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Life cycle of the amphipod Gammarus palustris Bousfield
Estuarine and
CoaStal
Marine Science
(1975)
3,
413-419
Life cycle of the Amphipod Gammarus palustris Bousfield
Colin P. Rem Department of Zoology, University of Maryland, College Park, Md 20742, U.S.A. Received 19 December I974 and in revised form 27 ‘january 1975
To elucidate the life cycle of the amphipod Gammarus palustris Bousfield, a section of the intertidal zone of a salt marsh along the lower Patuxent River, Maryland, was sampled from January 1972 to December 1973. Monthly collections revealed a gradual (if fluctuating) increase in abundance from March to October, after which numbers declined reaching a low in the midwinter months. The breeding season extended from February to October, with major peaks of activity occurring in March and August-September. Whereas the spring peak was produced by the over-wintering generation, the late summer peak was a product of the summer generation. The overwintering generation first revealed egg production in February and the first young appeared in March. These young matured during late May to late June. The over-wintering generation began to die-out in April, disappearing completely by early July, while the summer generation continued reproducing through July, August and September. Finally, it is surmised that the initial brood(s) of the earliest summer generation females have sufficient time to mature and reproduce before the end of the summer.
Introduction Bousfield’s (1969, 1973) and Teal’s (1962) reference to the frequent occurrence of Gummayus pahtris Bousfield within intertidal levels of American Atlantic coastal salt marshes stimulated a study of the species in the lower Patuxent River, Maryland. Although occurring periodically in plankton samples (Williams & Bynum, 19p), it is most often found among Spartina culms and under damp debris, stones, etc., from approximately mid-tide level to mean high-water neaps (Bousfield, 1973). A euryhaline species with a salinity preference ranging from about 5 to 20 %,,, G. palustris survives brief exposure to fresh water (at low temperatures) and sea water (at high temperatures). A portion of a salt marsh along the Ben Creek section of the lower Patuxent River was selected as the study area. Typified by a 3 m wide dense stand of Spartina alterniJora grading into a narrow zone of S. patens at higher levels, the entire marsh is inundated by each high tide. The marsh, with a bottom-type of decaying Spmtina and coarse to fine sand, ends at low water mark and gives way, oia a vertical drop of approx. 0.75 m, to a sandy, detritus-rich sediment. During the period of study, tidal amplitude had a mean range of 0.38 m and the salinity varied from 3.6 to 12.7 %,,. Water temperature followed the yearly cycle with a February low of 4.0 “C (1973) and a July-August high of 30.0 “C (1973). Dissolved oxygen concentrations were near saturation at all times. 413
2
4
6
8
10
12
I3 Dsc
I
21 Aus
I 123
30
Sept
.:.-::_ .
i_fi
360
- --
169
.
24
..
sap*
---
-...:-:.. ...a.. :::. .‘.‘... ..:.:.:.: . _‘_~.~_._~_~_
L-I ---.
%
Distribution
L ---. j... ,::;+g ( , , lI-i!b= 1
160
-. I:: . -:. k _.
394
3
--L
26 Mar
1
22oct
27 June
from Ben Creek, below this line the represent females in
I
----:.:.: :.: ..::: k ,.> 251
3
Bj ,...:. :: i--4 --
jw?
Figure I. The percentage size distribution of males, non-breeding females, breeding females and juveniles on monthly samples of Gammaruspalustris Maryland, rg~z-73. The horizontal dotted line (at 5 mm length) indicates the lower limit of malenon-breeding female distinction: size-group blocks of juveniles are centered; above it males are shown on the left and females on the right. The cross-hatched portions breeding condition (numerals give numbers in each sample).
i3 c
8
211
Life cycle of Gammarus palustris
Materials
aud methods
Samples were taken at monthly intervals from January 1972 to December 1973, their collection entailing the use of a D-net scraped vigorously over a 4.6 m2 area of S. alterniflora during high tide. The yields were placed in plastic bags holding 10% formalin and later examined in white enamel trays. All macroinvertebrates found were preserved in vials containing 70 ‘A alcohol and 5 ‘A glycerin. Those samples bearing large quantities of organic debris were sorted by Anderson’s (1959) sucrose floatation method. Every specimen of G. pahtris was measured to the nearest 0.5 mm from the tip of the rostrum to the tip of the telson while straightened out over a scale (curled animals were extended with forceps to achieve this measurement). According to Hynes (1954) this method slightly underestimates the length of the individual, but as all were treated alike the various samples are strictly comparable. Upon completion of this procedure, all individuals were categorized as juveniles, males, females or ovigerous females (see Table I for criteria). TABLE I. Criteria employed female G. palustris
to distinguish
Category Size (mm) Juvenile Male
2’0-4.5” 5’0-13.0
Female
5.0-9.5
Ovigerous female
4.0-6.5~
juvenile,
Morphological
male, female and ovigerous correlates
Immature individuals having no external sexual characteristics. Second antenae displaying whorls of simple setae extending onto the flagellae; relatively large 1st and 2nd gnathopods, the palm of the 2nd possessing an oblique palmer margin bearing a blunt spine tooth on the median concavity. Typically with relatively shorter, weaker and less setose 2nd antennae and weakly developed gnathopod propods; brood plates present on pereonoite segments 2 to 5. Eggs, occasionally marsupium.
enveloped
by mucus sacs, present in the
’ Specimens below 2.0 mm were excluded because of the differential effects of the preservative in releasing young held by ovigerous females. b While ovigerous females could be identified down to 4.0 mm, lack of reliable differences prevented separation of males and females below 5.0 mm.
Results and conclusions The percentage size distribution and abundance of males, females (ovigerous and nonovigerous) and juveniles were estimated for monthly samples of Gummarus pahtris from Ben Creek, 1972-73 (Figure I). These estimates reveal that the breeding seasonextends from February to October, with major peaks of activity occurring in March and August-September. The spring peak is produced by the over-wintering generation, whereas the late-summer peak is the product of the summer generation. In December of 1972 and 1973, the population was composed almost entirely of adults with an average female : male ratio of I : 3. The over-wintering generation first showed egg production in February (1.4% ovigerous females in 1972, 1.2% in 1973) attaining maximum output in March (42'7% ovigerous females in 1972, 37’3% in 1973) (Figure 2). The first young appeared in March, the greatest number occurring in April or May when they comprised 79.0 % of the tota! sample in 1972 and 81.9 % in 1973 (Figure 3). According to percentage size distribution (Figure I), by late May a few of the first young became mature, with many more young maturing by late June. The over-wintering generation began to die out during April and disappeared completely by late June-early July. The summer generation continued reproducing through July, August and September, ceasing such activity
416
C. P. Rees
Months
Figure a. The reproductive to December 1973.---,
of Gammarus pahttis
activity
during
January
1972
1973.
1972;---,
during the winter months of 1972-q and very likely 1973-74. However, a sizeable representation of juveniles was observed in January and February of 1972 and suggests greater reproductive activity in the winter of 197172. This appears to coincide with the fact that the winter of 1971-72 was particularly mild compared with the other years. The sex ratios of females to males ranged from 0.24 to 3.45 with females outnumbering males in II of 23 samplings (Figure 4). During the winter and early spring of both years the proportion of females steadily increased from a ratio value of about 1.0 in November to about 3.5 in April. Such a phenomenon has been noted in other gammarid amphipod populations and may be due to the naturally faster growth rate and earlier death of the males (Hynes, 1954). There is also the possibility that, in this species as in G. duebeni (Kinne, 1952, 1953), temperature determines sex and that the males are born earlier in the fall than females. Whatever the
t
20-
1’ 0
J
’
’
FMAMJJ
’
’
’
’
’
A
’
S
’
0
’
N
1
D
Months
Figure 3. Seasonal changes in the juvenile portion of the total population of 1972 to December 1973. -, 1972; - - -, 1973.
Gammaruspalustris during January
Life
cycle of Gammarus
palustris
417
JFMAMJJASOND Months
Figure 4. Seasonal changes in the female : male sex ratio of Gammarus pahstris during January 1972 to December 1973. --, 1972; - - -, 1973.
explanation, the predominance of females during the spring doubtlessly enhances the reproductive potential. An approximate I : I ratio prevails during late spring and early summer. The changing mean size of the young and adult portions of the summer generation can be tracked from March to August for both years (Figure 5). Adult males exhibited marked growth between May to June and July to September and noticeable decreasesbetween June to July and September to October. The decrease may be attributed to the rapid recruitment of young males belonging to the summer generation, to higher temperatures favouring the production of males (Kinne, 1952, 1953) or to predation. In connection with the last attribution, it may be noted that the June-July and September-October standard monthly samples yielded the greatest densities of the grass shrimp Palaemonetespugio and three Fundulus spp. Fraser (1973) and Nixon & Oviatt (1973) h ave established that these species
/ Adult;
I ,,l,lllllllllllxlllllll,J JFMAMJJASONDIJ
FMAMJ 1972
I Months
JASOND 1973
Figure 5. The mean size distribution of (a) males (- - -), non-breeding females (-), breeding females (. . . .) and (b) juveniles of Gammam pahsbis during January 1972 to December 1973. W, over-wintering generation; S, summer generation. (No samples were collected in January 1973.)
418
C. P. Rees
are frequent predators of intertidal gammarid amphipods and Van Dolah (pers. comm.) has evidence to suggest that Fund&s spp. selectively take larger specimens of G. pahtris (note: males exceed females in maximum size). While the mean size of the adult females fluctuated less than that of males, there was a similar decrease between the June to July sampling times. However, unlike the pattern for the males, the mean size fell slightly (1972) or remained constant (1973) between August to September only to rise the following month. This may be the combined result of a possible lag in female recruitment and the maturation of the ‘mid summer generation’. At any rate, during October the lower mean size of the females and a female : male sex ratio of 0.24 may have enabled the female portion of the total population to better withstand predation by P. pugio and Fund&s sp. The growth rate of a large fraction of the over-wintering generation can be followed from October to May for both years. During this period the males grew at an average of 0.63 mm/ month (3.77 mm in total) and the females 0.29 mm (1.76 mm in total). A comparison of growth increments between years, possible only from February to May, reveals a slower increase for both sexes in 1972. This is not surprising in view of the slightly lower temperatures during the spring of that year. The over-wintering generation of both years terminated reproduction in late June-early July, whence the summer generation became dominant. The average monthly size of the juvenile portion of the population is shown in Figure 5. During March to May, the progeny of the spring reproductive activity grew at an average of 0.42 mm/month in 1972 and 0.53 mm/month in 1973. Thereafter, the likelihood of repeated brooding and predation and the introduction of a late summer surge of reproductive activity frustrates further analysis. The late summer component, eventually yielding most of the over-wintering generation, grew at an average of 0.49 mm/month from August to December 1972 and 0.56 mm/month between the same months in 1973. While the growth rate slowed appreciably at the end of both years, it may be inferred from Figure 5 that the spring of each year is characterized by a marked increase. Although the data for the over-wintering generation suggest that it produces only one or two broods of young in the spring, the summer generation appears to reproduce a number of times. In support of the latter observation, it may be seen that during May to August (July may be excepted because of the possible effect of predation) there is a perceptible mean size and marked percentage increase of ovigerous females. During August-September, the situation is complicated by the possibility that the initial brood(s) of the earliest summer generation ovigerous females may have sufficient time to mature and reproduce before the end of the summer. In the wake of this possible supplement to the breeding population, it is not surprising that the total population reached a peak of abundance the following month. For the remainder of the year the falling temperatures prolonged the time to maturity and consequently, recruitment of adults decreased. A portion of juveniles produced in September and October appear to over-winter as immatures and swell the breeding population as water temperatures rise in the spring. Thus, a female starting breeding in February and producing successive broods from March to the time of its death in early July, may have already produced one brood in the previous year. Summary I. A portion of the intertidal zone of a salt marsh along the lower Patuxent River, Maryland, was sampled from January 1972 to December 1973 to elucidate the life cycle of the amphipod Gammarus palustris Boustield.
Life
cycle of
Gammarus palustris
419
2. Monthly collections revealed a gradual (if fluctuating) increase in abundance from March to October after which numbers began to decline reaching a low in the mid-winter months. 3. The breeding season of G. pahstris extends from February to October, with major peaks of activity occurring in March and August-September. The spring peak is produced by the over-wintering generation whereas the late summer peak is a product of the summer generation. 4. The over-wintering generation first showed egg production in February and the first young appeared in March. By late May a few of these specimens had matured and many more were mature by late June. The over-wintering generation began to die-out during April, disappearing completely by late June-early July, while the summer generation continued reproducing through July, August and September, invariably ceasing activity during the winter months. 4. Finally, it is surmised that the initial brood(s) of the earliest summer generation females have sufficient time to mature and reproduce before the end of the summer. Acknowledgements Thanks are due to Messrs R. Orth, M. Smith, K. Thoemke and R. VanDolah for field assistance, often under difficult weather conditions. This work was supported by N.S.F. Grant 2M-AXAF. References Anderson, R. 0. 1959 A modified flotation technique for sorting bottom fauna samples. Limnology and Oceanography 4, 223-225. Bousfield, E. L. 1969 New records of Gammarus (Crustacea: Amphipoda) from the Middle Atlantic Region. Chesapeake Science IO (I), 1-17. Bousfield, E. L. 1973 Shallow-water Gammaridian Amphipoda of New England. 312 p. Cornell Univ. Press, Ithaca, N.Y. Fraser, A. 1973 Foraging strategies in three species of Fund&s. MS. Thesis, University of Maryland. 54 PP. Hynes, H. B. N. 1954 The reproductive cycle of some British freshwater Gammaridae. Journal of Animal Ecology 24, 352-387. Kinne, 0. 1952 Zur Biologie und Physiologie von Gammarus duebeni Lillj.; III: ZahlenverhZltnis der Geschlechter und Geschlechtsbestimmung. Kieler Meeresforschungen 9, I 26-133. Kinne, 0. 1953 Zur Biologie und Physiologie von Gummarus duebeni Lillj., VII: Uber die Temperaturabhiingikeit der Geschlechtsbestimmung. Biologisches Zentralblutt 72, 260-270. Nixon, S. W. & Oviatt, C. A. 1973 Ecology of a New England Salt marsh. Ecological Monographs 43 (4), 463-498. Teal, J. M. 1962 Energy flow in the salt marsh ecosystem of Georgia. Ecology 43 (4), 6x4-624. Williams, A. B. & Bynum K. H. 1972 A ten-year study of meroplankton in North Carolina estuaries: Amphipods. Chesapeake Science I3 (3), 175-xgz.
CoaStal
Marine Science
(1975)
3,
413-419
Life cycle of the Amphipod Gammarus palustris Bousfield
Colin P. Rem Department of Zoology, University of Maryland, College Park, Md 20742, U.S.A. Received 19 December I974 and in revised form 27 ‘january 1975
To elucidate the life cycle of the amphipod Gammarus palustris Bousfield, a section of the intertidal zone of a salt marsh along the lower Patuxent River, Maryland, was sampled from January 1972 to December 1973. Monthly collections revealed a gradual (if fluctuating) increase in abundance from March to October, after which numbers declined reaching a low in the midwinter months. The breeding season extended from February to October, with major peaks of activity occurring in March and August-September. Whereas the spring peak was produced by the over-wintering generation, the late summer peak was a product of the summer generation. The overwintering generation first revealed egg production in February and the first young appeared in March. These young matured during late May to late June. The over-wintering generation began to die-out in April, disappearing completely by early July, while the summer generation continued reproducing through July, August and September. Finally, it is surmised that the initial brood(s) of the earliest summer generation females have sufficient time to mature and reproduce before the end of the summer.
Introduction Bousfield’s (1969, 1973) and Teal’s (1962) reference to the frequent occurrence of Gummayus pahtris Bousfield within intertidal levels of American Atlantic coastal salt marshes stimulated a study of the species in the lower Patuxent River, Maryland. Although occurring periodically in plankton samples (Williams & Bynum, 19p), it is most often found among Spartina culms and under damp debris, stones, etc., from approximately mid-tide level to mean high-water neaps (Bousfield, 1973). A euryhaline species with a salinity preference ranging from about 5 to 20 %,,, G. palustris survives brief exposure to fresh water (at low temperatures) and sea water (at high temperatures). A portion of a salt marsh along the Ben Creek section of the lower Patuxent River was selected as the study area. Typified by a 3 m wide dense stand of Spartina alterniJora grading into a narrow zone of S. patens at higher levels, the entire marsh is inundated by each high tide. The marsh, with a bottom-type of decaying Spmtina and coarse to fine sand, ends at low water mark and gives way, oia a vertical drop of approx. 0.75 m, to a sandy, detritus-rich sediment. During the period of study, tidal amplitude had a mean range of 0.38 m and the salinity varied from 3.6 to 12.7 %,,. Water temperature followed the yearly cycle with a February low of 4.0 “C (1973) and a July-August high of 30.0 “C (1973). Dissolved oxygen concentrations were near saturation at all times. 413
2
4
6
8
10
12
I3 Dsc
I
21 Aus
I 123
30
Sept
.:.-::_ .
i_fi
360
- --
169
.
24
..
sap*
---
-...:-:.. ...a.. :::. .‘.‘... ..:.:.:.: . _‘_~.~_._~_~_
L-I ---.
%
Distribution
L ---. j... ,::;+g ( , , lI-i!b= 1
160
-. I:: . -:. k _.
394
3
--L
26 Mar
1
22oct
27 June
from Ben Creek, below this line the represent females in
I
----:.:.: :.: ..::: k ,.> 251
3
Bj ,...:. :: i--4 --
jw?
Figure I. The percentage size distribution of males, non-breeding females, breeding females and juveniles on monthly samples of Gammaruspalustris Maryland, rg~z-73. The horizontal dotted line (at 5 mm length) indicates the lower limit of malenon-breeding female distinction: size-group blocks of juveniles are centered; above it males are shown on the left and females on the right. The cross-hatched portions breeding condition (numerals give numbers in each sample).
i3 c
8
211
Life cycle of Gammarus palustris
Materials
aud methods
Samples were taken at monthly intervals from January 1972 to December 1973, their collection entailing the use of a D-net scraped vigorously over a 4.6 m2 area of S. alterniflora during high tide. The yields were placed in plastic bags holding 10% formalin and later examined in white enamel trays. All macroinvertebrates found were preserved in vials containing 70 ‘A alcohol and 5 ‘A glycerin. Those samples bearing large quantities of organic debris were sorted by Anderson’s (1959) sucrose floatation method. Every specimen of G. pahtris was measured to the nearest 0.5 mm from the tip of the rostrum to the tip of the telson while straightened out over a scale (curled animals were extended with forceps to achieve this measurement). According to Hynes (1954) this method slightly underestimates the length of the individual, but as all were treated alike the various samples are strictly comparable. Upon completion of this procedure, all individuals were categorized as juveniles, males, females or ovigerous females (see Table I for criteria). TABLE I. Criteria employed female G. palustris
to distinguish
Category Size (mm) Juvenile Male
2’0-4.5” 5’0-13.0
Female
5.0-9.5
Ovigerous female
4.0-6.5~
juvenile,
Morphological
male, female and ovigerous correlates
Immature individuals having no external sexual characteristics. Second antenae displaying whorls of simple setae extending onto the flagellae; relatively large 1st and 2nd gnathopods, the palm of the 2nd possessing an oblique palmer margin bearing a blunt spine tooth on the median concavity. Typically with relatively shorter, weaker and less setose 2nd antennae and weakly developed gnathopod propods; brood plates present on pereonoite segments 2 to 5. Eggs, occasionally marsupium.
enveloped
by mucus sacs, present in the
’ Specimens below 2.0 mm were excluded because of the differential effects of the preservative in releasing young held by ovigerous females. b While ovigerous females could be identified down to 4.0 mm, lack of reliable differences prevented separation of males and females below 5.0 mm.
Results and conclusions The percentage size distribution and abundance of males, females (ovigerous and nonovigerous) and juveniles were estimated for monthly samples of Gummarus pahtris from Ben Creek, 1972-73 (Figure I). These estimates reveal that the breeding seasonextends from February to October, with major peaks of activity occurring in March and August-September. The spring peak is produced by the over-wintering generation, whereas the late-summer peak is the product of the summer generation. In December of 1972 and 1973, the population was composed almost entirely of adults with an average female : male ratio of I : 3. The over-wintering generation first showed egg production in February (1.4% ovigerous females in 1972, 1.2% in 1973) attaining maximum output in March (42'7% ovigerous females in 1972, 37’3% in 1973) (Figure 2). The first young appeared in March, the greatest number occurring in April or May when they comprised 79.0 % of the tota! sample in 1972 and 81.9 % in 1973 (Figure 3). According to percentage size distribution (Figure I), by late May a few of the first young became mature, with many more young maturing by late June. The over-wintering generation began to die out during April and disappeared completely by late June-early July. The summer generation continued reproducing through July, August and September, ceasing such activity
416
C. P. Rees
Months
Figure a. The reproductive to December 1973.---,
of Gammarus pahttis
activity
during
January
1972
1973.
1972;---,
during the winter months of 1972-q and very likely 1973-74. However, a sizeable representation of juveniles was observed in January and February of 1972 and suggests greater reproductive activity in the winter of 197172. This appears to coincide with the fact that the winter of 1971-72 was particularly mild compared with the other years. The sex ratios of females to males ranged from 0.24 to 3.45 with females outnumbering males in II of 23 samplings (Figure 4). During the winter and early spring of both years the proportion of females steadily increased from a ratio value of about 1.0 in November to about 3.5 in April. Such a phenomenon has been noted in other gammarid amphipod populations and may be due to the naturally faster growth rate and earlier death of the males (Hynes, 1954). There is also the possibility that, in this species as in G. duebeni (Kinne, 1952, 1953), temperature determines sex and that the males are born earlier in the fall than females. Whatever the
t
20-
1’ 0
J
’
’
FMAMJJ
’
’
’
’
’
A
’
S
’
0
’
N
1
D
Months
Figure 3. Seasonal changes in the juvenile portion of the total population of 1972 to December 1973. -, 1972; - - -, 1973.
Gammaruspalustris during January
Life
cycle of Gammarus
palustris
417
JFMAMJJASOND Months
Figure 4. Seasonal changes in the female : male sex ratio of Gammarus pahstris during January 1972 to December 1973. --, 1972; - - -, 1973.
explanation, the predominance of females during the spring doubtlessly enhances the reproductive potential. An approximate I : I ratio prevails during late spring and early summer. The changing mean size of the young and adult portions of the summer generation can be tracked from March to August for both years (Figure 5). Adult males exhibited marked growth between May to June and July to September and noticeable decreasesbetween June to July and September to October. The decrease may be attributed to the rapid recruitment of young males belonging to the summer generation, to higher temperatures favouring the production of males (Kinne, 1952, 1953) or to predation. In connection with the last attribution, it may be noted that the June-July and September-October standard monthly samples yielded the greatest densities of the grass shrimp Palaemonetespugio and three Fundulus spp. Fraser (1973) and Nixon & Oviatt (1973) h ave established that these species
/ Adult;
I ,,l,lllllllllllxlllllll,J JFMAMJJASONDIJ
FMAMJ 1972
I Months
JASOND 1973
Figure 5. The mean size distribution of (a) males (- - -), non-breeding females (-), breeding females (. . . .) and (b) juveniles of Gammam pahsbis during January 1972 to December 1973. W, over-wintering generation; S, summer generation. (No samples were collected in January 1973.)
418
C. P. Rees
are frequent predators of intertidal gammarid amphipods and Van Dolah (pers. comm.) has evidence to suggest that Fund&s spp. selectively take larger specimens of G. pahtris (note: males exceed females in maximum size). While the mean size of the adult females fluctuated less than that of males, there was a similar decrease between the June to July sampling times. However, unlike the pattern for the males, the mean size fell slightly (1972) or remained constant (1973) between August to September only to rise the following month. This may be the combined result of a possible lag in female recruitment and the maturation of the ‘mid summer generation’. At any rate, during October the lower mean size of the females and a female : male sex ratio of 0.24 may have enabled the female portion of the total population to better withstand predation by P. pugio and Fund&s sp. The growth rate of a large fraction of the over-wintering generation can be followed from October to May for both years. During this period the males grew at an average of 0.63 mm/ month (3.77 mm in total) and the females 0.29 mm (1.76 mm in total). A comparison of growth increments between years, possible only from February to May, reveals a slower increase for both sexes in 1972. This is not surprising in view of the slightly lower temperatures during the spring of that year. The over-wintering generation of both years terminated reproduction in late June-early July, whence the summer generation became dominant. The average monthly size of the juvenile portion of the population is shown in Figure 5. During March to May, the progeny of the spring reproductive activity grew at an average of 0.42 mm/month in 1972 and 0.53 mm/month in 1973. Thereafter, the likelihood of repeated brooding and predation and the introduction of a late summer surge of reproductive activity frustrates further analysis. The late summer component, eventually yielding most of the over-wintering generation, grew at an average of 0.49 mm/month from August to December 1972 and 0.56 mm/month between the same months in 1973. While the growth rate slowed appreciably at the end of both years, it may be inferred from Figure 5 that the spring of each year is characterized by a marked increase. Although the data for the over-wintering generation suggest that it produces only one or two broods of young in the spring, the summer generation appears to reproduce a number of times. In support of the latter observation, it may be seen that during May to August (July may be excepted because of the possible effect of predation) there is a perceptible mean size and marked percentage increase of ovigerous females. During August-September, the situation is complicated by the possibility that the initial brood(s) of the earliest summer generation ovigerous females may have sufficient time to mature and reproduce before the end of the summer. In the wake of this possible supplement to the breeding population, it is not surprising that the total population reached a peak of abundance the following month. For the remainder of the year the falling temperatures prolonged the time to maturity and consequently, recruitment of adults decreased. A portion of juveniles produced in September and October appear to over-winter as immatures and swell the breeding population as water temperatures rise in the spring. Thus, a female starting breeding in February and producing successive broods from March to the time of its death in early July, may have already produced one brood in the previous year. Summary I. A portion of the intertidal zone of a salt marsh along the lower Patuxent River, Maryland, was sampled from January 1972 to December 1973 to elucidate the life cycle of the amphipod Gammarus palustris Boustield.
Life
cycle of
Gammarus palustris
419
2. Monthly collections revealed a gradual (if fluctuating) increase in abundance from March to October after which numbers began to decline reaching a low in the mid-winter months. 3. The breeding season of G. pahstris extends from February to October, with major peaks of activity occurring in March and August-September. The spring peak is produced by the over-wintering generation whereas the late summer peak is a product of the summer generation. 4. The over-wintering generation first showed egg production in February and the first young appeared in March. By late May a few of these specimens had matured and many more were mature by late June. The over-wintering generation began to die-out during April, disappearing completely by late June-early July, while the summer generation continued reproducing through July, August and September, invariably ceasing activity during the winter months. 4. Finally, it is surmised that the initial brood(s) of the earliest summer generation females have sufficient time to mature and reproduce before the end of the summer. Acknowledgements Thanks are due to Messrs R. Orth, M. Smith, K. Thoemke and R. VanDolah for field assistance, often under difficult weather conditions. This work was supported by N.S.F. Grant 2M-AXAF. References Anderson, R. 0. 1959 A modified flotation technique for sorting bottom fauna samples. Limnology and Oceanography 4, 223-225. Bousfield, E. L. 1969 New records of Gammarus (Crustacea: Amphipoda) from the Middle Atlantic Region. Chesapeake Science IO (I), 1-17. Bousfield, E. L. 1973 Shallow-water Gammaridian Amphipoda of New England. 312 p. Cornell Univ. Press, Ithaca, N.Y. Fraser, A. 1973 Foraging strategies in three species of Fund&s. MS. Thesis, University of Maryland. 54 PP. Hynes, H. B. N. 1954 The reproductive cycle of some British freshwater Gammaridae. Journal of Animal Ecology 24, 352-387. Kinne, 0. 1952 Zur Biologie und Physiologie von Gammarus duebeni Lillj.; III: ZahlenverhZltnis der Geschlechter und Geschlechtsbestimmung. Kieler Meeresforschungen 9, I 26-133. Kinne, 0. 1953 Zur Biologie und Physiologie von Gummarus duebeni Lillj., VII: Uber die Temperaturabhiingikeit der Geschlechtsbestimmung. Biologisches Zentralblutt 72, 260-270. Nixon, S. W. & Oviatt, C. A. 1973 Ecology of a New England Salt marsh. Ecological Monographs 43 (4), 463-498. Teal, J. M. 1962 Energy flow in the salt marsh ecosystem of Georgia. Ecology 43 (4), 6x4-624. Williams, A. B. & Bynum K. H. 1972 A ten-year study of meroplankton in North Carolina estuaries: Amphipods. Chesapeake Science I3 (3), 175-xgz.