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2008 SOUTHEASTERN NATURALIST 7(2):207–218
Morphological and Developmental Differences in Three
Species of the Snapping Shrimp Genus Alpheus
Heather R. Spence1 and Robert E. Knowlton2,*
Abstract - Living, freshly collected individuals of three species of snapping shrimps
were studied to determine any differing morphological, developmental, and ecological
features: Alpheus heterochaelis, collected from Beaufort, NC; A. angulosus,
found mainly in Jacksonville, FL, but also at one site in Beaufort; and A. estuariensis,
collected at another Jacksonville site. Structural characteristics of these superficially
similar species are summarized, with particular attention to coloration. Adult A.
angulosus individuals have blue-green 2nd antennal fl agella (vs. tan in the other two
species) that are significantly shorter than those of A. heterochaelis. Alpheus angulosus
and A. estuariensis bear smaller eggs (<1 mm, regardless of embryonic stage)
than A. heterochaelis (>1 mm), and the former species displays the zoea larval form
typical of alpheids (vs. abbreviated larval development in A. heterochaelis).
Perhaps the most persistent and widespread noise in shallow-water tropical
and subtropical seas is that produced by “snapping shrimps,” decapod
crustaceans belonging to the family Alpheidae. In most alpheids, the chelate
first pereiopods (“claws”) are asymmetrical, one member of the pair (the
major chela) being much larger and more heavily calcified than the other
(Fig. 1). The noise is produced by vast numbers of individuals, each one
rapidly closing the dactylus, or movable finger, of its major claw onto the
pollex, or immovable finger, of the propodus, in the manner of a pistol hammer
(hence the alternative name “pistol shrimp”) (Knowlton and Moulton
1963, Versluis et al. 2000).
Many alpheid species are morphologically very similar and consequently
“difficult to recognize using traditional systematic methods” (Knowlton
1986). Preserved specimens have been relied upon in the process of defining
species, but some distinguishing characteristics, such as color, cannot be
considered as they are lost in the process of preservation (Knowlton 1986).
Alternative methods are required to discern some species.
A number of morphologically similar species from the so-called Alpheus
edwardsii group are found along coastlines of the southeastern
United States. Alpheus heterochaelis Say (big-claw snapping shrimp) was
first described in 1818, but since then new species have been delineated
among specimens originally thought to be A. heterochaelis. One of these
1Department of Biology, University of Massachusetts, North Dartmouth, MA
02747-2300. 2Department of Biological Sciences, George Washington University,
Washington, DC 20052. *Corresponding author - firstname.lastname@example.org.
208 Southeastern Naturalist Vol.7, No. 2
is Alpheus estuariensis Christoffersen (1984); another is Alpheus angulosus
McClure (2002), initially named Alpheus angulatus (McClure 1995).
Although the A. heterochaelis neotype is from northern Florida (Amelia
Island), A. angulosus is actually the most common species collected in
Florida locations, and A. heterochaelis is more commonly found in North
Carolina (McClure 1995).
The morphological differences recognized among these three species
are very subtle, and verbal descriptions, especially color, can be vague.
Field identification is important because the species are sympatric, yet
some of the most significant morphological differences documented are
not useful distinguishing features in identifying live shrimps. For example,
the angular carapace characteristic of A. angulosus (McClure 1995) is obscured
by the pereiopods.
Developmental differences between A. heterochaelis, A. angulosus, and
A. estuariensis have not yet been established. Knowlton (1970) described the
developmental pattern of A. heterochaelis from North Carolina and postulated
that then-assumed A. heterochaelis from Florida (now known to be A.
angulosus) was a different species based on observations of smaller eggs in
the Florida shrimps.
Snapping shrimps are characterized by unique behaviors and interesting
interactions with other species and abiotic aspects of the environment.
Aspects of behavioral-ecological investigation have included mechanics of
the snap (e.g., Versluis et al. 2000), sociality (Duffy et al. 2002), aggression
(Knowlton and Keller 1982), chemical and visual signaling (Hughes 1996),
and habitat selection (Corfield and Alexander 1995). Animals thought to be
A. heterochaelis have been and continue to be used as subjects of many
investigations, making it all the more important to facilitate correct identifi-
cation of this and its “look-alike” species.
The purpose of our research project was to discern and describe developmental,
ecological, and additional morphological differences between
Alpheus populations from two geographically separated areas: southern
North Carolina and northern Florida. These lines of investigation, centered
around the examination of live specimens, together can provide a further and
clearer basis for distinguishing the three species.
Figure 1. Dorsal view of male A.
heterochaelis, showing “balaeniceps-
type” minor chela.
2008 H.R. Spence and R.E. Knowlton 209
Shrimp were collected at two sites in each state. Both North Carolina
sites were located at the mouth of the Newport River (estuary), Beaufort, approximately
0.4 km apart and on nearly opposite sides of Beaufort Inlet. The
primary collection location was near the Duke University Marine Laboratory
(DUML), on the western side (“Research Cove”) of Pivers Island Road. The
other North Carolina site was “Duncan’s Green” (DG), at the west end of
Front Street in Beaufort. The collection locations in Florida, about 10 km
apart, were Fort George Inlet (FGI), and Round Marsh (RM), both along
the shore of the St. Johns River (estuary), Jacksonville. The FGI collection
site was near the mouth of the river, in Huguenot Memorial Park, near the
south side of the bridge (FL Route A1A) linking Fort George and Little Talbot
Islands. The RM site was near a salt marsh in the backwaters of the St.
Johns River, near an observation platform in the Fort Caroline portion of the
Substrata at the collection sites consisted of a mixture of sand and mud in
different proportions (of sand/silt/clay), and always included, to various degrees,
Crassostrea virginica (Gmelin) (oyster) shells and some live oysters.
At DUML, shell clumps were sitting on a muddier substrate, sloping toward
the water, than the other locations. The FGI site was very fl at, with a sandier
substrate supporting rocks (mainly concrete slabs and other rubble from reconstruction
of the bridge) encrusted with oysters. At RM, the oyster shells
were comparatively more plentiful but generally looser (i.e., not in clumps).
The salinity at RM was lower (25 ppt) than at the other sites (35 ppt).
Collection and maintenance
Shrimp were collected manually during low tide, at DUML, FGI, and
RM, during two different seasons: July 1–5, 2004 (hereinafter designated
“summer”), and November 15–17 and December 4–5, 2004 (“fall”); on
March 14, 2005, a brief collection was made at DG. Animals were mostly
found either by turning over shell clumps or rocks located around the mean
low-water mark or pushing a hand-held dip net through loose shells. On site,
each collected shrimp was placed into a plastic bag half-filled with seawater
from the site, along with 1–2 oyster shells to provide a shelter for the shrimp.
Multiple animals found under the same rock were usually two in number and
of opposite sex, presumed to be a mating pair, thus kept in the same bag.
The bags and their contents were transported by plane to the laboratory
at George Washington University, where they were catalogued (organized
based on location, each animal assigned a number) and placed in 20-cmdiameter
glass bowls individually (except for summer-collected paired
animals; these were placed together in plastic 20-cm x 12-cm aquaria). Collection
water was replaced with a solution of artificial sea salt mix made to
the salinity of the water in which the shrimp were collected. Oyster shells
210 Southeastern Naturalist Vol.7, No. 2
that were free of macroscopic encrusting organisms (to reduce the risk of
bacterial infiltration) were positioned in each tank. Each animal was sized
(carapace length measured) and characterized in terms of sex, “handedness”
(side bearing the major chela), and any unusual features.
Temperature, salinity, and pH were kept within normal ranges while the
animals were maintained in the laboratory. Water was aerated with pumps
and air stones, supplemented with potassium iodide to facilitate molting, and
changed approximately every 3 days. Laboratory lights were turned on and off
in concert with the natural photoperiod to the extent possible. Shrimp were fed
TetraMin tropical fish fl akes or shrimp pellets every few days, generally preceding
water changes to minimize fouling of the tank water. Shrimp that died
were fixed using 4% formalin and preserved in 70% ethyl alcohol for future
reference and morphological study. Voucher specimens were deposited into
the US National Museum of Natural History (USNM), Washington, DC, as
follows: A. angulosus—two specimens (mating pair), USNM 1098194, Fort
George Inlet of St. Johns River, Jacksonville, FL, coll. R.E. and M.K. Knowlton,
3 July 2004 (died in lab 12 July 2004); A. estuariensis—one specimen,
USNM 1098195, Round Marsh of St. Johns River (Timucuan Preserve: Fort
Caroline), Jacksonville, FL, coll. R.E. Knowlton, 2 July 2004 (died in lab 7
September 2004); A. heterochaelis—two specimens (mating pair), USNM
1098193, “Research Cove” near Duke University Marine Laboratory, Beaufort,
NC, coll. H. Spence, 1 July 2004 (died in lab 20 July 2004).
Morphological features of adult shrimp and developmental stages were
determined through observation and photography of our collected living
material, supplemented by examination of all available preserved specimens
of the three species stored in the USNM.
Digital photographs, made using an MTI 3CCD camera and FlashPoint
FPG 3.10 program through a Leica MZ12 microscope and analyzed with
program ImageJ 1.20s, were taken as quickly as possible after collection to
document natural coloration. We found that using a bowl of about the same
diameter as the animal, combined with drawing off some of the sea water in
the bowl to about the animal’s height, was reasonably successful in immobilizing
a shrimp long enough to photograph it without desiccating it. Ventral
views could be obtained by inverting the animal contained within a covered
Petri dish. Turning off or dimming the lights between taking photographs
also helped the shrimp stay still, as did the use of backlighting.
Reproductive activity, such as the presence of eggs on pleopods
(swimmerets) of females, or ripe ovaries, was noted at time of collection.
Embryos of A. heterochaelis and A. angulosus in various stages of
development were examined and photographed (as above), referenced
with preserved specimens in USNM collections and Knowlton’s (1973)
description of A. heterochaelis development. Egg characteristics, such as
2008 H.R. Spence and R.E. Knowlton 211
approximate number, size, shape, color, stage of embryonic development
(indicated by percentage of egg area occupied by yolk and appearance of
compound eyes), were recorded upon arrival at the laboratory and tracked
subsequently until hatching or loss. Early larval characteristics were noted
for a single live specimen.
Overall, 77 individuals were collected. The summer collection yielded a
total of 51 shrimps: 24 from Florida (19 A. angulosus from FGI, 5 A. estuariensis
from RM) and 27 A. heterochaelis from North Carolina (DUML).
Included among them were 12 ovigerous females: 5 A. angulosus and 7 A.
heterochaelis. The fall collection yielded a total of 22 specimens: 15 from
Florida (11 A. angulosus from FGI, 4 A. estuariensis from RM) and 7 A.
heterochaelis from North Carolina (DUML); of these, 5 A. angulosus females
bore eggs. In March, 4 shrimps were found from DG, the only site
where both species were collected: 3 individuals of A. angulosus (including
a mating pair) and one large female A. heterochaelis. The latter was found
closer to the water and deeper into the mud than the former, which were, as
in Florida (FGI), under more shallowly situated rocks.
All alpheids collected from DUML clearly matched species descriptions
for A. heterochaelis (e.g., McClure 1995, Williams 1984), while those from
FGI and RM generally matched the species descriptions for A. angulosus and
A. estuariensis, respectively (e.g., McClure 1995, 2002). Individuals of A.
heterochaelis (in summer collection) were generally larger (mean carapace
length ± standard deviation = 10.1 ± 2.5 mm; number of individuals = 24) than
A. angulosus (8.2 ± 1.1 mm, n = 19), although there was some overlap; those
of A. estuariensis were consistently smaller (7.1 ± 0.5 mm, n = 4). The difference
in means between A. heterochaelis and each of the other two species was
significant (vs. A. angulosus: t = 3.06, df = 41, P < .01; vs. A. estuariensis: t =
2.33, df = 26, P < .05), but between A. angulosus and A. estuariensis, it was not
(t = 1.85, df = 21, P > .05).
At DG, where A. heterochaelis or A. angulosus were sympatric, an overall
color difference between these species was discernable. While some A. angulosus
individuals (collected at FG) and A. estuariensis (from RM) were seen
to have diffuse blue pigment on their uropods (Fig. 2a), bright blue spots, with
orange on anterior margins, were found to be a major distinguishing feature of
A. heterochaelis (from DUML) (Fig. 2b). Also, there is a fl attened triangular
area of the carapace at the base of the A. angulosus rostrum (Fig. 3a), but not
in the other two species (Fig. 3b). The minor chela of A. angulosus is visibly
broader than that of A. heterochaelis (Figs. 4a, b), and it does not bear the
row of long setae (“balaeniceps-type” claw) characteristic of A. heterochaelis
males (Fig. 1; also noted and illustrated in McClure 1995, McClure and
212 Southeastern Naturalist Vol.7, No. 2
Figure 2. a. Tan to pale blue tail fan of A. estuariensis (also characteristic of A. angulosus).
b. Tail fan of A. heterochaelis, with characteristic bright blue spots on uropods.
Figure 3. a. Rostrum of A. angulosus, exhibiting triangular base (indicated by arrow)
and fl anked by eyes. b. Rostrum of A. estuariensis, which lacks triangular base (as
does the rostrum of A. heterochaelis).
Figure 4. a. Anterior region of A. angulosus exhibiting relatively wide minor chela
and paler coloration after being kept in the laboratory. b. Anterior region of female
A. heterochaelis, showing thinner (vs. A. angulosus) minor chela and tan antennal
fl agella. c. Anterior region of A. estuariensis, exhibiting characteristic slender minor
chela, tan antennae, and angular dactylus of major chela.
Wicksten 1997). The long and slender minor chela of A. estuariensis (Christoffersen
1984, McClure 1995) is distinguishable (Fig. 4c). Another important
feature of A. estuariensis is its diffusely banded color pattern (Christoffersen
1984); on the dorsal side of each abdominal segment, the anterior margin is
lighter than the posterior one (Fig. 5).
a b c
2008 H.R. Spence and R.E. Knowlton 213
Figure 5. Dorsal view of A. estuariensis
abdominal segments, showing characteristic
Figure 6. Dorsolateral view of A. angulosus
head, showing blue antennal fl agella (one
indicated by arrow).
Figure 7. Eggs of A. angulosus (top) and
A. heterochaelis (bottom) about halfway
through embryonic development. Egg
size: top, 0.71 x 0.60 mm; bottom, 1.12 x
Figure 8. Stage II zoea larva of A. angulosus.
Total length = 2.56 mm.
214 Southeastern Naturalist Vol.7, No. 2
The most significant and consistent new characteristics separating A. angulosus
from the other two species are color of both pairs of antennal fl agella
and length of the 2nd pair: blue-green and short, respectively, in A. angulosus
(Fig. 6); tan (red-brown) and long, respectively, in A. heterochaelis and A.
estuariensis (Figs. 4b and 5). The proportion of antennal fl agellum length
to carapace length was found to differ significantly between A. heterochaelis
and A. angulosus with means of 4.2 ± 0.7 and 3.2 ± 0.8, respectively (t =
3.04, df = 18, P < .05). Alpheus estuariensis (3.4 ± 0.9, n = 3) was not included
in the length analysis due to low sample size.
In general, shrimps kept in the laboratory gradually lost the overall dark
coloration present at collection, becoming pale tan to virtually translucent
(Fig. 4a). This “blanching” phenomenon was markedly greater in A. heterochaelis
and A. angulosus than in A. estuariensis. However, even after
extended periods in the lab, the antennae of all A. angulosus individuals
retained their blue-green color, and those of A. heterochaelis and A. estuariensis
their tan color.
In the fall collection of A. angulosus, the pleopods of females were observed
to bear viable but numerically few eggs in various stages of embryonic
development; A. heterochaelis females were not gravid in the fall. There was no
obvious difference in egg number per female between the summer A. angulosus
(FL) and A. heterochaelis (NC) populations. The number of eggs found on a
given ovigerous female ranged from a few to over 200. Eggs of A. heterochaelis
in the earlier stages were about twice as big as similarly developed eggs of
A. angulosus (Fig. 7); this relationship persisted throughout later stages (e.g., A.
heterochaelis, 1.53 x 1.21 mm, vs. A. angulosus, 0.75 x 0.60 mm). The eggs of
both species contained green yolk, but there was one instance of brown-colored
yolk in A. angulosus. Based on measurements of eggs attached to pleopods of
A. estuariensis females preserved in the USNM collection, sizes at comparable
stages are about 0.5 mm (early) and 0.9 x 0.7 mm (close to hatching).
More often than not, the ovigerous females kept in the lab did not retain
eggs on their pleopods, but a single live larva was found to have hatched
from one of the A. angulosus eggs (fall collection). Although the larva was
photographed (Fig. 8) and examined upon discovery, the first instar was
presumed to be missed since, in alpheids with extended larval development,
it typically is only a matter of hours before the molt to the second instar occurs
(Knowlton 1973). The larva swam around for a few days after hatching,
but did not survive past “Stage II.” Compared to descriptions and figures
of A. heterochaelis larvae (Knowlton 1973), the two species at “Stage II”
exhibited the following similarities: antennal scales with terminal segments,
stalked compound eyes, three pairs of maxilliped exopods, visible rudiments
of other thoracic appendages, telson with 7 + 7 plumose setae, and a median
notch. Larval features of A. angulosus that were different include smaller
size, the lack of pleopod rudiments on the abdomen, presence of a large red
chromatophore at the base of the telson, less residual yolk, and possibly a
more strongly notched telson.
2008 H.R. Spence and R.E. Knowlton 215
Our collection data, albeit limited to four sites, are consistent with Mc-
Clure and Wicksten’s (1997) observation that, between Alpheus angulosus
and A. heterochaelis, one or the other species was generally much more
common at each of their sampling localities. In previous field work (R.E.
Knowlton, unpubl. data) at the Beaufort sites, A. angulosus was rarely found
at DUML (one individual, compared to 19 A. heterochaelis), but was more
abundant at DG (10 animals, vs. 30 A. heterochaelis), confined mainly to
a small area of predominantly loose oyster shells over a rather sandy substratum;
in contrast, A. heterochaelis was almost always under larger shell
clumps partially embedded in mud (at both sites).
In our study, A. angulosus was found to be more difficult to distinguish
visually from A. heterochaelis than from A. estuariensis. Alpheus angulosus
is described as distantly related to A. heterochaelis and A. estuariensis, being
more closely related to A. armillatus, which has a conspicuous banded color
pattern (Mathews et al. 2002). However, since several species are currently
confused with A. armillatus, and some of them are present in Florida and
elsewhere along the southeastern US coast (Mathews 2006), the affinities
and actual distribution range presently remain undetermined.
The main new morphological finding of our study is the difference in
antennal fl agellum color and length between A. angulosus and the other two
species. While freezing has been used to preserve coloration for description
(McClure 1995), examination of live animals, preferably recently collected
ones, reveals important taxonomic characters that are not likely to be distorted.
Especially among Alpheus spp., differences in coloration have been
shown to be of systematic importance (Knowlton and Mills 1992).
Previous morphological descriptions generally matched our findings
(summarized in Table 1), but further clarification is desirable for functional
use in identification. Antenna length and color, plus chela morphology,
are probably the easiest means of identification of these three species.
Chela morphology, which exhibits a certain degree of sexual dimorphism
(McClure and Wicksten 1997), is especially useful if shrimp are found in
mating pairs; thus, males and females of the same species can be compared
to each other.
The A. angulosus larva that hatched exhibited the “zoea” larval form
typical of most species of Alpheus (Knowlton 1973), as well as caridean
shrimp in general. Based on observations of larvae captured in plankton
and/or reared in the laboratory, alpheid species have typically been shown
to exhibit an extended period (circa 2–3 weeks) of larval development involving
at least 4, and probably more (about 9), instars (Knowlton 1970).
In contrast, A. heterochaelis hatches as a larger (>l mm, regardless of
216 Southeastern Naturalist Vol.7, No. 2
stage), more advanced larva that passes through only 3 instars in 4–5 days
(Knowlton 1973). The smaller eggs and larva of A. angulosus (Table 1),
however, are consistent with extended post-embryonic development, being
the result of a shorter period of embryonic growth and morphogenesis;
based on egg size, A. estuariensis also appears to demonstrate this pattern.
The fundamental differences found between A. heterochaelis and A. angulosus
with regard to egg size and pattern of larval development indicate
strong differences in reproductive biology. Interspecies habitation of the
same burrow has been observed for other species of snapping shrimp, and
linked to facultative symbiosis with interspecific communication (Boltaña
and Thiel 2001), but was not observed between males and females of different
species in the present study.
Traditional taxonomic practices, such as careful observation of preserved
adult specimens, are certainly of value in discerning some differences among
species. But with regard to morphologically similar Alpheus spp., such as
those described above, it becomes all the more important to consider additional
characters (e.g., color) based on living animals in different ontogenetic
phases, and to investigate ecological-behavioral features (e.g., habitat preferences),
some of which may be found to be unique enough to be helpful in
locating and identifying particular species in the field. The variety of features
described here also are interrelated with each other (e.g., morphogenesis) and
Table 1. Key morphological features differentiating the principal southeastern US Alpheus spp.,
based on this study and Christoffersen (1984), Knowlton (1973), McClure (1995), McClure and
Wicksten (1997), and Williams (1984). Unless otherwise indicated, characters refer to adults.
Character A. angulosus A. estuariensis A. heterochaelis
Antennal fl agella: Blue, short (Fig. 6) Tan, long Tan, long (Fig. 4b)
color, length (Figs. 4c, 5)
(of 2nd antenna)
Base of rostrum Widens into fl attened Triangular area Triangular area
triangular area lacking lacking
on carapace (Fig. 3a) (Fig. 3b)
Major chela: Present Absent Absent
Minor chela: Short, broad (Fig. 4a) Long, very Long, “balaeniceps” in
propodus and slender male (Figs. 1, 4b)
dactylus (Fig. 4c)
Uropods: color Tan to pale blue Tan to pale blue Bright blue spots bordered
(Fig. 2a) with orange (Fig. 2b)
Egg size (regardless Less than 1 mm Less than 1 mm More than 1 mm (Fig. 7)
of embryonic stage) (Fig. 7)
Larva (1-day old): 2.5–2.6 mm, pleopods Unknown 4.6–4.8 mm, pleopods
total length, pleopod absent (Fig. 8) biramous but
2008 H.R. Spence and R.E. Knowlton 217
the ecological roles of the species, and are important considerations for research
involving complexes of superficially similar alpheid species.
We wish to thank William Kirby-Smith at the Duke University Marine Laboratory
and Craig Morris and Daniel Tardona of the National Park Service at Fort
Caroline for invaluable collection guidance, as well as Diana Lipscomb for guidance
in using the microscope digital camera. Marilyn Schotte facilitated the museum work.
Henry Merchant, Melissa Hughes, Nancy Knowlton, Lauren Mathews, Martin Thiel,
and Arthur Anker provided indispensable input. We would also like to thank SuMin
Hong, Matthew Lowery, and Tara Scully for their help in maintaining the animals in the
laboratory, and Tony Chan for providing some useful observations in an unpublished
preliminary study. We dedicate this paper to the late Paul Spiegler, who encouraged
and inspired us with his love of natural history. Financial support for this project was
received from the Enosinian Scholars Program at George Washington University.
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