Arthur G. Humes and Hugo A. Janmeback.
Najadicola ingens (Koenike), a water-mite parasitic in fresh-water clams.
Psyche 57(3):77-87, 1950.

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PSYCHE
VOL. 57 SEPTEMBER, 1950 No. 3
NAJADICOLA INGENS (KOEiNIKE),
A WATER-MITE PARASITIC IN FRESH-WATER CLAMS Department of Biology, Boston University The distribution of Najadicola ingens (Koenike 1895) Piersig 1897, a hydrachnid parasite of fresh-water clams in North America, is known only from a few scattered records. Relatively few speci- mens have been reported up to the present. Koenike (1895) de- scribed the species from specimens found in Anodonta fragilis Lamarck and Unio complanata (Solander)' and sent to him by Dr. Tyrrell of Ottawa. The collection locality was given only as Canada. Wolcott (1899) collected this mite from Anodonta fragilis at "26" Lake, near Charlevoix, Michigan, and Intermediate Lake, Ellsworth, Michigan, from Unio gibbosus Barnes, U. ligamentinus Lamarck, and Anodonta footiana Lea at Grand Rapids, Michigan, and from U. luteolus Lamarck at Long Lake, Kalamazoo, Michigan. He ex- amined 3500 clams representing 60 species from Michigan, Wiscon- sin, Nebraska, New York, Illinois, Iowa, and Pennsylvania, but found not more than 16 specimens, all from the Michigan localities just mentioned. Wolcott (1918) stated that the species is generally distributed, but did not give additional distribution records. Marshall (1929) reported 16 adults and a few nymphs in two specimens of Anodonta in a small lake on Bruce peninsula, near Georgian Bay, Ontario. Kelly (1 899) examined 161 4 clams representing 44 species from Illinois, Iowa, and Pennsylvania, but reported no N. ingens, although he found several species of Atax (now Unionicola). In view of the abundance indicated below of N. ingens in. New Eng- 'The name Unio is now properly applied to certain European clams, the North American species formerly included having been placed in other genera.
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Pu&e 57:77-87 (1950). hup ttpsychu einclub org/S7/57-077.htd



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78 Psyche [September
land, the apparent scarcity in other parts of the United States and Canada would seem to be only because of the inattention of in- vestigators to this species and lack of collections. During the five months from June 14 to November 19, 1949, 3077 fresh-water clams from 74 localities in New England, Quebec, and New Brunswick, representing 14 species, were examined for N. ingens. The mite was found in 43 or 58% of the localities. Al-
together 906 specimens of N. ingens were collected, comprising 494 males and 412 females. Specimens of the clam hosts, together with about 50 mites of both sexes and from each of the three hosts re- ported below, have been deposited in the Museum of Comparative Zoology, Harvard University. The assistance of Mr. William J. Clench of the Museum of Comparative Zoology in identifying the clams is gratefully acknowledged.
Mitels were collected from clams in the following localities which are indicated on the accompanying map (Fig. 1) : Massachusetts :
(1) Sampson Pond, South Carver ; (2)Monponsett Pond, Halifax ; (3) Unionville Pond, Holden ; (4) Carbuncle Pond, Oxford ; (5) Lake Chaubunagungamaug, Webster ; (6) Wickaboag Pond, West Brookfield; (7)Sandy Pond, Ayer; (8) Ward Pond, Ashby ; (9) Nabnassett Pond, Tyngsboro ; (10) Mashpee Pond, Mashpee; (1l)Buckmaster Pond, Westwood; (12)Lake Winneconnet, Taunton; (13)Quarter Mile Pond, Medford ;
(14) Forge Pond, Westford; (1 5) Bare Hill Pond, Harvard ; (1 6) Warner's Pond, Concord ; (1 7) Heart Pond, South Chelmsford ; (1 8) Lake Cochichewick, North Andover ; (1 9) Winter Pond, Winchester ; (20)Lake Attitash, Amesbury ; (21) Fort Pond, Lancaster ; (22) Hoosicwhisick Pond, Milton; (23) Hemenway Pond, Milton.
Rhode Island : (24) Worden's Pond, South Kingston. Vermont : (25) Island Pond, Brighton ; (26) Bomoseen Lake, Castle- ton; (27)Lake Champlain at Sandbar State Forest Park, Mil- ton.
New Hampshire : (28) Crystal Lake, Eaton Center ; (29) Province Lake, Effingham ; (30) Wash Pond, Hampstead ; (3 1) Island Pond, Hampstead ; (32) Great Pond, Kingston ; (33)Angle Pond, East Hampstead ; (34) Country Pond, Newton. Maine : (35) Long Pond, at Long Pond; (36) Madawaska Lake, Stockholm; (37)stream at Ouelette ; (38) Eagle Lake ; (39)



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Humes and Jamnback-Parasitic Water-mite
COLLECTION
LOCALITIES FOR
NAJADICOLA INGENS
Pig. 1. Localities in New England, Quebec, and New Brunswick where Nu jadicola ingens was collected.




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80 Psyche [September
Park's Pond, Clifton ; (40) Unity Pond, Unity ; (41) China Lake, China.
Quebec : (42)lMassawippi Lake, at Ayer's Cliff. New Brunswick: (43) Cranberry Lake, Harvey Station. Of the 14 species of clam hosts 3 were parasitized by N. ingens and 11 were negative. The 3 positive species were Elliptio complan- atus (Solander) (2054 specimens of which 231 or 11.2% were positive), Anodonta cataracta Say (625 specimens of which 242 or 38.7% were positive), and Lampsilis radiata (Gmelin) (152 speci- mens of which 18 or 11.8% were positive). The negative species included 17 Ligumia nasuta (Say), 3 Lampsilis cariosa (Say), 3 Lampsilis ~entric~osa (Barnes), 19 Lampsilis ochracea (.Say), 79 Anodonta implicata Say, 59 Anodonta marginata Say, 38 Alasmi- donta undulata (Say), 1 Alasmidonta heterodon (Lea), 25 Strophi- tus rugosus Swainson, 1 Leptodea fragilis Rafinesque, and 1 Prop- tera alata Say.
The highest degree of parasitism in any single locality was 87.3% for Anodonta cataracta at Wash Pond, N.H., and 57% for Elliptio complanatus at Crystal Lake, N.H.
Judging from the relative number of individual clams parasitized, the preferred host seems to be Anodonta cataracta, with 1 out of every 3 clams containing mites. Elliptio complanatus appears to be next in preference, with 1 out of every 9 parasitized. Lampsilis radiata seems least preferred of the three hosts. The high figure of 11.8% is misleading, since it is based upon parasitized clams from only two localities. One, Massawippi Lake, Quebec, produced 18 clams of which 16 were parasitized; the other, Lake Champlain at Sandbar State Forest Park, Vt., comprised 23 L. radiata of which only 2 contained mites. The remaining 134 L. radiata were negative. The samples of clams from the various localities were not of equal size. At several localities only a few clams were collected. Table I shows the number of clams of each of the 3 host species examined in the 43 localities. The numbers in parentheses indicate the number of individuals which were parasitized by N. ingens. The average number of clams examined of all 14 species in the 43 positive local- ities was 51 (1-253) and in the 31 negative localities 28 (3-134). Only 2 of the negative localities comprised more than 100 clams. Excluding these the average number of clams examined in 29 neg- ative localities becomes 21 (3-80). These data indicate the possibil-



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Humes and Jamnback-Parasitic Water-mite
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Table I. The total number of clams examined and number with N. ingens for each of the three host species in the 43 positive localities.



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82 Psyche [September
ity that mites in certain localities may have been overlooked because of the small sampling.
Commonly 1 male and 1 female mite were found in each par- asitized A. cataracta and E. complanatus. ,Single males and single females were frequently found in these clams, but very rarely two of the same sex. This was not the case in L. radiata, where at least two of each sex were present, one clam having 4 males and 8 females. The average num'ber of mites per clam was 1.7 in E. cornplanatus and 1.8 in A. cataracta.
The smaller and presumably younger individuals of E . complan- atus and A. cataracta were more frequently parasitized, as shown in Figs. 2 and 3. The arithmetical mean length for unparasitized E. cornplanatus was 6.9 cm. and for parasitized individuals 5.6 cm. The arithmetical mean length for unparasitized A. cataracta was 7.1 cm. and for parasitized individuals 6.2 cm. The largest par- asitized clam was an A. cataracta 11.2 cm. in length. The smallest was an E. complanatus 2.7 cm. in length. The position of N. ingens in the host clams varied with the host species. Almost invariably when there was a male and a female present they occurred in the same suprabranchial chamber, usually in the anterior half of the gill area. Immature mites or single males were generally located di~tally~between the gill lamellae. The mites occurred in any of the four suprabranchial chambers, but there was noticed a distinct difference in location in E. complanatus and A. cataracta. In A. cataracta the mites occurred with about equal fre- quency in the right and left outer suprabranchial chambers. In only 4 individuals were mites found in the inner gill chambers. In E. complanatus the mites occurred nearly equally in the right and left inner suprabranchial chambers. In only 6 individuals were mites found in the outer gill chambers. Since mites were found in the outer chambers of E. cornplanatus and in the inner chambers of A. cataracta, they are probably not mechanically prevented from entering those chambers. The position of the mites seems to have little relation to the use of the gill as a marsupium, since in A. cataracta mites were found in both gravid and non-gravid gills. According to Baker (1928) the specialized long-term breeding clams like Anodonta have the gills modified to provide better water circula- tion than in the short-term breeders like Elliptic. This may have some bearing on the difference in location of the mites in the two clam genera. It is of interest to note that both these genera use the



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19501 Humes and Jamnback-Parasitic Water-mite 83 two outer gills as marsupia! In the 17 positive Lampsilis radiata the mites occurred nearly equally in the four suprabranchial cham- bers. One clam had mites in all four of its suprabranchial chambers. Ninety-one per cent of the parasitized clams had papillae of var- ious sizes on the walls of the suprabranchial chambers. As many as 40 papillae were counted in one suprabranchial chamber contain- ing a pair of mites. The largest papillae measured were 3 mm. in diameter and 5 mm. in length. Fig. 4 shows a specimen of Anodonta cataracta from which the left valve and mantle have been removed. The left outer suprabranchial chamber has been opened to show the papillae, a male and a female N. ingens, and several small, dark egg masses.
In only 10 clams were papillae found without mites. Pre- sumably these clams had previously been parasitized by N. ingens. The papillae were often arranged in an incomplete ring around the mites and their egg masses, perhaps acting as a barrier to prevent the eggs from being carried out of the gill chambers, either via the excurrent siphon in Anodonta or into the mantle cavity by way of the incomplete connection of the innermost gill lamella and visceral mass in Elliptic. The papillae occur mostly within the supra- branchial chambers, but in a few cases in E. complanatus extended down 011 the side of the visceral mass.
The presence of the mites apparently interferes with the use of the gills as marsupia. In many gravid, parasitized A. cataracta the marsupia containing mites were asymmetrically developed. The anterior half of the parasitized gill contained few or no glochidia, while the unparasitized gill of the opposite side was normally de- veloped.
The factors involved in regulating the geographical distribution of N. inqens are difficult to determine. The mites occurred in small ponds, large lakes, and in slowly flowing streams. All clams ex- amined were taken from a depth of not more than 4 feet. The types of bottom included soft clay, silt, fine white sand, gravel, or small stones.
Both N. ingens and species of Unionicola were occasionally found in the same clam host. Adults of the two genera are easily differ- entiated, Najadicola being larger, cream-colored, and remaining within the suprabranchial chambers, while Unionicola is smaller, dark-colored, and crawling about over the surface of the gills and mantle.
The eggs of N. ingens are laid in masses in the suprabranchial



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Humes and Jamnback-Parasitic Water-mite




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86 Psyche ISeptember ,
chambers of the clams, not inserted into the mantle or gills as in Unionicola. The greatest number of egg masses found in a single clam was 23 in one A. cataracts. Each egg mass contained approx- imately 260-300 eggs, and was enclosed in gelatinous material, lying unattached in the suprabranchial chamber. The color of the eggs
varied from light cream when freshly laid, through orange, to nearly black when about to hatch. Masses of light cream-colored eggs were hatched in 17 days in distilled water at summer room temperature. Sexual dimorphism is very pronounced in N. ingms, gravid females reaching a. diameter of 5 x 6 mm., 3 or 4 times that of males. The color varies from honey yellow to light reddish brown, Micro- scopically the sexes may be recognized by the position of the pair of triangular genital plates. Those of the male are contiguous medially, while in the female the two plates are separated. The average mm- ber of acetabula per plate in 10 males was 72 (39-142) and in 10 females 94 (59-122). The two acetabula located near the outer apex of the plate are larger than the others. Adult N. ingens can survive for several weeks at least outside the clam host. In distilled water at summer room temperature a male survived 36 days and a female 49 days. The female laid eggs during this period.
Pig. 4. Anoitonta catwacta with left valve and mantle removed and left outer suprabranchial chamber opened to show papillae, a male and a female N. zngens, and several small, dark egg masses. (xl.5)



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Humes and Jamnback-Parasitic Water-mite
Najadicola ingens is reported from 43 localities in Massachusetts, Rhode Island, Vermont, New Hampshire, Maine, Quebec, and New Brunswick, after an examination of 3077 fresh-water clams from 74 localities in New England and adjacent areas of Canada. The mite parasitizes principally two host clams, Anodonta cataracta and Elliptio complanatus, and occasionally Lampsilis radiata. The smaller and presumably younger A. cataracta and E. com- planatus are parasitized more often than larger individuals. I11 A. cataracta the mites occur mostly in the outer suprabranchial cham- bers, but in 3. complanatus in the inner suprabranchial chambers. The walls of the suprabranchial chambers containing mites almost invariably bear numerous papillae. The presence of mites in gravid gills apparently interferes with the normal use of the gill as a marsupium.
LITERATURE CITED
BAKER, P. C.
1928. The fresh-water Mollusca of Wisconsin. Part 11. Pelecypoda. Bull. Univ. Wisconsin, ser. no. 1527, gen. ser. no. 1301, pp. 1-495. KELLY, H. M.
1899. A statistical study of the parasites of the Unionidae. Bull. Illinois State Lab. Nat. Hist., 5, pp. 399-418.
KOENIKE, F.
1895. Nordamerikanische Hydrachniden. Abhandl. naturw. Ver. Bre- men, 8, pp. 167-226.
MARSHALL, R.
1929. Canadian Hydracarina. Publ. Ontario Fish. Res. Lab., 37-39, pp. 57-93.
PIERSIG, R.
1897. Bermerkungen zur Hydrachnidenkunde, Zool. Anz., 20, pp. 59-61. WOLCOTT, R. H.
1899. On the North American species of the genus Atax (Fabr.) Bruz. Trans. Amer. Micros. Soc., 20, pp. 193-259. 1918. The water-mites (Hydracarina).
In: Fresh-water Biology, by Henry B. Ward and George Chandler Whipple, John Wiley and Sons, New York, pp. 851-875.



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