a new water mite of the family thermacaridae from hot springs

A New Water Mite of the Family Thermacaridae from Hot SpringsAuthor(s): Rodger MitchellSource: Transactions of the American Microscopical Society, Vol. 82, No. 2 (Apr., 1963), pp.230-233Published by: Wiley on behalf of American Microscopical SocietyStable URL: http://www.jstor.org/stable/3224000 .

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A. S. KAPOOR A. S. KAPOOR

LEKANDER, B. 1949. The sensory line system and the canal bones in the head of some Ostario-

physi. Acta Zool., 30: 1-131. OMARKHAN, M. 1949. The lateral sensory canals of larval Notopterus. Proc. Zool. Soc.

Lond., 118: 938-972. PEHRSON, T. 1945. The system of pit organ lines in Gymnarchus niloticus. Acta Zool., 26:

1-8. SOLGER, B. 1880a. Neue Untersuchungen zur Anatomie der Seitenorgane der Fische. ii. Die

Seitenorgane der Selachier. Arch. Mikr. Anat., 17: 458-479. 1880b. Neue Untersuchungen zur Anatomie der Seitenorgane der Fische. iii. Die Seiten-

organe der Knochenfische. Arch. Mikr. Anat., 18: 364-390. WILSON, H. V. 1889-1891. The embryology of the sea-bass (Serranus atrarius). Bull. U. S.

Fish. Comm., 9: 209-227.

A NEW WATER MITE OF THE FAMILY THERMACARIDAE FROM HOT SPRINGS1

RODGER MITCHELL

Department of Biology, University of Florida, Gainesville, Florida

A curious new species of Thermacarus, discovered in 1958, was mentioned in a previous paper (Mitchell, 1960). The relations of this species were uncertain until the growth process in the previously described species of Thermacarus had been studied (Mitchell, 1962). It is now clear that the very small size of the new species is not a character of great phylogenetic significance, and that it is suf- ficiently close to the known species as to be placed in the same genus.

Thermacarus minuta Sp. Nov. (Figs. 1-5)

DESCRIPTION

Body encased by complete dorsal and ventral shields separated from each other by a narrow lateral membrane; dorsal shield with two setae on the anterior margin just mesal to the eyes, a seta posterior to each eye and a pair of setae posterior to the median eye; 5 pairs of setae along the lateral and posterior margins of the shield, and 3 pairs of setae in a row down the center of the shield.

Venter with from four to six setae posterior and lateral to the genital field and a pair of setae by the excretory pore.

Coxa I with several fine setae along the disto-mesal border, and a row of from six to nine stouter setae down the central area; most of the setae filiform, but as many as three of the anterior setae short and blunt; coxa II with five fine setae on the lateral margin; coxa III with a single heavy seta; coxa IV with two antero- laterally placed fine setae, one posterolateral fine seta, and a single stouter central seta.

Male genital field with the 12 to 15 acetabula crowded laterally, setae of the genital valves fine and concentrated posteriorly; female genital field large, crowded

1A portion of this work was supported by a grant from the American Philosophical Society.

LEKANDER, B. 1949. The sensory line system and the canal bones in the head of some Ostario-

physi. Acta Zool., 30: 1-131. OMARKHAN, M. 1949. The lateral sensory canals of larval Notopterus. Proc. Zool. Soc.

Lond., 118: 938-972. PEHRSON, T. 1945. The system of pit organ lines in Gymnarchus niloticus. Acta Zool., 26:

1-8. SOLGER, B. 1880a. Neue Untersuchungen zur Anatomie der Seitenorgane der Fische. ii. Die

Seitenorgane der Selachier. Arch. Mikr. Anat., 17: 458-479. 1880b. Neue Untersuchungen zur Anatomie der Seitenorgane der Fische. iii. Die Seiten-

organe der Knochenfische. Arch. Mikr. Anat., 18: 364-390. WILSON, H. V. 1889-1891. The embryology of the sea-bass (Serranus atrarius). Bull. U. S.

Fish. Comm., 9: 209-227.

A NEW WATER MITE OF THE FAMILY THERMACARIDAE FROM HOT SPRINGS1

RODGER MITCHELL

Department of Biology, University of Florida, Gainesville, Florida

A curious new species of Thermacarus, discovered in 1958, was mentioned in a previous paper (Mitchell, 1960). The relations of this species were uncertain until the growth process in the previously described species of Thermacarus had been studied (Mitchell, 1962). It is now clear that the very small size of the new species is not a character of great phylogenetic significance, and that it is suf- ficiently close to the known species as to be placed in the same genus.

Thermacarus minuta Sp. Nov. (Figs. 1-5)

DESCRIPTION

Body encased by complete dorsal and ventral shields separated from each other by a narrow lateral membrane; dorsal shield with two setae on the anterior margin just mesal to the eyes, a seta posterior to each eye and a pair of setae posterior to the median eye; 5 pairs of setae along the lateral and posterior margins of the shield, and 3 pairs of setae in a row down the center of the shield.

Venter with from four to six setae posterior and lateral to the genital field and a pair of setae by the excretory pore.

Coxa I with several fine setae along the disto-mesal border, and a row of from six to nine stouter setae down the central area; most of the setae filiform, but as many as three of the anterior setae short and blunt; coxa II with five fine setae on the lateral margin; coxa III with a single heavy seta; coxa IV with two antero- laterally placed fine setae, one posterolateral fine seta, and a single stouter central seta.

Male genital field with the 12 to 15 acetabula crowded laterally, setae of the genital valves fine and concentrated posteriorly; female genital field large, crowded

1A portion of this work was supported by a grant from the American Philosophical Society.

230 230



NEW WATER MITE FROM HOT SPRINGS 231

between coxae III-IV, 10 to 15 acetabula in the posterolateral angle of the genital field, chaetotaxy of the genital valve sparse and variable.

Legs relatively short, bearing short, stout setae; leg IV, male with few setae and a very slight distal expansion of segment 4; without distoventral stout setae on segments 4-5.

Q3~~~~l I--"

FIG. 1. FIG. 2. FIG. 3. FIG. 4. FIG. 5.

Thermacarus minuta Sp. Nov. Venter, male. Capitulum and right legs not shown. Venter, female. Legs not shown. Mesal surface, palp, amle. Leg IV, anterior surface, male. Leg IV, posterior surface, female.

Measurements are given in Table I. Specimens. Holotype male: hot spring on Loon Creek approximately six

miles downstream (northeast) from Loon Creek Ranger Station, which is 18 miles north of Sunbeam, Idaho. August 11, 1958. Paratypes; 25 males, 25 females: same data.

\- l^



RODGER MITCHELL

TABLE I

Dimensions and dorsal area of two Thermacarus species. It was shown that dorsal area and volume of Thermacarus are related in a linear fashion (Mitchell, 1962) and the more conveniently calculated areal figures (area=1/2 L 1/2 W7r) are used as an index of relative growth. The data on T. nevadensis were taken from Mitchell (1962). The means are in

parentheses

Area Increase Length Width Area in From in mm. in mm. mm.2 Earlier Stage

T. minuta nymphs (n =7) ................. 0.62-0.79 0.45-0.64 0.23-0.40

(0.69) (0.52) (0.29) males (n= 17) .................. 0.98-1.19 0.68-0.82 0.60-0.94 2.21

(1.07) (0.74) (0.64) females (n = 19) ................ 0.98-1.22 0.66-0.88 0.48-0.84 2.14

(1.09) (0.75) (0.62) T. nevadensis

young nymphs (n=12) .......... 0.67-0.89 0.48-0.63 0.26-0.43 (0.79) (0.56) (0.35)

old nymphs (n=7) .............. 1.29-1.68 1.09-1.47 1.30-1.80 4.77 (1.56) (1.36) (1.67)

males (n=20) .................. 2.35-3.22 1.72-2.21 3.2-5.6 2.81 (2.98) (2.00) (4.70)

females (n = 64) ................ 1.84-3.35 1.27-2.51 1.8-6.5 2.75 (2.81) (2.07) (4.59)

DISCUSSION

The two described species of Thermacarus, T. thermalis Sokolow and T. nevadensis, are very similar and differ from T. minuta in four major features: body chaetotaxy, genital field, secondary sexual characters of the legs, and their size.

There are many more setae on the dorsum and on coxa I in the new species but this in itself is not a character of great importance, especially in the set of coxal setae that show great variation and asymmetry. This intraspecific variability can be used to argue against attaching importance to it. Both the genital field and the secondary sexual features of the legs are labile structures that show both sexual and intraspecific variability in all species of Thermacarus.

Thus, the various qualitative features that are so distinctive in T. minuta cannot be considered as very satisfactory phylogenetic indicators either individually or collectively. There is little importance to be assigned to the immense difference in the size of T. minuta and the described species because the difference is due to a highly modified growth in one stage only.

The great size of adult T. nezadensis Marshall is almost entirely due to an extraordinary growth that occurs during the nymphal stage (Mitchell, 1962), so any evaluation of the significance of the size differences in Thermacarus must be based on nymphal growth features. Nymphs of the two American species can be distinguished on the basis of the provisional genital field. T. minuta had an average of 9.4 acetabulae (n=7) and ranged from seven to ten. A series of T. nevadensis nymphs from a hot spring by Harney Lake, Oregon, had from nine to 19 acetabulae with an average of 12.4 (n = 21). The size of the young T. nevadensis nymphs is the same as the few available T. minuta nymphs (Table I), and this suggests that growth prior to the nymphal stage is similar in both species. No direct data are available on this point.

T. nevadensis increases its dorsal area by a factor of 4.77 during the nymphal stage and by 2.75 times in the female teleiochrysalis, making a total areal increase of 13.11 times from nymph to adult female. The areal increase from nymph to

232



NEW WATER MITE FROM HOT SPRINGS

adult female in T. minuta is only a matter of some 2.14 times. Some indications of exoskeletal growth are apparent in T. minuta nymphs, but they cannot be clearly distinguished, and the limited sample available shows that there is not lilely to be extensive growth such as that exhibited by T. nevadensis. Thus, most of the T. minuta growth must occur during the nymphochrysalis and the increment at this stage, 2.14 to 2.21, is not much different than that of T. nevadensis (Table I).

A detailed T. minuta growth budget cannot be presented from the material at hand. It is possible to show that the adult size of these two Thermacarus species is due to differences in nymphal growth increments and not to growth processes at any other point in development.

On the basis of present collection records (Brues, 1928, 1930 and Mitchell, 1960), T. minuta would certainly appear to be a local endemic. T. minuta could be a northern species because all hot spring studies have carried out east, west, or south of central Idaho. Regardless of the real distribution of T. minuta, it is certainly a derivative of some line independent of the widespread holarctic stock that.gave rise to T. nevadensis and T. thermalis.

Regrettably, this divergent new species gives no further evidence of the relations of the three species of Thermacaridae with other water mite families.

LITERATURE CITED BRUES, CHARLES T. 1928. Studies on the fauna of hot springs in the Western United States

and the biology of thermophilous animals. Proc. Amer. Acad. Arts, Sci., 63: 139-228. 1932. Further studies on the fauna of North American hot springs. Ibid., 67: 185-303.

MITCHELL, RODGER. 1960. The evolution of thermophilous water mites. Evolution 14: 361-377.

1962. The growth of Thermacaruis nevadensis Marshall (Acari: Hydrachnellae). Zool. Anz. (in press).

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a new water mite of the family thermacaridae from hot springs

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