

Spawned Mortality Total P,

gametes loss

2.9 0.5

4.7 0.6

7.6 1.1 8.7

132

P. L. HORN

xapu!

pnool)

ENERGETICS

133

OF A CHITON

RESPIRATION

The relationship between oxygen consumption (R) and shell-free dry body weight ( W) can be expressed linearly as: log R = log a + b log W where a is a proportionality factor and b is the weight exponent. The parameters of this regression for each set of measurements of aquatic and aerial respiration are listed in Table VIII.

TABLE VIII Parameters respiration:

of the regression, log, R = log, a + b. log, W, for each set of measurements of aerial and aquatic R, oxygen consumption at STP (ml. h- ‘); W, shell-free dry body weight (g); r, correlation coefficient of regression; a and b are constants. High shore

Low shore

Temp. (“C)

a

b

Aerial respiration

21 21.5 17 13.5 9

0.134 0.147 0.123 0.089 0.072 0.039

0.654 0.760 0.784 0.698 0.875 0.677

Aquatic respiration

18 15.5 13.5 11 9

0.120 0.131 0.106 0.109 0.097

0.682 0.697 0.670 0.705 0.690

a

h

0.985 0.936 0.959 0.941 0.978 0.960

0.156 0.136 0.102 0.092 0.087 0.066

0.405 0.616 0.654 0.703 0.877 0.973

0.930 0.938 0.941 0.982 0.960 0.981

0.976 0.966 0.960 0.959 0.964

0.140 0.104 0.103 0.103 0.085

0.559 0.605 0.652 0.680 0.738

0.938 0.978 0.973 0.962 0.983

The effect of activity on respiration rates of chitons is unknown. All animals were stationary during the aerial respiration experiments, whereas in the aquatic experiments there was usually an initial short period of movement ( < 30 min). In many energetics studies, effects of activity have been ignored, but in others an arbitrary correction factor of x 2 has been applied to respiration during periods of movement (e.g. Carefoot, 1967; Trevallion, 1971; Wright & Hartnoll, 1981). A correction factor of this magnitude appears quite acceptable in the light of experimental evidence (Newell & Roy, 1973), and thus was applied to respiration for Chitonpelliserpentis during active periods. That is about 80 y0 of immersion time and 33 y0 of emersion time for low-shore chitons, and 50% and 13x, respectively, for high-shore chitons. Total annual energy equivalents for oxygen consumption are in Table IX.

134

P. L. HORN TABLE IX

Energy

losses via respiration

(R) in air and water; Aquatic respiration

Chiton group High shore Low shore

all units are kJ m -- ‘. yr Aerial respiration

61.0 260.0

‘.

Total

42.5 34.1

103.5 294.1

DEFAECATION

The relationship between body length and daily faecal production was best expressed as a log-log regression (Hughes, 1971a,b; Calow, 1975). The regression lines for each bi-monthly data set are plotted in Fig. 4. Faecal production by low-shore chitons was generally 50-100% greater than for similar sized high-shore animals at the same time of the year. For both groups, greatest faecal production occurred in autumn (March and

90 -

/MY /

60-

I IO

I 20 Body Length

I 30 (mm)

I 40

I 50

t 60

Fig. 4. Regression lines relating chiton body length to daily faecal production by high-shore ( -) and low-shore (--) chitons, measured six times during the year: for each line, n = 18; correlation coefficients (r) are given in parentheses below (high-shore, low-shore); Ja, January (0.83,0.77); Mr. March (0.72, 0.82); My, May (0.89, 0.84); Jy, July (0.69, 0.93); Se, September (0.65, 0.89); No, November (0.67, 0.88).

ENERGETICS

OF A CHITON

135

May), and it was least in spring (November). The defaecation rate of a chiton in May was generally about twice the rate exhibited in November. The organic content of faeces did not appear to be related to season, and the mean ash content of low-shore faeces (93.5 %, n = 18, SD = 1.64) was not significantly greater than that for high-shore faeces (92.8%, n = 18, SD = 2.81) (t-test, P > 0.05). However, energy content of faeces of both groups appeared to be seasonal. Peak values of 13-15 kJ . g ’ ash-free occurred during the winter (May to September) and minimum values (5-10 kJ . g- ’ ash-free) were recorded during summer (November to March). The mean calorific value of low-shore faecal organic matter (11.9 kJ *g - ’ ash-free) was not significantly greater than the mean high-shore value (10.2 kJ . g- ‘) (c-test, P > 0.05). Annual faecal production by high-shore chitons was 50.9 g * m - 2, and on the low shore, 185.5 g.mm2. The amount of mucus in faeces of C. pelliserpentis could not be measured, but studies of several aquatic gastropods have indicated that mucus can represent 6-19% of the organic dry weight (Glow, 1975; Kofoed, 1975; Edwards&Welsh, 1982). Descriptions in Bandel(1974) suggest that gastropod faeces generally contain more mucus than those produced by chitons. Hence, mucus in the faeces of C. pelliserpentis was assumed to comprise 8% of the organic residue dry weight in making further calculations. Faecal production minus mucus was estimated to be 34 kJ . m 2. yr ’ on the high shore, and 132 kJ . m - 2. yr - ’ on the low shore. PRODUCTION

OF MUCUS

Two components of mucus were estimated; faecal mucus, and trail mucus secreted by the foot. The dry weight of faecal mucus was assumed to be 8 y0 of the organic residue of faeces. On the high shore this amounted to 0.29 g. m- ‘. yr - ’ and had an energy equivalent of 7.0 kJ. On the low shore, estimated annual faecal mucus production was 0.96 g. m _ 2 and had a calorific value of 23.0 kJ. The dry weight of mucus secreted (M, mg * day ’ ) by a chiton of length L (mm) was best described by the following equation: log,M = 1.334 log,L - 3.167, Y= 0.783 . On the high shore, produced. Low-shore being 635 kJ. Hence, was 370 kJ . m - 2 on

15.13 g. me2 of mucus with a calorific value of 363 kJ were chitons produced 26.49 g * m - ’ annuahy, the energy equivalent estimated annual production of mucus (faecal and foot mucus) the high shore, and 658 kJ * m - 2 on the low shore.

EXCRETION

Excretion by chitons from both groups did not appear to be seasonally dependent (Table X), so annual excretory losses were estimated using the mean excretion rate. On the high shore, ammonia and urea losses amounted to 0.32 and 0.15 kJ m- 2,

P.L. HORN

136

respectively, and on the low shore, 0.61 and 0.20 kJ . m 2, respectively. Assuming that these two components made up only 50% of the nitrogenous excretion, as suggested by Nicol’s (1967) review, total excretory losses were estimated to be 0.9 kJ . m - 2 on the high shore, and 1.6 kJ . m - 2 on the low shore. It appears that excretion is not a significant element of the chiton’s energy budget.

TABLE X

Excretion of ammonia nitrogen and urea nitrogen (pg nitrogen excreted per gram dry weight of chiton per tidal cycle, standard error in parentheses): annual mean value (X) obtained by combination of all mdividual data points; nd, no data. Chiton

group

Temp. (“C)

Ammonia

nitrogen

Urea nitrogen

High shore

9.5 15.0 17.0

6.8 5.4 4.2 x = 5.5

(0.7) (1.5) (1.5) (1.1)

2.5 (0.7) nd 3.0 (0.9) X = 2.8 (0.8)

Low shore

9.5 15.0 17.0

9.5 5.7 8.8 x = 7.5

(2.0) (1.4) (2.3) (2.1)

1.3 (0.3) nd 3.9 (1.6) X = 2.6 (1.5)

TABLE XI

Comparison of annual energy budgets of individual high- and low-shore chitons with initial body lengths of 15 and 45 mm: numbers in parentheses are 95% confidence intervals; C,,,, measured consumption; C,, summed consumption (P + R + F + U); all other abbreviations are defined in the text; all values are in kJ. High shore Component

G? CS p, p, p, R F u

Low shore

15 mm

45 mm

15 mm

29.8 (3.1-79.6) 24.0 (16.0-34.3) 1.6 (0.7-2.8) 0.1 (0.1-0.1) 16.8 (11.1-23.9) 4.3 (3.3 - 5.6) 1.2 (0.8-1.8) 0.0 (0.0-0.1)

62.0 (6.2-165.6) 87.3 (65.9-t 14.8) 1.0 (0.5-1.8) 1.3 (0.9-1.8) 59.5 (46.0-76.5) 17.0 (13.2-22.3) 8.2 (5.1-12.0) 0.3 (0.2-0.4)

38.1 (11.6-83.1) 27.6 (18.3-39.8) 1.6 (0.8-2.7) 0.1 (0.1-0.1) 16.8 (11.1-23.9) 6.5 (4.8-8.8) 2.6 (1.5-4.2) 0.0 (0.0-O. 1)

45 mm 80.3 (24.1-175.1) 101.6 (74.3-137.1) 0.7 (0.4-1.2) 1.3 (0.9-1.8) 59.5 (46.0-76.5) 24.3 (17.8-32.8) 15.5 (9.0-24.4) 0.3 (0.2-0.4)

ENERGETICS OF A CHITON INDIVIDUAL

Annual

ENERGY BUDGETS

energy budgets for individual

chitons with initial lengths (on 1 January

of 15 and 45 mm, from both the high- and low-shore High-shore

chitons lost a smaller proportion

and defaecation differences

137

than

did similar

were not statistically

are shown in Table XI.

of their ingested energy through respiration

sized chitons

significant.

groups

1982)

from the low shore,

although

these

Small animals devote relatively more energy

to growth production, and relatively less to reproductive production, than do large animals. Large chitons lost about twice as much energy via defaecation, and about a third more via respiration, relative to small ones from the same group. ANNUAL ENERGY BUDGETS

The calculated values of all components high- and low-shore chiton groups are intervals around the values in Tables XI assumed variables (e.g. the calorific value variance.

of annual energy budgets (for 1982) for both shown in Table XII. The 95% confidence and XII were calculated assuming that any of mucus and faecal mucus content) had zero

TABLE XII Components of the annual energy budgets for chiton groups from the high and low zones: the upper and lower bounds identify the 95% confidence intervals around each component; C,,, = measured consumption; C,, P + R + F + U; all other abbreviations are defined in the text; all units are kJ m 2 yr ‘.

C,

CS

High shore

1258 471 47

721 532 392

Low shore

3316 1521 456

1510 1131 840

p,

p¶tl

R

F

u

32.9 18.6 8.5

7.5 5.3 3.6

493 370 279

135 103 80

51 34 20

1.2 0.9 0.7

60.9 36.2 18.8

11.8 8.7 6.1

828 658 525

397 294 215

210 132 74

2.0 1.6 1.3

PtY

DISCUSSION

The budgets for both chiton groups do balance; the 95% confidence intervals for summed consumption are entirely within the confidence intervals for measured consumption (Table XII). However, the error terms are so large that no statistically significant differences are apparent between individuals from the two shore levels despite apparent biological differences, particularly in the R and F components. Some components of the budget are more reliable than others. Biomass, growth production and reproductive production were based on comprehensive data. Respiration was also investigated comprehensively, but the correction for activity was untested. The assump-

P. L. HORN

138

tion that mucus comprised

8% of the organic

portion

of faeces was not investigated.

To estimate Pm it was assumed that mucus was produced was moving

or stationary.

Measured

consumption

constantly

whether the chiton

was the least reliably

calculated

aspect of the budget as it was based on four major variables (radular rasping rate, area cleared per rasp, time spent feeding, and the assumed caloritic value of the food available), each of which depends on other variables and assumptions. Previous studies of gastropods have shown that species living at different shore heights (Paine, 197 1) or in neighbouring sites (Hughes, 197 lb) can exhibit intraspecific differences in energy budget characteristics, although neither of these studies provided confidence intervals around their estimates. The present study suggests that the calculation of a comprehensive energy budget where a large number of measured parameters have to be combined will result in such a large total variance that statistically significant differences are unlikely to be identified. The finding that respiration accounts for only about a quarter of assimilated energy, and that production (almost exclusively mucus) is the major component of assimilation is contrary to the commonly held view that respiration accounts for the bulk of assimilated energy in animal energetics (McNeil1 & Lawton, 1970; Branch, 1981). However, in several molluscan energy budgets in which ingestion and defaecation were measured, the sum of tissue production and respiration was 40-80% less than the total assimilation calculated by difference (Carefoot, 1967; Leighton, 1968; Ansell, 1982; Edwards & Welsh, 1982). However, Edwards & Welsh (1982) found that the energy shortfall for the mudsnail Ilyana~~a obsoleta was accounted for completely by production of mucus, comprising 80% of assimilated energy. Such a Pm : A ratio supports those obtained for both groups of Chiton pelliserpentis (66 and 74%). Generally, in previous studies of molluscan energetics, production of mucus had been either ignored, or estimated and included in the U term. Since it is such a major component of the budget for C. pelliserpentis, mucus must have substantial survival value, although uncertainty surrounds the actual advantages imparted. Besides being a medium for more efficient locomotion,

secretion of mucus by molluscs has been shown to be essential for adhesion

(Grenon & Walker, 1981) keep gills moist during aerial exposure (Robbins, as an offensive agent (Branch & Branch, 1980) bind faecal pellets (Bandel,

1975) act 1974) and

have an enriching effect on the environment by becoming a medium for bacterial growth (Calow, 1974). The importance of considering production of mucus in studies of molluscan energetics has been demonstrated. Seasonal changes in the calorific value of tissue were due probably to changes in the proportion of lipid present. The reciprocal relationship between somatic calorific value and gonad index for low-shore chitons suggests that lipid is stored in the somatic tissue during the gonad resting phase and then transferred to the gonad during gametogenesis (as demonstrated for chitons by Lawrence et al., 1965). However, high-shore chitons exhibit a different pattern of caloritic variation, suggesting utilisation of lipid stocks at times of starvation (Nimitz & Giese, 1964; Lawrence & Giese, 1969). High-shore chitons ingest about 15% less energy than similar sized animals on the low shore.

ENERGETICS

OF A CHITON

139

Decrease in energy value of somatic tissue during summer may indicate an excess demand over ingestion, with the converse occurring during winter. Lower calorific values of somatic tissue of immature, relative to mature chitons suggests that an energetic advantage was gained by building a growing individual initially out of “cheap” material until it was capable of making a genetic contribution to the population (Paine, 197 1). Higher calorific values of mature, relative to developing gonad, and female, relative to male gonad, are explicable in terms of lipid stocks (Lawrence, 1976). Reproductive effort (defined as P,/P, + P,) increased with age in both groups as expected for molluscs (Browne & Russell-Hunter, 1978). However, although the highshore group comprised mainly older individuals, the value for total reproductive effort by this group was not significantly greater than that for the low-shore group (0.22 compared with 0.19) due to lower individual values (i.e. values of reproductive effort for 45 mm chitons were 0.58 and 0.68 on the high and low shores, respectively). However, these results were obtained from one season only, and both P, and reproductive effort of molluscs can vary considerably between years (Parry, 1982; Workman, 1983). It might be thought that since immersion time (i.e. available feeding time) is considerably less for high-shore, relative to low-shore chitons, then high-shore animals would be disadvantaged energetically. However, both groups have similar rates of growth, suggesting that availability of food was not a limiting factor to growth on the high shore as shown for some molluscs (Workman, 1983; Underwood, 1984). High-shore C. pelfiserpentis have almost completely compensated for this shortfall in feeding time by spending a greater proportion of their immersion and movement times feeding, an adaptation displayed also by the trochid Mekzgraphiu aethiops (Zeldis & Boyden, 1979). Also, members of the high-shore group lose a smaller proportion of their energy via respiration as a result of reduced temperature dependence of metabolism (Horn, 1985). Further, although individual consumption per year was about 15% less on the high, relative to the low shore, individual faecal production was about 50% smaller, suggesting that high-shore chitons have a more efficient assimilation, ingest a smaller inorganic proportion, or can absorb inorganics. The adaptations outlined above, along with apparent differences in the storage of energy in somatic tissue, appear to enable high-shore Chitonpelliserpentis to maintain maximal growth in the face of disadvantages related to living high on the shore.

ACKNOWLEDGMENTS

I thank Drs. R. G. Creese and A. J. Underwood for their constructive criticisms of this manuscript.

140

P. L. HORN REFERENCES

ANSELL, A. D., 1982. Experimental

studies of a benthic predator-prey relationship. II. Energetics ofgrowth and reproduction, and food-conversion efficiencies, in long-term cultures of the gastropod drill Polinices alderi (Forbes) feeding on the bivalve Tellina tenuis da Costa. J. Exp. Mar. Biol. Ecol., Vol. 61, pp. l-29. BANDEL, K., 1974. Faecal pellets of Amphineura and Prosobranchia (Mollusca) from the Caribbean coast of Columbia, South America. Senckenbergiana Maritima, Vol. 6, pp. 1-31. BAXTER, J.M. & A.M. JONES, 1978. Growth and population structure of Lepidochitona cinereus (Mollusca : Polyplacophora) infected with Minchinia chitonis (Protozoa : Sporozoa) at Easthaven, Scotland. Mar. Biol., Vol. 46, pp. 305-313. BOYDEN, C. R. & J. R. ZELDIS, 1979. Preliminary observations using an attached microphonic sensor to study feeding behaviour of an intertidal limpet. Estuarine Coastal Mar. Sci., Vol. 9, pp. 759-769. BOYLE, P.R., 1970. Aspects of the ecology of a littoral chiton, Sypharochiton pelliserpentis (Mollusca : Polyplacophora). N.Z. J. Mar. Freshwater Res., Vol. 4, pp. 364-384. BRANCH, G. M., 1981. The biology of limpets: physical factors, energy flow, and ecological interactions. Oceanogr. Mar. Biol. Annu. Rev., Vol. 19, pp. 235-380. BRANCH, G.M. & M. L. BRANCH, 1980. Competition between Cellana tramoserica (Sowerby) (Gastropoda) and Patiriella exigua (Lamarck) (Asteroidea), and their influence on algal standing stocks. J. Exp. Mar. Biol. Ecol., Vol. 48, pp. 35-49. BROWNE, R.A. & W.D. RUSSELL-HUNTER, 1978. Reproductive effort in molluscs. Oecologia (Berlin), Vol. 37, pp. 23-27. CALOW, P., 1974. Some observations on locomotory strategies and their metabolic effects in two species of freshwater gastropods, Ancylusjluviatilis Mull. and PlanorbO contortus Linn. Oecologia (Berlin), Vol. 16, pp. 149-161. CALOW, P., 1975. Defaecation strategies of two freshwater gastropods, Ancylusjluviatilis Mull. and Plunorbis contortus Linn. (Pulmonata) with a comparison of field and laboratory estimates of food absorption rate. Oecologia (Berlin), Vol. 20, pp. 50-63. CAREFOOT, T. H., 1967. Growth and nutrition of Aplysiu punctutu feeding on a variety of marine algae. J. Mar. Biol. Assoc. U.K., Vol. 47, pp. 565-589. CONWAY, E. J., 1947. Microd@sion analyses and volumetric error. Crosby Lockwood, London, 357 pp. EDWARDS, S. F. & B. L. WELSH, 1982. Trophic dynamics of a mud snail (Ilyanassu obsoleta (Say)) population on an intertidal mudflat. Estuarine Coastal ShelfSci., Vol. 14, pp. 663-686. ELLIOTT, J.M. & W. DAVISON, 1975. Energy equivalents of oxygen consumption in animal energetics. Oecologia (Berlin), Vol. 19, pp. 195-201. GRENON, J. F. & G. WALKER, 1981. The tenacity of the limpet Patella vulgata L.: an experimental approach. J. Exp. Mar. Biol. Ecol., Vol. 54, pp. 277-308. HORN, P. L., 1982. Adaptations of the chiton Sypharochiton pelliserpentis to rocky and estuarine habitats. N.Z. J. Mar. Freshwater Res., Vol. 16, pp. 253-261. HORN, P. L., 1984. Beachrock erosion due to feeding by Chiton (Syphurochiton) pelliserpentis at Mudstone Bay. Kaikoura. New Zealand. Mauri Ora. Vol. I I. pp. 57-61. HORN, P. L., 1985. Respiration in air and water of the chiton Chiton pelliserpentis from high and low zones of a sheltered shore. N.Z. J. Mar. Freshwater Res., Vol. 19, pp. 1 I-19. HUGHES, R.N., 1970. An energy budget for a tidal-flat population of the bivalve Scrobicularia plana (da Costa). J. Anim. Ecol., Vol. 39, pp. 357-379. HUGHES, R.N., 1971a. Ecological energetics of the keyhole limpet Fissurella barbadensis Gmelin. J. Exp. Mar. Biol. Ecol., Vol. 6, pp. 167-178. HUGHES, R.N., 197 1b. Ecological energetics of Nerita (Archaeogastropoda, Neritacea) populations on Barbados, West Indies. Mar. Biol., Vol. 11, pp. 12-22. HYMAN, L. H., 1967. The Invertebrates, Vol. VI. Mollusca I. McGraw-Hill, New York, pp. 70-142. JOHNS, P.M., 1960. Chiton pelliserpentis (Mollusca : Amphineura); a study in the taxonomy of a species in relation to its breeding biology and ecology. M. SC. thesis, University of Canterbury, Christchurch, New Zealand, 180 pp. KOFOED, L. H., 1975. The feeding biology ofHydrobia ventrosa (Montagu). II. Allocation of the components of the carbon budget and the significance of the secretion of dissolved organic material. J. Exp. Mar. Biol. Ecol., Vol. 19, pp. 243-256.

ENERGETICS OF A CHITON

141

LAWRENCE.J. M., 1976. Patterns of lipid storage in post-metamorphic marine invertebrates. Am. Zooi., Vol. 16, pp. 741-762. LAWRENCE,J. M. & A. C. GIESE, 1969. Changes in the lipid composition of the chiton, Kntharina funicata, with the reproductive and nutritional state. Physiol. ZooL, Vol. 42, pp. 353-360. LAWRENCE,A.L.. J.M. LAWRENCE& A.C. C&SE, 1965. Cyclic variations in the digestive gland and glandular oviduct of chitons (Mollusca). Science, Vol. 147, pp. 508-5 10. LEIWTON,D.L., 1968. A comparative study offood selection and nutrition in the abalone, Haliotis rufescens Swainson, and the sea urchin, S@ong,vlocentrotuspurpuratus (Stimpson). Ph. D. thesis, University of California, San Diego, U.S.A., 197 pp. McN~IL.~., S. & J.H. LAWTON,1970. Annual production and respiration in animal populations. Nature (I.ondoN), Vol. 225. pp. 472-414. NEWELL,R.C. & A. ROY, 1973. A statistical model relating oxygen consumption of a mollusc (Littoriaa iittorea) to activity, body size and environmental conditions. Physiol. Zool., Vol. 46, pp. 253-275. NICOL, J. A.C.. 1967. The biology of marine animals. Pitman, London, second edition, 699 pp. NIMITZ, M. A. & A.C. GIESE, 1964. Histochemi~al changes correlated with reproductive activity in the chiton Katharina tunicata. Q. J. Microsc. Sci., Vol. 105, pp. 481-495. 0~TAWAY,J. R., 1975.Tidal tank system in operation at the Edward Percival Marine Laboratory, Kaikoura. Mauri Ora, Vol. 3, pp. 31-36. PAINE, R.T., 1966. Endothermy in bomb calorimetry. Limnol. Oceanogr., Vol. 11, pp. 126-129. PAINE, R.T., 1971. Energy flow in a natural population of the herbivorous gastropod 7’egulufunebrulis. Limnol. Oceanogr., Vol. 16, pp. 86-98. PAINE, R.T. & R. L. VADAS, 1969. Calorific values of benthic marine algae and their postulated relation to invertebrate food preference. Mar. Biol., Vol. 4, pp. 79-86. PARRY, G. D., 1982. Reproductive effort in four species ofintertidal limpets. Mar. Biol., Vol. 67, pp. 267-282. PETR~XW~CZ, K. & A. MACFADYEN,1970. Productivity of terrestrial animals;principles and methods. IBP Handbook No. 13. Blackwell, Oxford, 190 pp. PLATT,T. & B. IRWIN, 1973. Calorific content of phytoplanktoi~. Limnot. Oceunogr., Vol. 18, pp. 306-310. ROBBINS,B.A., 1975.Aerial and aquatic respiration in the chitons ~~~~f~~~~~z~ ca~~ornicaand ~~)n~ce~~u li~eata. V&ger {SuppI.], Vol. 18, pp. 98-102. SOI_rrHW,4RD, A.J., 1964. Limpet grazing and the control of vegetation on rocky shores. In, Grazing in terrestrial and marine environments, edited by D.J. Crisp, Blackwell, Oxford, pp. 265-2’73. SO~J~HWOOD, T. R. E., 1978. Ecological methods. Chapman and Hall, London, 524 pp. STEEL, R. G. D. & J. H. TORRIE, 1980. Principles and procedures of statistics: a biometrical approach, second edition. McGraw-Hill International Book Co., 633 pp. STKICKLAND,J. D. H. & T. R. PARSONS,1972. A practical handbook of seawater analysis. Bull. Fish. Res. Bourd Gun., Vol. 167, pp. I-311. TREVALLION,A., 1971. Studies on Tellina tenuis da Costa. III. Aspects of general biology and energy flow. J. Exp. Mar. Biol. Ecol., Vol. I, pp. 95-122. UNDERWOOD,A. J., 1984. Microalgal food and the growth of the intertidal gastropods Nerita atramentosa Reeve and Bembicium nanum (Lamarck) at four heights on a shore. J. Exp. Mar. Biol. Ecol., Vol. 79. pp. 217-291. WORKMAN,C., 1983. Compa~sons of energy partitioning in contrasting age-structured populations of the limpet fatella ~Ja~gutaL. J. E.xp. Mar. Biol. Ecol., Vol. 68, pp. 81-103. WKIOHT,J. R. & R.G. HARTNOLL,1981. An energy budget for a population of the limpet Patella vulgata. J. Mar. Biol. Assoc. U.K., Vol. 61, pp. 627-646. ZELDIS, J. R. & C. R. BOYDEN,1979. Feeding adaptations of Melugraphia aethiops (Gmelin), an intertidal trochid mollusc. J. Exp. Mar. Biol. Ecol., Vol. 40, pp. 267-284.