Chesapeake Science Vo!. 8, No. 3, p. 193-199 September, 1967

Some Factors Influencing Reburrowing Activity of Soft-Shell Clam, Mya arenaria'

H. T . PFITZENMEYER AND K. G. DROBECK

University of Maryland, Natural Resources Institute Chesapeake Biological Laboratory, Solomons, Maryland 20688

ABSTRACT: Three size-groups of soft-shell clams, Mya arenaria, averaging 45, 60, and 74 mm in shell length were placed in aquaria and their rate of reburrowing measured. The substrate in each aquarium consisted of sand particles: (I) less than 0.5 ram, (II) 0.5 to 1 ram, (III) 1 to 2 mm, (IV) 2 to 4 ram, and (V) an equal combhmtion of the previous 4 sizes. New speci- mens were tested at every 5 C rise or fall in ambient water temperature and observations were made periodically for 48 hours.

The three variables, water temperature, clam size, and sediment particle size were highly significant (1% level), in influencing clam reburial activities by the end of the second hour through the termination of the experiment 48 hours later. Optimum water temperatures for reburrowing were between 8.8 and 21 C. After 48 hours, 62% of the small, 39% of the medium, and 21% of the large clams had reburrowcd in all experimental tests combined. The aqua- rium with less than 0.5 mm substrate particle size had the greatest percentage of reburrowed clams of each size. As the particle size increased, progressively less reburrowing took place. Most reburrowing took place between 4 and 8 hours after the start of the experiment.

I n t r o d u c t i o n

Soft-shell clams in Maryland are en- tirely subtidal and must be harvested by the hydraulic clam dredge. The efficiency of this mechanical operation is very high, collecting about 95% of the clams in its path with practically no shell breakage. The gear is not selective and all size clams "tre removed from the bottom. Some sorting does take place on the endless conveyor belt, which carries the dredged clams to the operator in the boat where he select~s the sizes he wishes. The unde- sired clams, too small legally (under 2") or too large for the market, are permitted to fall back overboard where they come to rest on the bottom not far from the point they were dredged. On occasion when the density of clams is too great or the conveyor belt is unattended, al l size-groups are redeposited into the water.

During periods of calm water and low

1Contribution No. 330, Natural Resources In- slitut.e, University of Maryl,md.

turbidity, live clams along with large ac- cumulations of exposed shells have been seen resting on the bottom in areas where commercial dredgers have worked. The fate of these live clams was not known. Some were observed actively trying to re- burrow while others appeared to remain quiescent. However, after a few days the majority of clams were not visible on the bottom. These dredged bottoms attract concentrations of predatory fish and crus- taceans which feed on the exposed clams and other benthic invertebrates that have been dredged from the sediments and re- deposited back into the water.

It was not known what percentage of clams were able to burrow or if all size- groups were capable of doing it. Baptist (1955) studied clams less than 22 mm in shell length and found that there appeared to be a demarcation line of juvenile clams at about 13 mm below which they were very active while those larger were progressively less active in reburrowing. The present experiment was designed to

193

194 II . T. P F I T Z E N M E Y E R AND K. G. DROBECK

disclose the behavior of larger clams in three size-groups averaging 45, 60, and 74 mm in shell leng-th. Two other environ- mental variables, water temperature and bottom particle size, were also tested to determine what influence, if any, they had upon clam reburrowing.

The authors wish to acknowledge Y. J. Schwartz and D. G. Cargo of the Chesapeake Biological Laboratory for their assistance with the analysis of the data and reading the manuscript.

Methods

Beach sands sieved through individual screens of the U. S. Standard Sieve Series comprised tile test. aquaria sediments. Par- tiele sand grain sizes according to the Wentworth scale within each aquarium were: (I) very fine to medium (less than 0.5 mm), (II) coarse (0.5 ~o 1 mm), (III) very coarse (1 to 2 ram), and (IV) gran- ule (2 to 4 ram), or (V) consisted of a mixture of the previous four substrates in equal amounts.

The five graded sediments were placed in 15 gallon, plexiglass aquaria 1 x 1 x 2 feet long. A sufficient volume of each graded sand was used to have six inches of substrate in each aquarium. WaW.r was pumped directly from the Patuxent River into a reservoir, then by gravity flow through separate lines into each aquar- ium, entered on the long side at the sand- water interface and overflowed from the top of the aquarium. The flow of water into each aquarium during the experiment was maintained at about one liter per minute. Prior to initiating each series of experiments, the aquaria were permitted to fill and overflow with continuously run- ning seawater for approximately one hour. At the termination of each experiment, all clams were removed from the sediments, the aquaria and sand flushed with fresh- water and then air dried so that there was no appreciable accumulation of organic matter from one experiment to another.

Specimens for each test were obtained, by means of a Maryland hydraulic clam dredge, from the same location near the mouth of the Patuxent River, approxi-

mately 1 ~ miles from its entrance into the Chesapeake Bay. No more than one hour elapsed from dredging to the start of experiment. Thi r ty clams were selected and placed in each aquarium, 10 clams each between 35 to 50 mm, 51 to 65 mm, and 66 to 75 mm. The middle group, or medium-size clams, were marked on both valves with red chalk. The smaller clams could be distinguished on sight from the larger clams. Although not a permanent marking, the chalk remained on the shell throughout the duration of the experi- ments, which was 48 hours.

The 30 test clams, 10 from each size- group, were randomly dropped on the water surface of each aquarium and per- mitted to settle to the bottom. On settling, those which landed against the side of the aquarium wall were carefully moved so that they were not in direct contact with it or any other clam.

Periodic observations were made on the progress of reburrowing in each aquarium to note the number of clams of each size- group; (1) not reburrowed, (2) less than half reburrowed, (3) more than half re- burrowed, (4) reburrowed. An imaginary dorso-ventral line through the umbone and perpendicular to the longitudinal axis di- vided the clam in half. When the shell was not visible, or the posterior edge was even with the water-sediment interface, the clain was classified 'ts reburrowed. In some instances, the total number of visible clams had to be subtracted from the imm- ber placed in the aquarium to determine tile number reburrowed, since the siphon tip or shell on many reburrowed clams was not visible.

To test the effects of water temperature upon the burrowing activity of the three size groups of clams, experiments were conducted at approximat.ely every 5 C rise or fall in ambient water temperature. The aquaria temperatures noted in this experi- ment are mean values over the test period and usually remained within a degree of the outside river water temperatures.

The data was subjected to an analysis of variance with multiple classification, following the methods of Snedeeor (1948).

REBURROWING ACTIVITY OF SOFT-SHELL CLAM 19,~

T . t m ~ 1. Analysis of var iance of soft-shell clam reburrowing ac t iv i ty in re la t ion to tempera ture , sediment , and size dur ing specific t ime intervals .

Source of variation

Time interval in hours

F-values

1 2 4 8 21 24 48

Main eilects Temperature (T) 0.4 13.1"* 16.1"* 11.9"* 4.4** 4.3** 3.1"* Sedintent (SE) 0.7 14.4"* 20.0** 64.1"* 124.0"* 151.0"* 79.6** Size (SI) 2.6 61.8"* 72.7** 117.0"* 102.0"* 126.0"* 69.2**

Firs t Order In te rac t ions T X SE 0.1 1.3 1.1 0.9 0.8 0.8 0.6 T X SI 0.2 4.6** 2.5* 2.0* 1.6 1.8" 1.4 S E X SI 0.4 5.5** 5.0** 6.5** 5.0** 4.7** 2.0

* Significant at the 5% level. ** Significant a t the 1% level.

TAt3L~: 2. N u m b e r of soft-shell clams which reburrt)wed at var ious water t empera tu res (means) and time intervals . N u m b e r of clams are indicated which did not complete reburrowing or exhibited no �9 ~ctivity af ter 48 hours.

Water Temperature C. Time (hours) Total clams

3.8 5.2 8.8 13.0 13.9 18.5 21.0 25.7 27.2

1 0 0 0 6 5 l l 6 N . O . 6 34 2 1 3 5 23 18 11 33 3 4 101 4 N.O. 14 30 30 38 19 23 13 10 177 8 27 47 20 29 16 15 10 23 24 211

21 N.O. 3I 28 3 2i 19 9 N.O. N.O. 111 24 45 0 3 0 0 0 8 26 18 100 48 10 4 12 N.O. N.O. 14 N.O. 19 12 71

Tot . Relmr. 83 99 98 91 98 89 89 84 74 805 Par t . Rebur. 39 30 30 35 26 20 36 31 20 267 No Act iv i ty 28 21 22 24 26 41 25 35 56 278 Tot . Clams 150 1,50 150 150 150 150 150 150 150 1350

N.O. = No observat ion.

To perform this analysis, the observations had to be coded by adding a factor of ten.

Results and Discussion

Statistically, during the first hour of the experiment, no significant difference ill water temperature, bottom sediment, or size of the clam existed in relation to rate of reburrowing (Table 1). The com- paratively small amount of activity ex- hibited by the clams during this time can be attributed to the stress placed upon them by the method of collection. With an increase in reburial activity, by the second hour and until the termination of the ex- periment 48 hours later, all three variables influenced reburrowing significantly (1% level). Tlle following discussion will at-

tempt to elaborate more fully on the in- fluences of each upon the activity of re- burrowing.

WATER TEMPERATURE

Results were obtained at nine different temperature test ranges varying from 3.8 to 27.2 C (Table 2). Smnmarizcd in this table are the combined observations from all tanks at the specified times and water temperatures thereby eliminating any dif- ferences which occurred because of sedi- ment variation or clam size. The number of clams reburrowed, partly reburrowed, and those which showed no activity after 48 hours is noted also. The specific water temperature ranges were not statistically analysed per se, but for the first 4 hours

196 ~ . T. P F I T Z E N M E Y E R AND K, G. DROBECK

TABLE 3. Average percent of large (L), medium (M) and small (S) clams which reburrowed at the given times in the five aquaria. Various tem- perature experiments are grouped. Maximum number per test, 10 clams.

Aquaria C!am _ _ I/ours SiZe 1 2 4 8 21 24 48

L 0 2 11 34 63 62 60 I M 4 16 40 72 90 89 95

S 18 38 69 86 98 97 98

II L 0 0 4 19 53 50 61 M 1 9 29 58 83 80 77 S 10 36 72 87 93 93 97

III L 0 1 3 10 63 22 35 M 1 6 19 30 55 49 55 S 6 20 55 65 93 &3 88

L 0 0 0 0 1 1 0 IV M 0 0 8 2 5 4 1

S 0 0 1 7 20 19 36

V L 0 0 5 12 40 38 48 M 0 3 18 39 65 58 68 S 2 20 42 52 ~ 83 90

the most active rate of reburrowing took place at medium temperatures or between 8.8 and 21 C. During the first hour more clams dug in at 18 C than at any other temperatures. At subsequent observations, the activity spread to the adjoining water temperatures so that after 24 hours mos~ activity took place at the extreme tem- peratures. The percentage of clams which wcrc reburied at the termination of each temperature experiment were compara- tively equal. These most inhibiting tem- peratures during the initial four hours were 3.8, 25.7, and 27.2 C.

In a study of pumping rates of Mya arenaria subjected to various temperatures in the laboratory, Harr igan (1956) found the highest activity occurred at 16 and 20 C. He believed this to be the range of optimal temperature for water circu- lation. Pfitzenmeyer (1962) found that when the water temperatures were within the 15 to 18 C range in the Chesapeake Bay, clam larvae were found in greatest abundance. Most reburrowing activity in this experiment began at water tempera-

tures in these corresponding ranges which substantiates further the optimum temper- atures for this boreal species in the Chesa- peake Bay.

The number of clams which began to reburrow, but for some unknown reason did not complete this activity, is noted in Table 2. Overcrowding could be considered, since the clams were placed in the con- filled area, but 30 clams per two square feet does not approach a density of 219 clams per 1 �89 square feet reported in New England by Dow and Wallace (1950). Natural densities of marketable clams (2 inches or greater in shell length) have been found in Maryland greatly exceeding 30 clams per 2 square feet. Therefore, it is doubtful tha t overcrowding prohibited reburrowing in these experiments. Experi- mental clams exhibited practically no lateral movement, but dug vertically in the spot where they were resting. Occa- sionally a few clams, after partially or completely reburrowing, dug back out of the sediment. This happened most com- monly between the 21 and 24 hour ob- servation period and was observed with all size clams (Table 3). A possible expla- nation for this behavior could be tha t the clams reached the bottom of the aquaria which prevented them from attaining their natural depth. They then dug back out attempting to find a more suitable loca- tion and in most instances were reburrowed by the next observation period.

S E D I M E N T

The aquaria which contained the finest sediment (<0.5 mm) had the greatest number and percentage of reburrowed clams of each size (Table 4). As the particle grain size of the sediments increased, pro- gressively less reburrowing took place. Al- though sorting by wave action occurs nat- urally among sediments, it is believed that it does not take place similar to the particle sizes chosen in these controlled experi- ments. One might speculate tha t entirely different results would be obtained if the sediments were made up of a wider range of grain sizes. An at tempt to simulate

REBURROWING hCTIVITY OF SOFT-SHELL CLAM 197

TABLE 4. Combined number and percentage (parentheses) of the three size groups of soft-shell clams which reburrowed in the sediments of the five aquaria.

Sediment

Clam size (mm) I Med. to fine

<0.5

II III IV V Coazse V. coarse Granule Mixture 0.5-1.0 1.0-2.0 2.0-4.0 > 0 . 5 - 4 . 0

134 (29) 67 (14) 2 (0.4) 102 (22) 247 (53) 158 (35) 13 (2) 184 (40) 374 (83) 308 (68) 60 (13) 289 (64) 755 (55) 533 (39) 75 (5) 575 (42)

Large 65-75 174 (38) Medium 50-65 310 (68) Small 35-50 386 (85) Total 870 (64)

65

45

25

, s I ~176

I .."" if ...."

I ..." I .-*'~

if .~176

/ ,x~/~. ~ "*

/ ...--" / /

! / / / e~.

/ .," I /

2.4 I-2 .5-I <.5

SAND PARTICLE SI7E (ram)

Fig. 1. Total percentage of three size groups of clams completely reburrowed in four different particle size sand substrates after 48 hours.

more natural conditions was the reason for including in these experiments a pro- portionate combination of sediments fl'om each of the four aquaria to make up the sediments of the fifth. The total number of clams (575t to reburrow in this sediment was between the total number which re- burrowed in aquarium III in very coarse sand (533) and aquarium II in coarse sand (755), or the median size sediments tested.

Although more clams reburrowed in the <0.5 mm grain sediment (aquarium I) than in any of the other sediments tested, this cannot be taken conclusively as the optimum sediment size. Ideally, still finer sediment should have been included in the experiment, but some information on the optimum sediment may be gained from the series used. In Table 4, if one observes the differences in the percent reburrowed

from one a(luarium to the other, the dif- ference between aquarium I and II is small compared with the differences be- tween aquaria II and III, and III and IV. This indicates that the most desirable sand grain size is very close to the <0.5 mm particle diameter. The reburrowing curve, especially for the small clams in Figure 1, also levels off at the <0.5 mm grain sediment. Also from the total number of clams to reburrow in each aquarium, it is evident that sediment is a significant factor (1% level) affecting reburrowing rate. Saila, Flowers, and Cannario (1966) also found that populations of the hard clan], Mercenaria mercena~ia, were velT much reduced in sediment particle size >2 mm in diameter.

If the sand particles are closely homog- enous in size, then compaction occurs, and the substrate is too hard for the clam foot and shell to penetrate. When the par- ticle sizes are very large, such as in aquar- ium IV, then the interstitial spaces are large and the grains of sediment do not fit close enough together for the foot to anchor itself properly prior to pulling the shell into the sediment. The granules also are too large and heavy to be moved by the water currents which are ejected by the clam from the pedal opening during the reburrowing activity.

SIZE

The number of small clams (av. 45 Iron) that reburrowed was greater at all ob- servation periods than the other two sizes, irrespective of temperature or bottom tex- ture (Table 3). This size also had the greatest percentage that completed reburial at the termination of the experiment. The

198 II. T. PFITZENMEYER AND K. G. DROBECK

100,

80

60

uu 19

a . 4 0

20

Ll I l 1 [ l

12 4 8 21 24 48

HOURS

Fig. 2. Cumulative percentages by time of clams which reburrowed in all five aquaria.

medium-size clams (av. 60 mm) were the next most successful and the large ones (av. 74 mm) the least successful in re- burrowing.

The strong influence which bottom type plays upon the number of clams to re- burrow is seen in Table 4. A particular size clan: does not exhibit any special preference for a certain sediment, but the increase or decrease in activity with chang- ing bottom type commonly varies with all sizes. This is more clearly seen in Figure l, which plots the total percentage of the three sizes of clams which reburrowed in the four different sediment grain sizes. The fifth sediment, which consisted of a combination of these four, was not used in this figure. The data for Figure 1, ob- tained after 48 hours and combining the observations made at the various temper- aturcs, indicates the maximum percentage of clams to relmrrow was 85% of the small size, 68% of the medium size, and 38% of the large size.

The comparative rate which the smaller clams reburrow is seen in Table 3. The medium and large clams are not more lethargic, since many of these could be seen attempting to reburrow almost im- mediately, but not to any measurable degree.

As the elan: matures and becomes larger, it is possible that their reburrowing activity becomes more difficult to perform. The larger the clam, the further the pedal opening is from the substrate surface and the clam mass is greater for the foot to pull into the sediment. A detailed study on the hydraulics and fluid-muscle system used in reburrowing is reported for Mga and other bivalves by Trueman (1966).

TIME

The cumulative percentages of clams which reburrowed when plotted against time generally followed a sigmoid shaped curve as illustrated in Figure 2. Only 4% of the total number of clams to reburrow did so by the end of the first hour. The effects of the physiological shock from being dredged and held out of water for a short period of time during transport to the laboratory partly contributed to this time lag. The percentage of reburrowed clams rapidly increased and 65% accom- plished this activity by the end of 8 hours. After 8 hours the activity decreased, and 26% became reburrowed between 9 and 24 hours and only 9% from 24 to 48 hours after the beginning of the experi- ment. The total number of all sizes of clams which reburrowed in each test never exceeded 66% of the 150 clams used in each experiment (Table 2).

DISCUSSION

These experimental observations sug- gest that many elams deposited on the bottom by the commercial dredgers rebur- row back into the sediment. The rate of reburrowing is dependent on the size of the clam, nature of the substrate, and water temperature. Smaller clams are capable of reburrowing sooner while larger clams take a progressively longer time to complete this activity. There is a marked decrease in reburrowing capability as the sediment particle size increases. The water temperature at which most reburrowing occurred during the first hour was 18 C.

Before attempting to apply this data to a field clam dredging operation, several factors, which could not be measured in

REBURROWING ACTIVITY OF SOFT-SHELL CLAM 199

this cont ro l led exper iment , mus t be t a k e n into considera t ion . The consis tency, con- tour, and compos i t ion of the b o t t o m ; pres- ence of vege t a t i on ; w a t e r dep th and cur- ren ts ; o ther benth ic species and dens i t ies ; and p reda t ion are some of the va r i ab l e s which would change wi th local i t ies to a f - fect the number of c lams to rebur row.

LITERATURE CITED

BAPtiST, J. P. 1955. Burrowing ability of juve- nile clams. U. ;% Fish and Wildli]e Serv., Spec. Sci. Rept. Fish No. 140, 13 p., Wash., D. C.

Dew, R. L.. A.~D D. E. WALLAC~. 1950. The story of the Maine clam (Mya :trenaria). Maine Dept. Sea and Shore Fish., August, Maine. 27 p.

H.~I~RIGAN, R.E . 1956. The effect of temperature on the pumping rate of the soft-shelled clam, Mya arenaria. MS Thesis. George Washington Univ., Wash., D. C., 54 p.

I~FITZENMEYER, H. T. 1962. Periods of spawning and setting of the soft-shelled clam, Mya are- naria, at Solomons, Maryland. Chesapeake Sci. 3 : 114-120.

SAILA, S. B., J. M. FLOWERS, A.ND M. T. CAN.~'ARI0. 1966. Factors affecting the relative abundance of Mercenaria merce~a~ia in the Providence River, Rhode Island. Abstract of paper pre- ~nted at National Shellfisheries Association, Norfolk, Virginia, 1966.

SNEDECOR, G.W. 1948. Statistical methods. Iowa State College Press, Ames, Iowa. 485 p., 4th Ed.

TaUE.~fA~', E. R. 1968. Bivalve mollusks: Fluid dynamics of burrowing. Science, 152(3721):523- 525.