North American Recent Soft-shelled Turtles (Family Trionychidae) Part 20

Food Habits

Previous authors, most of whom allude to published statements preceding their own, characterize soft-sh.e.l.led turtles as carnivorous and mention such food items as crawfish, insects, worms, snails, clams, frogs, tadpoles, fish, and occasional vegetable matter. Stockwell (1878:403) wrote that the relative lengths of portions of the digestive tract indicate "a purely carnivorous diet."

In an examination of the contents of 11 stomachs of _spinifer_ from Michigan, Lagler (1943:304) found that crawfish (47%) and insects (52%), princ.i.p.ally burrowing mayfly naiads (_Hexagenia_), and dragonfly naiads, comprised the bulk of the diet with cryptogams, vegetable debris, snails and fish remains present in small amounts.

Breckenridge (1944:186) wrote that 18 specimens of _spinifer_ in Minnesota contained 44 per cent crawfish, 29 per cent aquatic insects, 8 per cent fish, and 19 per cent unidentified material. Surface (1908:123) found crawfish in the only two stomachs of specimens he examined from Pennsylvania. Penn (1950) summarized the results of those authors, and estimated that crawfishes comprised 58 per cent (46% by volume) of the diet of softsh.e.l.ls. In Indiana, three stomachs examined by Newman (1906:131) in late June contained: 1) nine crawfish, 2) four crawfish, 22 dragonfly naiads, 3) nine dragonfly naiads, few plant buds. Neill (1951a:765) found crawfishes in the stomachs of five _spinifer_ from the Savannah River, Georgia. Evermann and Clark (1920:595) wrote that _spinifer_ in Lake Maxinkuckee feeds princ.i.p.ally on crawfishes. Shockley (1949:257) mentioned bottom organisms and small fishes as food. Clark and Southall (1920:16) stated that "Its princ.i.p.al food, to judge from a few specimens examined, consists of crayfishes."

Cahn (1937:183) wrote that the food of _muticus_ in Illinois consists princ.i.p.ally of crawfish, fish, frogs, tadpoles, larger insect larvae and nymphs, and aquatic mollusks. The kinds of fish eaten were _Notropis heterolepis_, _N. spilopterus_, _N. hudsonius_, _Lepomis machrochirus_, _Morone chrysops_, _Perca flavescens_, _Catostomus commersonnii_, and _Hypentelium nigricans_; Cahn (_loc. cit._) also stated that the mollusks eaten by _muticus_ are both gastropods and small, thin-sh.e.l.led bivalves. In regard to the feeding habits of _spinifer_ in Illinois, Cahn (_op. cit._:193) listed the following items in decreasing order of abundance as revealed by examinations of stomachs: crawfish, minnows, fry of larger fish, frogs, tadpoles, earthworms, insects (often beetles), and mollusca (_Pisidium_, _Viviparus_, planorbids). The kinds of fish mentioned were: _Notropis heterodon_, _N. heterolepis_, _N. hudsonius_, _Catostomus commersonnii_, _Lepomis humilis_, _L. macrochirus_, _Semotilus atromaculatus_, _Notemigonus crysoleucas_, _Umbra limi_, and _Micropterus salmoides_. Cahn (_loc. cit._) also found the remains of a six-inch brook trout (_Salvelinus_) in the stomach of a 13-inch _spinifer_ from Wisconsin.

Aga.s.siz (1857:399) found larvae of neuropterous insects in the stomach of one specimen of _muticus_, and fragments of _Anodonta_ and _Paludina_ (= _Campeloma_) in the stomach of one _ferox_. The expanded crushing surfaces of the jaws in some large individuals of _ferox_ may be an adaptation to mollusc-feeding (Schmidt and Inger, 1957:36).

Surface (1908:123) found _spinifer_ to have fragments of beetles in one of two specimens examined, and large quant.i.ties of corn in another from Ohio. Webb and Legler (1960:27) reported 23 chrysomelid beetle larvae (_Donacia_) in one specimen of _T. ater_. Evermann and Clark (1920:595) reported several _spinifer_ taken on hooks baited with gra.s.shoppers in water 14 feet deep in Lake Maxinkuckee, Indiana. Hay (1892:144) wrote of _muticus_ that "If there are potatoes growing near the water the turtles find their way to them and devour the stems, of which they are very fond." Wright and Funkhouser (1915:123) stated that young _ferox_ in the Okefinokee Swamp feed on fish and frogs, and according to the natives, larger specimens take waterfowl, a statement that Smith (1956:159) was probably reiterating when he mentioned that the diet included "perhaps young birds." Parker (1939:88) wrote that of two _spinifer_ from Reelfoot Lake, Tennessee, one contained coleopteran remains, and the other an aquatic beetle and two large tipulid larvae. Wied-Neuwied (1865:54) wrote that Lesueur found worms, snails, remains of _Paludina_ (= _Campeloma_), fruits and even hard nuts in stomachs of _muticus_.

Holbrook (_in_ Hay, 1892:145) mentioned that _spinifer_ feeds on fish and such reptiles as it can secure. There are no published statements known to me that report reptiles in the diet of American softsh.e.l.ls.

Carr (1952:425) erroneously cited Strecker (1927:9) and attributed "a young lined snake" to the diet of _T. s. emoryi_; Strecker, however, referred to _Kinosternon flavescens_. In conjunction with raising softsh.e.l.ls on turtle farms, Mitsukuri (1905:261) mentioned that first and second year-old turtles (_Trionyx sinensis_) must be transferred to separate ponds or they will be eaten by adults; perhaps corresponding cannibalistic tendencies exist in confined, natural habitats in American softsh.e.l.ls.

Captives eat essentially the same things that free-living individuals do, plus sc.r.a.ps of meat (Strecker, 1927:9; Gloyd, 1928:135; Pope, 1949; Conant, 1951:156, 160). Lagler (1943:303) mentioned a young _spinifer_ that fed on water fleas (_Daphnia_) and canned fish. Conant (_op. cit._:160) wrote that no captive was observed to take vegetable matter.

Food, mostly in intestines, of two adult females of _T. s. emoryi_ collected on June 12-14, 1959, from the Rio Grande at Lajitas, Brewster County, Texas, was examined. One female, KU 51961, contained little food and mostly plant fragments; because the stomach or intestine was not full of plant fragments, this food probably was ingested incidentally to the few insects present. Another female, KU 51955, contained insects, which were identified by Dr. George W.

Byers, Department of Entomology, University of Kansas, as follows: 1) Coleoptera, Dryopidae, genus _Helichus_, most numerous, 350 to 400 individuals; 2) Coleoptera, Scarabaeidae, genus _Phyllophaga_, one individual; 3) Odonata, Coenagrionidae, fragments, probably one individual; 4) Hymenoptera, Sphecidae, subfamily Bembicinae, one individual; 5) Ephemeroptera; fragments of naiad; and 6) a few plant seeds, pieces of slender roots, weed stems and a couple of fragments of tree bark. The scarab and wasp probably fell into the water and were eaten.

TABLE 6. Kinds of Insects Found in Stomachs and Intestines of 11 Specimens of Trionyx m. muticus (Eight Adult Males and Three Immature Females, 9.0 to 12.5 cm. in Plastral Length) From Douglas County, Kansas. Frequency of Occurrence (Approximate Number of Individual Insects/Number of Stomachs in Which Found) Is Given for Each Item Listed. Fragments of Insects Represent Adults Unless Otherwise Noted.

==========================================================+=========== FOOD ITEM | Frequency ----------------------------------------------------------+----------- Orthoptera | Locustidae | 1 | Ephemeroptera | Unknown (naiad) | 1 | Odonata | Anisoptera (naiad) | 3/3 Zygoptera (naiad) | 4/2 | Plecoptera | Unknown (naiad) | 2/1 | h.o.m.optera | Cicadellidae | 20/7 | Hemiptera | Lygaeidae | 1 | Neuroptera | Corydalidae (_Corydalis_ larva) | 1 | Trichoptera | Hydropsychidae? (incl. 18 larvae and 4 pupae) | 23/9 Unknown (incl. 1 larva) | 4/4 | Lepidoptera | Noctuidae? (larvae) | 2/1 Pyralidoidea (larva) | 1 Unknown | 1 | Coleoptera | Carabidae (incl. 1 larva) | 3/3 Cerambycidae? | 1 Chrysomelidae | 1 Cicindelidae (larva) | 1 Elateridae (larva) | 1 Hydrophilidae? (larvae) | 4/2 Scarabaeidae (incl. _Phyllophaga_) | 9/6 | Diptera | Anthomyiidae | 1 Asilidae | 1 Bibionidae (_Bibio_) | 5/2 Calliphoridae (puparium) | 1 Empididae | 1 Mycetophilidae | 1 Tipulidae (incl. _Tipula bicornis_ and _T. triplex_?) | 9/4 Unknown (5 muscoid, 3 acalyptrate, and 1 cyclorrhaphous | types) | 9/4 | Hymenoptera | Apoidea | 1 Formicidae (incl. _Camponotus_) | 11/4 Ichneumonidae (one questionable) | 4/3 Tenthredinidae | 1 Unknown (small wasps) | 3/2 ----------------------------------------------------------+-----------

Food from the digestive tracts of 11 specimens of _T. m. muticus_ from the Kansas River at Lawrence, Douglas County, Kansas, were examined (Table 6). The turtles (KU 55296-306, eight adult males and three immature females, ranging in plastral length from 9.0 to 12.5 cm.) were collected in June, 1958, by Mr. Robert R. Patterson. All turtles were caught on hook and line in a period of about four or five hours at dusk. Patterson frequently fished below the bridge at Lawrence and observed that heads of softsh.e.l.ls were often seen there about dusk and that the turtles seemed to prefer a rather shallow, quiet-water area of swirls and eddies for feeding. The stomachs, and to a lesser degree, the intestines, were nearly full. Some turtles contained plant fragments, princ.i.p.ally elm seeds. The kinds of food in this sample were also identified by Dr. Byers and were mostly insects, the most frequent item being trichopterans; many of the insects eaten undoubtedly fell into the water. The remains of spiders were found in four stomachs and crawfish fragments in five.

Stomachs of two adults of _muticus_ from Lake Texoma, Oklahoma, were opened. The stomach of one (OU 27593) was full of naiads of the burrowing mayfly _Hexagenia_; that of the other female (OU 27594) contained exoskeletal remains of crawfish. The two specimens were drowned in gill nets between the hours of 11 a. m. and 7 p. m., on July 10, 1954; the intact condition of the mayfly naiads indicated recent feeding.

The species of American softsh.e.l.ls are mainly carnivorous. The presence of vegetable matter (mentioned in previous paragraphs) in the digestive tracts of many specimens and True's statement (1893:152) that soft-sh.e.l.led turtles include a variety of vegetable matter in their food indicates omnivorous habits. Duellman and Schwartz (1958:272) stated that _ferox_ is omnivorous and Carr (1952:430) made a similar statement for _spinifer_. The diet seems to be determined by the food supply available, which may vary seasonally or with adverse conditions such as flooding; under normal environmental conditions, however, vegetable matter probably is ingested incidentally to other food. There is no indication of a preference in food habits according to species and subspecies. Most of the food seems to be obtained by active foraging that is triggered primarily by movement of the prey; the sense of smell is probably secondary.

Reproduction

_Size of males at s.e.xual Maturity_

Elsewhere (1956:121) I have shown that males of _spinifer_ from Lake Texoma, Oklahoma, and scattered localities in Texas and Louisiana are s.e.xually mature when they reach a plastral length of 9.0-10.0 centimeters. Adult males have distinct, convoluted, non-pigmented vasa deferentia and elongate testes, the maximal measurements of which are about 10 by 30 millimeters. Testes of hatchlings are approximately 4.0 by 0.4 millimeters (TU 13698.12, plastral length 3.2 cm., measured with ocular micrometer). I am not aware of seasonal changes in size of the testes.

In reading the discussion that follows, it is well to remember that males having the cloaca extending beyond the posterior edge of the carapace are regarded as s.e.xually mature. As an indication of geographic variation in _spinifer_, I have listed the measurements of the 10 smallest males for only those subspecies of which there are numerous records (Table 7). Corresponding data for _T. muticus muticus_ are also listed for comparison.

TABLE 7. Size at s.e.xual Maturity of the 10 Smallest Males of T. m.

muticus and Selected Subspecies of T. spinifer. The Extremes Precede the Mean (in Parentheses).

========================+======================= SPECIES AND SUBSPECIES | Plastral length (cm.) ------------------------+----------------------- _T. s. spinifer_ | 8.8-10.3 (9.6) _T. s. hartwegi_ | 9.6-10.5 (10.2) _T. s. pallidus_ | 9.1-11.2 (10.5) _T. s. guadalupensis_ | 9.3-10.8 (10.1) _T. s. emoryi_ | 8.2-9.0 (8.8) _T. m. muticus_ | 8.2-9.2 (8.7) ------------------------+-----------------------

The data indicate that the size at which s.e.xual maturity is attained in _emoryi_ (about 8.0-9.0 cm.) is less than in any other subspecies of _T.

spinifer_ (about 9.0-10.0 cm.), and, more importantly, corresponds to that of _T. m. muticus_. Although the mean for _T. s. spinifer_ is slightly less than in the remaining subspecies, I doubt that there is any significant difference according to subspecies in the size at which s.e.xual maturity is attained in the subspecies _spinifer_, _hartwegi_, _asper_, _pallidus_ and _guadalupensis_. The corresponding size in _T.

m. muticus_ and _T. s. emoryi_ heightens the morphological resemblance between these forms. The only s.e.xually mature male of _T. ater_, which morphologically resembles _emoryi_ and _muticus_, is 9.5 centimeters in plastral length. I do not know the size at which _T. ferox_ attains s.e.xual maturity. The smallest s.e.xually mature individual examined by me was 12.0 centimeters; probably _ferox_ attains s.e.xual maturity at a larger size than _spinifer_ or _muticus_. The relative size of attainment of s.e.xual maturity in _ferox_, _spinifer_, and _muticus_ corresponds to the maximum size of the three species; _ferox_ is the largest species and _muticus_ is the smallest (Table 2).

_Size of Females at s.e.xual Maturity_

Breckenridge (1955:6) wrote that the development of the mottled pattern "undoubtedly indicates a stage in the attainment of s.e.xual maturity"; I have mentioned (1956:121) that the mottled pattern is apparent on females before s.e.xual maturity is attained. To my knowledge females have no external characters which appear at the time of attainment of s.e.xual maturity.

s.e.xually mature individuals of _ferox_ have been described in various terms: 31-1/4 pounds (Goff and Goff, 1935:156); six pounds, lengths of carapace 10-1/2 and 13 inches (Hamilton, 1947:209); greatest width of head 3-1/2 inches (Wright and Funkhouser, 1915:120). A 10-1/2 inch carapace presumably represents the smallest turtle and corresponds to a plastron approximately 22.0 centimeters in length. There is no other information available concerning size at s.e.xual maturity in _T. ferox_.

There is little published information concerning the size at s.e.xual maturity in _T. spinifer_. Cahn (1937:193) wrote that _spinifer_ in Illinois "must attain a carapace length of about 24 centimeters [plastral length approximately 18.0 cm.] before the females become s.e.xually mature"; this statement is the basis for Smith's mentioning a length of 9-1/2 inches (1956:162). Evermann and Clark (1920:595) recorded the lengths of carapace of some females (presumably all adult) from Lake Maxinkuckee, Indiana, as 11, 11-3/4, 12-1/2, and 13 inches; the smallest measurement corresponds to a plastral length of approximately 21.0 centimeters.

The data concerning reproduction presented in succeeding paragraphs is based princ.i.p.ally upon examinations of turtles in the TU collections; I am indebted to Dr. Fred R. Cagle for permission to dissect these turtles. Females are regarded as s.e.xually mature when they have oviducal eggs or corpora lutea or ovarian follicles exceeding 15 millimeters in diameter. Hatchlings of _spinifer_ have ovaries that measure approximately 6.0 0.3 millimeters, and straight oviducts 0.2 millimeters in width (TU 5988, plastral length 3.5 cm. measured with ocular micrometer). In the size at which s.e.xual maturity is attained there seems to be much individual variation as well as geographic variation.

Females of _T. s. emoryi_ from the Rio Grande in the Big Bend region of Texas are s.e.xually mature when the plastron is approximately 16.0 centimeters (16.2 cm., KU 51960), and are the smallest adult females of _spinifer_ that I have seen; these females are representative of the population from which the smallest adult males of _spinifer_ are known and which is unique in showing s.e.xual differences in coloration.

A female (TU 3697), having a plastral length of 16.0 centimeters, which was obtained in the Rio Grande near Eagle Pa.s.s, Texas, in mid-July, is immature; the ovaries are compact having the largest follicles 2.5 millimeters in diameter, and the oviduct is wrinkled and convoluted but only six millimeters wide. Of three females of _emoryi_ from the Pecos River, Terrell County, Texas, having plastrons 17.4, 18.3 and 18.8 centimeters in length and obtained on June 11, the largest and smallest are immature, and internally resemble TU 3697. TU 14453.2 (18.3 cm.) is s.e.xually mature having large corpora lutea and enlarged ovarian follicles. KU 53754, from the Rio Salado in central Coahuila, Mexico, having corpora lutea and a plastral length of 20.3 centimeters, is s.e.xually mature.

Females of _T. s. guadalupensis_, measuring 14.5, 15.7, 16.3, 16.5, 16.8, 17.0, 19.0, and 20.0 centimeters in plastral length and obtained from June to September, are immature. The female measuring 19.0 centimeters indicates the approach of s.e.xual maturity in having swollen and convoluted oviducts seven to ten millimeters in width, but compact ovaries having the largest follicles 4.0 millimeters. The other _guadalupensis_ whose measurements are given above have oviducts that do not exceed four millimeters in width, and ovarian follicles that do not exceed two millimeters in diameter. TU 10187, obtained in July, plastral length 19.5 centimeters, is s.e.xually mature having corpora lutea and enlarged follicles. Two other _guadalupensis_, 21.5 and 22.0 centimeters (Pl. 12, top), having ovaries with enlarged ovarian follicles (the largest in one, only 11 mm.) are considered s.e.xually mature.

Concerning the subspecies _pallidus_, females (all collected in June or July) measuring 15.7, 16.3, 17.3, 17.5, 18.7, 19.5, 20.8 and 21.3 centimeters in plastral length are immature having solid, compact ovaries with the largest follicles not exceeding two millimeters in diameter; oviducts are straight not exceeding three millimeters in greatest width, except those turtles measuring 17.3 and 21.3 centimeters in which the oviducts are swollen and convoluted and, respectively, five and eight millimeters in greatest width. The smallest s.e.xually mature _pallidus_ is 19.8 centimeters in length; recorded lengths of other adult females are 23.5, 26.8 and 30.5 centimeters.

Of especial interest are three large female _pallidus_, measuring 24.8, 27.5, and 28.0 centimeters, which appear to be immature; two of these (TU 13303-04) are from the Sabine River, collected in July, and the other specimen is without data (presumably from the Sabine River).

The oviducts are large, swollen and convoluted, resembling those in s.e.xually mature individuals. The ovaries, however, are relatively solid and compact having approximate measurements of 125 6 millimeters (TU 13303) and 85 10 millimeters (TU 13304), and follicles not exceeding five millimeters in diameter.

Females of _spinifer_ from the lower Mississippi Valley of Louisiana having plastral lengths of 15.0, 15.5, 16.7, 17.5, 18.0, 19.5, 20.0, 20.4, and 20.8 centimeters are considered immature; the ovaries are compact and solid having follicles not exceeding three millimeters in diameter, and the oviducts, swollen and convoluted in the larger individuals, do not exceed six millimeters in width. The ovaries of the specimen 19.5 centimeters in length mentioned immediately above had been removed prior to my examination; the oviducts, however, were relatively straight and only five millimeters in width. Three females 23.0, 25.5, and 25.8 centimeters in length are s.e.xually mature. TU 5518, measuring 21.5 centimeters in length and obtained in June, indicates the onset of s.e.xual maturity in having large convoluted oviducts, but the ovaries are solid, compact, measuring 85 13 millimeters, and the largest follicles are only 4.5 millimeters. A larger turtle (TU 13080), 24.5 centimeters, obtained in July, has juvenal ovaries (largest follicles five mm.); the oviducts are enlarged and convoluted as in adult females.

Of two _T. s. asper_ collected from the Escambia River in June and July, one 18.0 centimeters in plastral length is immature, whereas the other, 27.0 centimeters long, is adult. A female _T. s. hartwegi_, measuring 20.7 centimeters, is adult having enlarged follicles and corpora lutea (TTC 719, Pl. 36, bottom).

In summary, females of all subspecies of _spinifer_, except some _emoryi_, may be s.e.xually mature when the plastron is 18.0 to 20.0 centimeters in length; probably all physiologically normal females are adult when 22.0 centimeters long. In general, females are s.e.xually mature at a plastral length of approximately 20.0 centimeters, a measurement that corresponds to a length of carapace of approximately 28.0 centimeters or about 11 inches. Females representative of that population of _emoryi_ inhabiting the Rio Conchos and the Rio Grande in the Big Bend region of Texas are adult when the plastron is approximately 16.0 centimeters in length, and are thus the smallest s.e.xually mature females of the species _spinifer_. Oviducts are large (at least eight mm. in width, undistended), swollen and convoluted prior to the first ovulation.

Of interest are the large females (for example, TU 13303, plastral length 28.0 cm.) that seemingly have immature, relatively small, ovaries (the oviducts are large and convoluted as in adult females). Possibly such ovaries represent a regression and are in senile turtles, but I am inclined to believe that the development of these ovaries has been arrested probably owing to hormonal unbalance, and that they have never been functional.

The size of adult females of _T. ater_ is unknown but probably approximates that of _T. spinifer_ or is slightly less. Females of _ater_ 15.5 and 17.2 centimeters in length are immature; the largest female, the holotype, is 18.3 centimeters in length, and was not dissected.

Females of _T. muticus_ are s.e.xually mature when smaller than _T.

spinifer_. Two turtles, 13.8 and 14.0 centimeters in length, have large convoluted oviducts about 10 millimeters in width and ovarian follicles nine to twelve millimeters in diameter, and seem to be near s.e.xual maturity. The smallest s.e.xually mature female (subspecies _muticus_) is TU 14436, measuring 14.4 centimeters in plastral length and having oviducal eggs. Recorded lengths of other adult females are 16.3, 16.5, 17.2 (subspecies _muticus_), and 18.0 centimeters (subspecies _calvatus_). Two females having plastral lengths of 17.5 (subspecies _muticus_) and 16.0 centimeters (subspecies _calvatus_) seem s.e.xually immature. These turtles collected in April and May have ovarian follicles not exceeding three millimeters in diameter.

_s.e.xual Activity_

Observations by Mitsukuri (1905:263), Conant (1951:160) and Legler (1955:98), const.i.tute the extent of our knowledge concerning courtship and copulation. Legler observed a male _spinifer_ and a female _muticus_ in captivity; the male was the aggressor, following at the rear or above the female, and at times nipping at the anterior part of her carapace.

During these movements, the posterior edge of the female's carapace was turned up slightly whereas that of the male was turned down; the turtles frequently surfaced to breathe. Occasionally the female followed the male. On the bottom the male crawled onto the female's carapace from the rear, remaining in a somewhat posterior position as described by Conant (_loc. cit._), and seemingly not clasping the female with his feet.

Copulation probably occurs in this position; Mitsukuri (_loc. cit._) mentioned that copulation in _Trionyx sinensis_ occurs at the surface of the water. The male remains in the coital position for approximately 15 seconds and then slowly drifts to one side and swims away. Legler observed five coital unions in one-half hour, each preceded by courting movements.

Legler's observations indicate that the courtship patterns of _muticus_ and _spinifer_ are similar, and that interspecific matings are possible.

I have not noted any hybrid.

Risley (1933:689) mentioned differential movements of the s.e.xes of _Sternothaerus odoratus_ in conjunction with the breeding cycle. Such movements are revealed by trapping procedures that yield deviations from the expected 1:1 s.e.x ratio. That differential s.e.xual movements probably occur in _Trionyx_ is indicated by my trapping 17 males in a group of 19 _spinifer_ in hoop-nets in Lake Texoma in the period June 14-July 12, 1954. On June 24-26, 1959, a field party from the University of Kansas collected 15 softsh.e.l.ls in hoop-nets at the mouth of the Rio San Pedro, near Meoqui, Chihuahua; all turtles were males. On June 17-18, 1959, the same expedition trapped 11 males in a group of 13 turtles in the Rio Conchos, near Ojinaga, Chihuahua. Earlier, June 12-14, 1959, 39 softsh.e.l.ls were trapped in the Rio Grande near Lajitas, Brewster County, Texas. Of these turtles, however, 19 were adult males and 20 were females; eight females were adult (s.e.xually mature) all having oviducal eggs (Fig. 23). One of the two females from Ojinaga, KU 51174, is s.e.xually mature (plastral length, 16.5 cm.) having oviducal eggs; the other is immature (plastral length, 8.0 cm.). The only softsh.e.l.l taken on June 21, 1959, 8 mi. N and 16 mi. W Ojinaga, KU 51173 (plastral length, 16.3 cm.) is a female having oviducal eggs. The two females from Lake Texoma are immature (plastral lengths, 9.8 and 12.4 cm.).

The results of trapping may indicate that females frequent shallow water for a short time before the period of deposition of eggs, but disperse to deep water after such periods or between them. The movements of immature females probably approximate those of adult males; the absence of immature females in the Meoqui series, and near absence (only one) in the Ojinaga series perhaps is due to fortuitous collecting methods or to slightly different diurnal movements between adult males and immature females. Females approaching s.e.xual maturity and those s.e.xually mature but not having oviducal eggs ready for deposition possibly remain relatively sedentary in deep water; such females possibly represent those absent in the 13.0-15.9 size group (Fig. 23). Certainly, factors other than those pertaining to egg deposition may cause mature egg-laden females to live in shallow water, or explain the deviations from the expected 1:1 ratio.

[Ill.u.s.tration: FIG. 23. Size distribution of 39 _Trionyx spinifer emoryi_ (19 males and 20 females) collected in the period June 12 through June 14, 1959, from the Rio Grande, near Lajitas, Brewster County, Texas. Solid squares represent s.e.xually mature specimens.

Females approaching s.e.xual maturity or those not ready for egg deposition (13.0-15.9 cm. size group) are possibly sedentary in deep water.]