Caecidotea communis (Say 1818)

Asellus communis Say

Asellus communis Say, 1818, pp. 427–428.

Not Asellus militaris Hay, 1878, p. 90.

Not Asellus communis Say.—Racovitza, 1920, pp. 79–95, figs. 52–73.

Asellus communis was the first species of North American Asellus to be described. The description was extremely brief and no details were given of the male sexual pleopods; furthermore, no drawings were included. In view of the inadequacy of the original description, it is uncertain if any of the several subsequent redescriptions, none of which referred to original type material, in fact applied to A. communis (cf. Smith, 1874; Richardson, 1905; Racovitza, 1920; Van Name, 1936).

The description by Racovitza (1920) is of some importance since it was original in the sense that it was not based on previous descriptions and was very detailed. It also provided an extensive bibliography up to 1920 for the species. The description, however, was based on two male specimens and one ovigerous female sent to Racovitza by the United States National Museum from a collection made by W. P. Hay from the edge of the Potomac River in Virginia, a locality some 125 miles from the region where A. communis had apparently been collected by Say for the original description. The decision to regard these specimens as conspecific with A. communis appears to have been quite arbitrary on the part of Racovitza; indeed it seems that Racovitza did not even sight Say’s description, as indicated by his remarks (p. 79) under the heading “Type de l’espèce.”

Unfortunately, no specimens identified by Say appear now to exist. Say did not mention in the original description that types had been set aside, but there is a brief note following a comment on the habitat of the species (p. 427, “Cabinet of the Academy”), from which we may reasonably conclude that identified material had been placed in the collections of the Academy of Natural Sciences of Philadelphia. None of this material can now be found, according to information received from Mr. C. W. Hart, Jr., the Academy of Natural Sciences of Philadelphia (personal communication, January 1966), who made a search on my behalf.

In the absence of material named by Say, it is therefore impossible to determine with absolute certainty the identity of A. communis or its conspecificity or otherwise with the species described by Racovitza. Despite this uncertainty, however, it is clear that the name A. communis has been the most frequently used of all specific names when referring to epigean freshwater isopods in North America. Van Name (1936, p. 456), for example, states that it is “by far the most abundant and widely distributed isopod in the eastern half of the United States, also in southern Canada.”

In order to provide a solution to the identity of A. communis, and as rather precise details were given by Say of the area from which we may conclude he obtained his specimens, the decision was taken to create a neotype. This decision, it is felt, is in accord with the provisions of Article 75, Neotypes, of the International Code of Zoological Nomenclature Adopted by the XV International Congress of Zoology (1961) in that the neotype is designated in connection with revisory work and is essential for the identification of one of a number of closely similar species. Confirmation has been received from four colleagues who work on or are interested in the taxonomy of North American Asellus species that this procedure is not one they object to (Drs. E. L. Bousfield, T. E. Bowman, R. Prins, and H. R. Steeves III).

With reference to the distribution of A. communis, Say noted (p. 427) that it inhabits “small streams of fresh water, under stones,” and (pp. 427–428) is “a very common species in our fresh water, particularly in rivulets under stones. It is frequently introduced with the Schuylkill water into Philadelphia.” Bearing this information in mind, a collection was made at Valley Forge, about 20 miles northwest of Philadelphia, on 14 April 1967 from Valley Forge Creek, a small, moderately fast-flowing, stony-bottomed tributary of the Schuylkill River. This locality is proposed as the restricted type locality. Of the three male specimens collected, the largest was selected and designated the neotype.

Comparison of Racovitza’s (1920) description with the description given herein of the neotype of A. communis indicates that two species are involved, that is the species described by Racovitza is different from A. communis. The material from which Racovitza received three specimens in 1920 and upon which specimens he based his description is fortunately still in existence and has been used as the type collection for a new species A. racovitzai (see under this species).

Asellus militaris was described by Hay in 1878. In a later publication, however, he noted (1882, p. 241) that the species should be synonymized with A. communis. Probable syntype material of A. militaris still exists and this, on examination, proved to be conspecific with A. intermedius (see discussion under this species).

TYPE MATERIAL.—Neotype: adult , catalog number 7300, labeled “Asellus communis Say Neotype ().” Topotypes: two adults , catalog number 7301, labeled “Asellus communis Say topotypes (2 ).” All specimens in the collection of the Academy of Natural Sciences of Philadelphia and in jar labeled “Asellus communis Say neotype and topotypic material collected from Valley Forge Creek, near Philadelphia, Pa., 14 April 1967 by W. D. Williams.”

DESCRIPTION OF NEOTYPE.—Body: Length, 11.0 mm; maximum width, 4.0 mm. Color of live specimens mottled pale and dark brown. Surface smooth.

Head (Figure 1A): About twice as wide as long. Front margin distinctly concave. Eyes moderately large and quite distinct. Lateral margins of head with numerous simple spines of various lengths.

Thoracic terga: Roughly rectangular, posterior ones slightly larger than anterior ones, all with short to long spines on lateral and posterior margins. Second to seventh terga with anterior angles forming small lobes increasing in size posteriorly. First tergum (Figure 1B) without such lobes but coxa of first peraeopod prominent.

First antenna (Figure 1C): Flagellum 16-merous and tip not quite reaching to distal end of last segment of peduncle of second antenna; penultimate 3 segments bearing aesthetascs. Flagellum and peduncle subequal in length. All segments of peduncle more or less subequal in length. First peduncle segment about twice as long as wide; second and third segments respectively 3 and 4 times as long as wide.

Second antenna (Figure 1D): Length (6.0 mm) just over half (0.55) that of body. Flagellum 53-merous and about twice length of peduncle. First, second, and third segments of peduncle stout, each with several strong simple spines, and about as wide as long; fourth segment as long as first three combined, 3 times as long as wide; fifth segment about 1.5 times length of fourth, about 6 times as long as wide.

Lips: Upper lip (Figure 1E) subquadrate with dense fringe of fine setae distally. Lower lip (Figure 1F) bilobed, each lobe more or less triangular and fringed with long fine setae distally and marginally.

Mandibles: Each with a large, well-developed 3-segmented palp, the last 2 segments of which form a weak claw with its inner margins bearing many spines each with a fine setose ‘comb’ distally. Left mandible (Figure 2A) with 4-toothed incisor process and lacinia; spine row beneath lacinia of 15 unilaterally plumose spines. Right mandible of neotype missing, but that of a topotype (Figure 2B) with a 4-toothed incisor and a spine row beneath incisor of 16 finely pectinate to unilaterally plumose spines.

First maxilla (Figures 2C, D): Inner plate with 5 large plumose spines terminally and numerous fine, small, simple spines laterally. Outer plate with 11 variously dentate stout spines on distal margin, one long, fine, plumose spine near lateral distal angle, some small spines on outer proximal margin, and a fringe of fine setae on proximal part of inner margin.

Second maxilla (Figure 2E): Outer plate of 2 subequal laminae; outer lamina with about 22 long to short, pectinate or dentate spines on distal margin; inner lamina with 15 such spines on distal margin. Inner plate bearing a number of simple, plumose or pectinate spines on distal edge and with a medial dorsal row of about 36 long, simple spines.

Maxilliped (Figure 2F): Palp large with many slender spines on inner margins of segments and a few strong spines on outer margins. Masticatory lobe with several simple or plumose spines distally and 5 coupling hooks medially. Epipodite subquadrate, outer basal angle almost a right angle.

First peraeopod (Figures 3A, B): Relatively short and stout, subchelate. Dactylus as long as palm of propodus, with numerous teethlike spines on inner margin and ending in a distinct claw. Propodus almost as long as wide, subquadrate; palm with a single large triangular projection as tall as width of opposing part of dactylus and situated near midpoint, a smaller projection between larger projection and point of attachment of dactylus, 2 very strong teethlike spines proximally, and a submarginal row of spines on inner and outer surfaces. Carpus small, as long as wide, triangular. Merus larger than carpus, slightly wider than long, subtriangular. Ischium about twice as long as merus, length about 1.5 times width. Basis subrectangular, about 1.5 times as long as ischium, and twice as long as wide.

Second peraeopod (Figure 3C): Longer than but not as robust as first peraeopod; not subchelate. Dactylus about half as long as propodus with 5 teethlike spines on inner margin and a similar terminal spine. Propodus about 4 times as long as wide, and bearing distally a sclerotized triangular process. Carpus trapezoidal, twice as long as greatest width. Merus subtriangular, almost as long as wide, with some very long and strong spines at anterodistal angle. Ischium subrectangular, twice as long as merus and two-thirds as wide as long. Basis subrectangular, 1.5 times as long as ischium, about twice as long as wide.

Third peraeopod (Figure 3D): Similar to second peraeopod.

Fourth peraeopod (Figure 3E): Slightly shorter than second or third peraeopod. Dactylus half as long as propodus and with 4 teethlike spines on inner margin. Propodus 3 times as long as wide, notched at point one-third of length from distal end on inner margin with several long spines proximal to notch and a single triangular projection on distal margin. Carpus bent in long axis and forming with dactylus and propodus an almost subchelate structure. Otherwise rather similar to second peraeopod.

Fifth peraeopod (Figure 4A): Longer than fourth peraeopod. Dactylus about two-fifths as long as propodus and with 4 teethlike spines on inner margin. Propodus about 5 times as long as wide; anterior margin not notched but with several long spines; distal margin with triangular projection and anterodistal angle with a strong spine. Carpus more or less straight along long axis, about twice as long as wide, and three-quarters length of propodus. Merus slightly longer than wide, with a few robust spines at posterodistal angle. Ischium three-fourths as wide as long, and as long as carpus; posterior margin with several long spines. Otherwise rather similar to second peraeopod.

Sixth peraeopod (Figure 4B): Slightly longer than fifth peraeopod. Propodus about 4 times as long as wide. Carpus 2.5 times as long as greatest width. Otherwise similar to fifth peraeopod.

Seventh peraeopod (Figure 4C): Slightly longer than sixth peraeopod. Carpus about twice as long as wide. Otherwise rather similar to sixth peraeopod.

First pleopod (Figure 5A): Total length of appendage 1.26 times as long as second pleopod. Sympod subrectangular, about three-fourths as wide as long, inner margin with 5 hooklike protuberances for coupling. Distal segment also subrectangular, but outer margin very slightly concave; twice as long as wide, and about 1.33 as long as sympod; distal margin and distal half of outer margin bearing numerous short and simple spines; inner proximal angle with single spine.

Second pleopod (Figures 5B–E): Sympod subsquare with single spine near inner distal angle. Proximal segment of exopod with 3 setose and 1 simple spine on outer margin. Distal segment of exopod ovate with 23 long setose spines marginally and also many very fine setae arranged in groups of about 3 to 5 on surface of segment near inner margin. Endopod narrow, slightly curved medially, about as long as both segments of exopod, two-thirds length of sympod, and about 3 times as long as greatest width; prominent inner and outer apophyses occur basally. Cannula of endopod long and simple, extending beyond caudal process. Caudal process prominent, rounded, sclerotized, without associated hooks. Mesial process not evident.

Third pleopod (Figure 6A): Sympod small. Exopod forming large operculum for remaining pleopods, ovate; suture between proximal and distal segments running obliquely and proximally; outer and distal margins of distal segment with many relatively long plumose spines, inner submargin with several short simple spines; outer margin of proximal segment with many simple spines, and short simple spines also present along suture with distal segment. Endopod small and ovate.

Fourth pleopod (Figure 6B): Sympod small. Exopod ovate and with a row of relatively long simple spines and very short fine setae along outer proximal margin. Endopod ovate, smaller than exopod.

Fifth pleopod (Figure 6C): Exopod subrectangular, about 1.5 times as long as wide, and with several long simple spines (but no fine setae) along outer proximal margin.

Uropod (Figures 4D, 6D): Slightly shorter (0.89) than telson. Peduncle about twice as long as greatest width, with many marginal spines. Exopod two-thirds (0.69) length of peduncle; endopod about as long as (0.92) peduncle and 3 times as long as greatest width.

Telson (Figure 6D): Subcircular, as long as wide; apex obtusely pointed, and lateral and posterior margin with numerous simple and relatively short spines.

MATERIAL EXAMINED.—Apart from the neotype and the two topotypes (in part), the following material was examined and considered to be A. communis; such differences as occurred from the neotype were considered to be insignificant at the species level.

NOVA SCOTIA: Argyle stream, Yarmouth County, 3 , coll. E. L. Bousfield, 28.vi.1958 (NMC); stream near Doctor’s Cove, Shelboume County, 1 , coll. E. L. Bousfield, 28.vi.1950 (NMC); Pubnico Lake, Yarmouth County, 5 , coll. E. L. Bousfield, 21.vi.1956 (NMC).

ONTARIO: Lynn River, 1 , coll. Ontario Dept. P. & D., 10.vi.1955 (NMC); Chaffey’s Locks, Leeds County, 1 , coll. I. M. Smith, 4.ix.1965 (ROM).

COLORADO: Independent Reservoir, Boulder County, 4 , coll. S. J. Herrmann, 18.iv.1967; Longmount Power Station Lake, Boulder County, 7 , coll. S. J. Herrmann, 18.iv. 1967; Mirror Lake, Larimore County, ∞ , coll. S. J. Herrmann, 26.iv. 1967; Kid’s Lake, Larimore County, 30 , coll. S. J. Herrmann, 26.iv.1967; Meadow Lake, Larimore County, ∞ , coll. S. J. Herrmann, 26.iv. 1967; Rainbow Lake, Larimore County, ∞ , coll. S. J. Herrmann, 26.iv.1967; Sunset Lake, Boulder County, ∞ , coll. S. J. Herrmann, 26.iv.1967; Willow Lake, Larimore, ∞ , coll. S. J. Herrmann, 26.iv.1967.

MAINE: Bangor, 2 , coll. J. Brower, 21.iv.1962.

MARYLAND: Hall’s Creek, Dunkirk, Calvert County, 2 , coll. R. H. Greenfield and W. H. Ball, 25.vi.1934 (USNM).

MASSACHUSETTS: Cambridge, 1 , coll. Wheatland, April 1860 (MCZ); Cambridge, 8 , coll. Wheatland, 21.iv.1860 (MCZ); (?) Cambridge, 2 , collector and date not marked (MCZ); Salem, 11 , coll. Boston Society of Natural History, date not marked (MCZ); Beaver Brook, Danvers, ∞ , coll. H. W. Winkley, date not marked (USNM); Red Brook Pond, 3 , coll. E. L. Bousfield, 11.ix.1963 (NMC); Witch Brook and Crocker Pond, 8 coll. E. L. Bousfield, 24.ix.1965 (NMC).

NEW JERSEY: Swamp back of Orange Mountains, ∞ , coll. E. G. Mitchell, 1906 (USNM); Lakehurst, 1 , coll. D. Barr, 16.v.1962 (ROM); New Lisbon, 3 , coll. D. Barr, 16.v.1962 (ROM); Lakehurst, 2 , coll. D. Barr, 17.v.1962 (ROM).

PENNSYLVANIA: Conestoga River Lancaster County, 6 , coll. H. W. Levi, June 1948 (MCZ).

VERMONT: Lake Champlain,* 2 , coll. E. L. Bousfield, 19.vi.1956 (NMC).

WASHINGTON: Echo Lake* King County, 2 , coll. E. L. Bousfield, 20.vii.1955 (NMC).

WEST VIRGINIA: Halltown, 6 , coll. B. Bryan, date not marked (USNM).

Collections marked by an asterisk contained more than one species. Many of the collections in addition to males included juveniles and nonovigerous and ovigerous females, but since it is at present not possible to identify such material, no note of their occurrence is made. These two comments apply generally to all collections referred to in subsequent pages of this paper.

GEOGRAPHICAL DISTRIBUTION.—Figure 7 shows the geographical distribution of all localities detailed above (except those of uncertain position) as well as the position of the restricted type locality. It indicates that the species mainly occurs in the northeastern part of the United States and the southeastern part of Canada. There are two disjunct regions, however, where the species has also been recorded: Colorado, where it was recorded from eight different localities in the Denver area; and the State of Washington, where the species was recorded together with A. racovitzai in Echo Lake, King County. Material from both regions was examined with particular care, but as far as the present author could discern all specimens appeared clearly to be conspecific with material of A. communis from the eastern part of North America.

The material from Echo Lake was collected by Dr. E. L. Bousfield, and his notes upon the locality are of considerable interest; he wrote (personal communication, 1 September 1967):

Echo Lake is the type locality of Crangonyx richmondensis occidentalis H. & H., one of a species complex that is usually found together with A. communis in the east.… Crangonyx pseudogracilis Bousf., formerly thought to be endemic to eastern North America, has also turned up in material from Oregon and Washington [cf. Bousfield, 1961], and indicates that fresh-water peracaridans may have much wider distributions than formerly believed.

In this connection, the records of “A. communis” by Hatch (1947) are also of some interest; while no certainty can be attached to his specific determinations, the mention of material from the Lake Washington drainage basin (p. 171) with a single prominent tooth on the inner margin of the posterior surface of the propodus of the male first peraeopod suggests that he too might have been dealing with a “typically” eastern species, for males of A. occidentalis, the only widespread western species known, lack such a tooth.

One cannot rule out in the case of the Washington material missorting of labels (the material was not sorted by Bousfield personally); but this eventuality could hardly have occurred in the case of the material from Colorado, which was collected only a short time before my receipt of it. Artificial transference from the eastern States cannot perhaps be ruled out for the Colorado localities, since all are situated in a region with extensive numbers of artificial water bodies and reservoirs, and such waters are frequently recipients of food stocking (invertebrates) on the part of angling associations. Asellus is well known to be a nutritive fish food, but appears naturally to be rare or absent in most of the region to the north of Colorado. Thus, Dr. W. N. Rosine of Augustana College, South Dakota, wrote (personal communication, 11 May 1967):

I have collected amphipods rather extensively in South Dakota, Nebraska and Minnesota and have come across isopods only once.… I must say that over the years I have been rather surprised by the lack of isopods in this part of the county… it seems to me that if they were even occasionally present around here then I would have found them at some time or another. Collecting in Colorado produced the same experience. Visits in that State to literally hundreds of lakes, springs and streams yielded only two collections in the early 1950s.

ECOLOGY.—From the limited locality data upon labels in the collections of material examined, it seems that A. communis may occur in a wide variety of inland waters: from creeks, rivers, ponds, lakes, reservoirs, and, in one instance, from a swamp. At least with regard to choice of macrohabitat the species appears to be wide ranging. It does not, however, seem to be present in any of the Great Lakes, as it was never recorded in any of the numerous collections that I have examined from these waters. Hatchett (1947) commented upon the ecology of “A. communis” in Michigan at length, but as the characters he used to identify Michigan species of Asellus (number of segments in flagellum of first antenna, head shape) are variable and show no well-defined specific differences, the identity of his species remains uncertain. For the same basic reason, several other reports containing ecological data on “A. communis” must also be ignored.

FURTHER DESCRIPTION ().—This account, unless specifically noted otherwise, is based only upon material from the eastern part of North America, but the variation recorded is nevertheless inclusive for western material.

Body: The smallest male with well-developed secondary sexual characters was 4.0 mm long; the largest male examined was 18.0 mm.

First antenna: Flagellum 6- to 17-merous, the number of articles depending to some extent upon the size of the specimen; flagellum tip reaching to midpoint of last peduncle segment of second antenna or almost to distal end of this segment.

Second antenna: Length from just over half to same length as body, the fraction showing a rough inverse correlation with absolute body size. This relation is indicated in Figure 8 in which are plotted the appropriate values for the largest male in each of 27 collections, including those from Colorado and Washington (indicated differently). Flagellum 36- to 82-merous, the number of segments generally increasing with the length of the specimen.

Mouthparts: See Table 1.

First peraeopod: 2 to 4 (usually 3) very strong teethlike spines at proximal end of palm. Some variation occurs in the shape of the palm and its triangular process (cf. Figure 9).

First pleopod: Total length of appendage 1.0 to 1.47 (usually 1.1) times as long as second pleopod. Inner margin of sympod with 3 to 6 coupling hooks. Outer margin of distal segment straight to slightly concave.

Second pleopod: One to a few simple spines near inner distal angle of sympod. Proximal segment of exopod with 2 to 4 spines on outer margin, and distal segment with 19 to 24 marginal spines. Although no gross variations in the morphology of the tip of the endopodite occur, there are nevertheless minor morphological variations occurring between specimens from the same locality and also from different localities; Figure 10 illustrates the range of such variation. As indicated, the cannula may vary in length and may not extend beyond the caudal process or may distinctly do so. The caudal process itself may be no more than a slightly rounded distal protuberance, or may be quite prominent and even in some cases obtusely pointed. The typical morphology is as illustrated in Figures 5D,E for the neotype. For purposes of comparison, Figure 10 also indicates the structure of the endopoite tip of a specimen from Echo Lake, Washington, and Independent Reservoir, Colorado.

Uropod: See Table 2.

[Except A. kenki, males only; except where indicated, data from examination of all material available—details in text]

[Except where indicated, males only, and data from examination of all available material]