North American Recent Soft-shelled Turtles (Family Trionychidae) Part 22

Females of _muticus_ are s.e.xually mature when the plastron is 14.0 to 16.0 centimeters long, which corresponds to a carapace 19.6 to 22.4 centimeters (about 7-3/4 to 8-3/4 inches) long (average CL/PL approximately 1.4, see Fig. 13). The smaller adult females probably mature s.e.xually in their sixth year, but most probably do so when seven years old. Accordingly, some _T. spinifer emoryi_, which are s.e.xually mature at a plastral length of 16.0 centimeters, are also s.e.xually mature in their seventh year, whereas most _spinifer_ (s.e.xually mature at a plastral length of 18.0 to 20.0 cm., corresponding to a length of carapace of 25.2 to 28.0 cm. or about 10 to 11 inches) probably become s.e.xually mature in their ninth year, and some when eight years old. Most males of _spinifer_ are s.e.xually mature when the plastron is 9.0 to 10.0 centimeters long (length of carapace 12.6 to 14.0 cm. or 5 to 5-1/2 inches), whereas males of _muticus_ and some _T. spinifer emoryi_ are s.e.xually mature at a plastral length of 8.0 to 9.0 centimeters (length of carapace 11.2 to 12.6 cm. or 4-1/2 to 5 inches). The smaller adult males are probably s.e.xually mature in their fourth growing season. Breckenridge (_op.

cit._:7, Tab. II) commented on the abundance of females between five and 12 inches in length, and males that ranged in length from five to seven inches. The abundance of turtles in these size ranges is probably due, in part, to a slowing of the rate of growth indicating the approach of s.e.xual maturity; the abundance of the smallest males is especially in accord with the size at s.e.xual maturity of males (about five inches).

The largest acceptable record of size of _spinifer_ is 18 inches in length of carapace (Breckenridge, 1957:232). Stockwell (1878:402), however, wrote that females of _spinifer_ attain "an extreme length of from twenty-four to twenty-eight, and, in rare instances, thirty inches, with an average length of carapace of fifteen to eighteen,"

and True (1893:152) mentioned lengths of two feet or even more.

Turtles 17 to 18 inches long are doubtless rare and probably about 60 years old. A specimen of _ferox_ lived the longest time in captivity--25 years (Pope, 1949:304). Individuals of _ferox_ probably exceed the maximum recorded length of carapace of 18-1/2 inches (Aga.s.siz, 1857:401). The head of a _ferox_ having a width of 3-1/2 inches (Wright and Funkhouser, 1915:120) corresponds to a length of carapace of approximately 22-1/2 inches (PL/HW == 4.9; CL/PL == 1.3).

De Sola and Abrams (1933:12) wrote that _ferox_ in the Okefinokee Swamp, Georgia, attains a length of two feet. The largest female of _muticus_ of which I have record is 21.5 centimeters in plastral length (KU 2308), a measurement corresponding to a carapace about 13 inches long.

Mortality

Man, in one sense or another, is a great enemy of soft-sh.e.l.led turtles. Those caught by fishermen are destroyed because of the erroneous belief that they are harmful to fish populations. Some are drowned in hoop-nets or gill nets used by commercial fishermen. Many softsh.e.l.ls are used by man for food. Herald (1949:118-19) reported the results of spraying an area with DDT and mentioned a 10-inch individual of _ferox_ that was eating a dead bluegill, and which "probably died as a result of ingesting contaminated food."

Predation on eggs probably accounts for most mortality. Hamilton (1947:209) reported tracks of spotted skunks, racc.o.o.ns and foxes seen about destroyed nests, and Cahn (1937:183) incriminated skunks and racc.o.o.ns. Goldsmith (1945:449) reported a racc.o.o.n that unearthed seven nests in one night. Little and Keller (1937:221) wrote of egg sh.e.l.ls found in the sand (probably not as a result of hatching), and Muller (1921:182) reported egg sh.e.l.ls around dug-up nests, suggesting such predators as "ground moles," racc.o.o.ns and crows. Chesser (_in_ Harper, 1926:416) said that in the Okefinokee Swamp the jackdaw (fish crow), racc.o.o.n, bear and domestic dogs will eat the eggs. Wright and Funkhouser (1915:122) recorded a young _ferox_ in the stomach of a water moccasin (_Agkistrodon piscivorus_), and suggested that young soft-sh.e.l.ls probably are food of larger snakes. Kellogg (1929:26) wrote that stomachs of two alligators each contained one soft-sh.e.l.led turtle. Newman (1906:136) found that young captives were eaten by individuals of _Chrysemys_ and _Sternothaerus_, and I found that they were eaten by _Kinosternon_. Mitsukuri (1905:261-62) stated that first- and second-year individuals of _T. sinensis_ are eaten by the adults.

Breckenridge (1960) wrote that a clutch of eggs probably failed to develop because of an "... unusually cool season." Evermann and Clark (1920:595) stated that "many young appear to perish during the first winter." They (_op. cit._:594) found two eggs submerged in two feet of water and it is supposed that they never hatched. Dundee (1950:139) reported remains of soft-sh.e.l.led turtles left on the mud of a dried swamp.

Parasites

Muller (1921:182) found maggots in a few eggs of a clutch, but thought that only the infertile and decomposing eggs were infested. I removed a hard, spherical cyst from the hind leg of a preserved softsh.e.l.l (TU). A captive hatchling (TU 17304) died as the result of a continuously enlarging and deepening hole on the top of its head; I could not discern a visible parasite with the naked eye. I found 25 leeches (_Placobdella parasitica_, largest about 13 mm.; identified by Dr. Kenneth B. Armitage, Department of Zoology, University of Kansas) in a.s.sociation with 11 _T. m. muticus_ (number per turtle not known) that were collected from the Kansas River at Lawrence, Douglas County, Kansas. Evermann and Clark (1920:596) reported a few nematodes in the stomachs of some _spinifer_, and three nematodes are listed by Harwood (1932:46, 60, 62, 66) in the same species. Hughes, Higginbotham and Clary (1941) have listed the known reptilian hosts of parasitic trematodes, and Hughes, Baker and Dawson (1941) have done the same for tapeworms. The species of parasites and their trionychid hosts are listed below.

TREMATODA _Trionyx ferox_: _Neopolystoma orbiculare_ _Vasotrema amydae_ _Neopolystoma rugosa_ _Vasotrema attenuatum_ _Polystomoides coronatus_ _Vasotrema robustum_ _Teloporia aspidonectes_

_Trionyx muticus_: _Crepidostomum cooperi_ _Opisthorchis ovalis_

_Trionyx spinifer_: _Hapalorhynchus ev.a.g.i.n.atus_ _Vasotrema amydae_ _Opisthorchis ovalis_ _Vasotrema attenuatum_ _Polystomoides coronatus_ _Vasotrema longitestis_ _Teloporia aspidonectes_ _Vasotrema robustum_

CESTODA _Trionyx ferox_: _Proteocephalus trionychinus_

_Trionyx spinifer_: _Proteocephalus testudo_

NEMATODA _Trionyx spinifer_: _Camalla.n.u.s trispinosus_ _Spiroxys amydae_ _Falcaustra chelydrae_

Economic Importance

Several authors have mentioned softsh.e.l.ls as a food item much sought after by man. The commercial value of these turtles has been summarized by Clark and Southall (1920:15-16). Softsh.e.l.ls are consumed in quant.i.ty only in small towns near the place of capture. They are found only occasionally in the markets of large cities because the turtles are little known and the palatability of their flesh is unappreciated. Also, they do not stand shipment so well as other turtles, and they are "not so meaty as the snapper; so there is more waste" (Clark and Southall, _loc. cit._). Little and Keller (1937:221) reported living individuals for sale at the market in Ciudad Juarez, Chihuahua; however my inquiry at markets in Juarez in the summer of 1959 disclosed no evidence of recent sale of soft-sh.e.l.led turtles. In the southeastern United States the demand is perhaps greater than in other regions. I have noted softsh.e.l.ls in the market at New Orleans, and Oliver (1955:19) has mentioned the sale of "some 146,600 pounds"

in one recent year in Florida. Over most of their range, however, there probably is no general demand for softsh.e.l.ls and no special efforts are made to capture them. Softsh.e.l.ls have been raised successfully on "turtle farms" in j.a.pan (Mitsukuri, 1905). True (1893:152) wrote that "The eggs also are considered very excellent."

Softsh.e.l.ls generally are condemned by fishermen because of the mistaken belief that they are detrimental to fish populations. Food of softsh.e.l.ls is princ.i.p.ally crawfish and insects. Fish comprise a small proportion of the diet (frequency 1.9% in Michigan, Lagler, 1943: Tab.

9). Most of the fishes eaten seem to be small minnows. Probably fish would comprise a larger percentage of the diet if they could be caught. I doubt that a softsh.e.l.l can pursue and capture a healthy fish in natural waters. Recently dead fish are eaten and perhaps fish eggs, and senile and decrepit fishes. There is no evidence that soft-sh.e.l.led turtles are active predators on any kind of fish. Of course in congested areas such as ponds of fish hatcheries, it is desirable to eliminate the turtles. The known food habits of soft-sh.e.l.led turtles suggest that they compete with game fishes for food, but there is no information on the intensity of compet.i.tion (Lagler, _op. cit._:305).

The combined statements of many authors in their general accounts of food habits (for instance, Babc.o.c.k, 1919:425) have tended to create the erroneous belief that soft-sh.e.l.led turtles harm waterfowl. To my knowledge the only basis for this belief is the statement of Wright and Funkhouser (1915:123) that according to the natives of the Okefinokee Swamp, the larger turtles "devour also such waterfowl as are unfortunate enough to be taken unaware by these reptiles." Perhaps an occasional waterfowl is eaten, but the present information on kinds of food eaten certainly does not warrant the destruction of soft-sh.e.l.led turtles. There may be some mortality in congested areas such as game refuges where young birds crowd the surface of the water.

The kind of bait successfully used in trapping softsh.e.l.l turtles suggests that they are of some value as scavengers.

EVOLUTIONARY HISTORY

Before attempting to reconstruct the history of soft-sh.e.l.led turtles in North America, it will be helpful to summarize the salient facts concerning the distribution and relationships of the living forms, and to comment on fossils.

Distribution

The geographic range of the family Trionychidae in North America is princ.i.p.ally in the eastern two-thirds of the continent and contributes to the well-known floral and faunal resemblance of eastern North America to that of eastern Asia (Schmidt, 1946:149) because _Trionyx ferox_ (see Fig. 18) resembles the species of the genus in Asia more closely than it does any North American species. The Recent distribution in America does not include the Neotropical region, whereas the geographical range in the Old World extends south of the equator (Fig. 1; Dunn, 1931:109, fig. 2; Gadow, 1909:333, fig. 72; Hay, 1908:35, fig. 16).

American softsh.e.l.ls occur in all river systems in the United States and the two adjacent river systems on the east coast of Mexico that drain into the Gulf of Mexico. Softsh.e.l.ls inhabit streams of the Great Plains and occur westward to the foothills of the Rocky Mountains in the western tributaries of the Mississippi River. Only _T. s.

spinifer_ occurs in the southern part of the Great Lakes-St. Lawrence drainage. Softsh.e.l.ls are absent from the Atlantic Coast drainage except the Hudson River and those rivers at least south of (and including) the Pee Dee River in South Carolina.

_T. s. emoryi_ is not known to be indigenous west of the Rio Grande drainage, and has probably been introduced across the Continental Divide via the Gila River in western New Mexico into the Colorado River drainage of Arizona (Miller, 1946:46); the species undoubtedly occurs in Mexico on the Sonoran side of the Colorado River opposite Baja California (Bogert and Oliver, 1945:417).

In the summer of 1959, I trapped turtles and with a specimen in hand inquired about softsh.e.l.ls occurring in the inland drainages of northern Mexico. From two collecting stations on the Rio Nazas in Durango, only specimens of _Pseudemys_ and _Kinosternon_ were obtained; local inhabitants had neither seen nor heard of softsh.e.l.ls.

Flooded conditions in August of 1959 permitted trapping in only one of the inland drainages of northwestern Chihuahua, the Rio Santa Maria; only specimens of _Kinosternon_ were obtained. Local residents near that river as well as those living near the Rio Casa Grandes and Rio del Carmen had not seen or heard of softsh.e.l.ls. A person that I judge to be a competent observer reported seeing a softsh.e.l.l in June of 1958 in the Rio Alamos (Arroyo Cuchujaqui) near Alamos, Sonora, in the Rio del Fuerte drainage on the west coast of Mexico. I was a member of a field party from the University of Kansas that visited that locality in late January of 1959; only specimens of _Pseudemys_ and _Kinosternon_ were collected. Possibly isolated populations occur in streams of the Pacific Coast drainage of northern Mexico. If so, they may have entered Pacific Coast drainages by stream capture across the Continental Divide. Several species of fish that are characteristic of the Rio Grande traversed the Sierra Madre Occidental at some former time (presumably via the Rio Conchos and Rio Papigochic) and occur in the Yaqui River drainage (Meek, 1904:x.x.xviii, xlvii; Miller, 1959:214-15, 217). Because of the probability that the Rio Nazas at some former time flowed north into the Rio Grande (Meek, _op.

cit._:x.x.xiv), it is notable that softsh.e.l.ls are absent in the Rio Nazas drainage; the Big Bend turtle, _Pseudemys scripta gaigeae_, occurs in both drainages.

Relationships

Characters of _Trionyx ferox_ suggesting a closer resemblance to some Old World members of the family than to the other three American species are: large size; marked difference between juvenal and adult patterns on the carapace; the marginal ridge; and the longitudinal ridgelike prominences on the carapace, especially in juveniles. Other characters of _ferox_ suggesting a corresponding, but less marked resemblance to Old World species of _Trionyx_ are: the large size of the eighth pair of pleurals; the absence of callosities on the epiplastron and preplastra; frequent fusion of the hyoplastra and hypoplastra (more than in _spinifer_ or _muticus_); and tolerance of marine waters (more than _muticus_ or _spinifer_). Some fossils also suggest alliance with _ferox_ and some Old World members of the genus in their large size, large eighth pair of pleurals, and occurrence in marine deposits; several Old World species have been reported at sea (_Pelochelys_, _T.

triunguis_, _T. sinensis_). _T. ferox_ is monotypic and has the most southeasterly displaced, geographic range.

Because _ferox_ resembles softsh.e.l.ls from the Old World more closely than it does any American species, _ferox_ is a.s.sumed to be more closely related to Old World softsh.e.l.ls than to any American species, and, because of resemblance to some fossils, _ferox_ is a.s.sumed to resemble most closely the primitive, ancestral stock of softsh.e.l.ls that occupied North America. _T. spinifer_, _T. muticus_ and _T. ater_, which resemble each other more closely than any of them resembles _T. ferox_ or any Old World species, are considered autochthonous in North America. _T.

spinifer_ and _T. muticus_ are distinct, sympatric species. Burt (1935:321) suggested that the two species "may be variants of the same species." _T. ater_ is weakly differentiated from _T. spinifer emoryi_.

The species, _ferox_, _spinifer_ and _muticus_ are well-differentiated and were considered by Aga.s.siz (1857), Gray (1869) and Baur (1893) as belonging to three different genera.

In the widely distributed _T. spinifer_, the subspecies _spinifer_, _hartwegi_ and _asper_ closely resemble one another; _asper_ seems most distinct, whereas _spinifer_ and _hartwegi_ are terminal populations of an east-west cline in one character. The subspecies _pallidus_, _guadalupensis_ and _emoryi_ resemble one another more closely than any resembles any of the subspecies mentioned immediately above; _T. s.

pallidus_, however, is annectent. _T. s. pallidus_ and _guadalupensis_ represent terminal populations of clines in several characters, some of which occur in _emoryi_, but that subspecies is more distinct from _pallidus_ and _guadalupensis_ than those subspecies are from each other. _T. s. emoryi_ is the most variable subspecies. _T. ater_, known only from a restricted area in central Coahuila, is most closely related to _T. s. emoryi_, and possesses some characters judged to represent the attenuation of the geographic cline in _pallidus_, _guadalupensis_ and _emoryi_ mentioned above. Some characters of _ater_ show alliance to the species _muticus_. Of _T. muticus_, whose geographic range is removed from that of _ater_, there are two subspecies. Four subspecies of _spinifer_ (_spinifer_, _hartwegi_, _pallidus_ and _asper_) intergrade in the Mississippi River drainage of Louisiana; few specimens, however, are typical of _asper_. The subspecies of _muticus_ do not show definite evidence of intergradation. To facilitate quick reference, the occurrence of some characters that are shared by, or are approximated in, two or more forms are listed in Table 10. In addition to external characters, some ratios emphasize the clinal relationship between _T. s.

pallidus_, _guadalupensis_, and _emoryi_ mentioned above. Of especial interest is the frequent resemblance of those subspecies and _T. ater_ to _T. ferox_ (dorsal pattern on limbs of adults, reduction in anterior tuberculation, wide head, narrow carapace, and short snout), and the less marked resemblance of _T. muticus_ to _T. ferox_; not shown in Table 10 is the resemblance of _ferox_ to _T. muticus calvatus_ in having thick, black-bordered postocular stripes. Some populations of _T.

s. emoryi_ resemble _T. muticus_ in the corresponding size at s.e.xual maturity and in having well-developed plastral callosities. It is notable that the occurrence of _ater_, and to a lesser extent that of _T. s. emoryi_, which resembles _ferox_ (and _muticus_), is in the southwestern United States and northern Mexico.

TABLE 10. Frequency of Selected Characters Among Species and Subspecies of Trionyx in North America. Characters of muticus Refer to the Typical Subspecies; Horizontal Dashes Connecting X's Indicate that Computations for Those Subspecies Were Combined; Vertical Dashes Indicate that the Subspecies Is Intermediate Between the Adjacent Subspecies.

Column headings:

A: _ferox_ B: _spinifer_ C: _hartwegi_ D: _asper_ E: _pallidus_ F: _guadalupensis_ G: _emoryi_ H: _ater_ I: _muticus_

=====================================+=================================== | Species and subspecies Characters +---+---+---+---+---+---+---+---+--- | A | B | C | D | E | F | G | H | I -------------------------------------+---+---+---+---+---+---+---+---+--- Juvenal pattern: | | | | | | | | | black spots, ocelli | | X | X | X | | | | | | | | | | | | | | white dots | | | | | X | X | X | X | -------------------------------------+---+---+---+---+---+---+---+---+--- Pattern on snout: | | | | | | | | | acute angle (reduced in _muticus_) | X | X | X | X | X | | | | X | | | | | | | | | triangular | | | | | X | X | X | X | -------------------------------------+---+---+---+---+---+---+---+---+--- Pattern on side of head: | | | | | | | | | contrasting marks | X | X | X | X | X | X | | | | | | | | | | | | non-contrasting marks (distinct | | | | | | | | | stripe in _muticus_) | | | | | | X | X | X | X -------------------------------------+---+---+---+---+---+---+---+---+--- Pattern on limbs of adults: | | | | | | | | | contrasting | | X | X | X | X | X | | | | | | | | | | | | reduced or absent | X | | | | | X | X | X | X -------------------------------------+---+---+---+---+---+---+---+---+--- Tuberculation (anterior edge of | | | | | | | | | carapace): | | | | | | | | | conical, equilateral | | X | X | X | X | | | | | | | | | | | | | reduced or absent | X | | | | | X | X | X | X -------------------------------------+---+---+---+---+---+---+---+---+--- Head (PL/HW, fig. 3): | | | | | | | | | wide | X | | | X | | X | X | X | | | | | | | | | | narrow | | X | X | | X | | | | X -------------------------------------+---+---+---+---+---+---+---+---+--- Carapace (CL/CW, fig. 4): | | | | | | | | | wide | | X | X | X | X | X | | | X | | | | | | | | | narrow | X | | | | | X | X | X | -------------------------------------+---+---+---+---+---+---+---+---+--- Level of Carapace Width (CL/PCW, | | | | | | | | | fig. 5): | | | | | | | | | middle of carapace | X | X | X | X | | | | | X | | | | | | | | | farther posteriorly | | | | | X | X | X | X | -------------------------------------+---+---+---+---+---+---+---+---+--- Snout (HW/SL, fig. 6): | | | | | | | | | long | | X---X | X | X---X | | | X | | | | | | | | short | X | | | | X---X | X | X | -------------------------------------+---+---+---+---+---+---+---+---+---

Fossils

The known occurrence of fossil trionychids throughout the world indicates a former distribution more widespread than the family has today; the princ.i.p.al difference in the former and present distributions is the lack of living softsh.e.l.ls in Europe.

I have not studied in detail the many fossil remains but such examination as I have made of them suggests that many of the characters used as a basis for distinguishing fossil forms in North America are subject to individual variation or are of no diagnostic value in the living species (Hummel, 1929:769). Knowledge of the variation in the living species of the Old World would aid in adequately appraising the North American fossils. Some osteological characters of the three living American species (excluding _ater_) together with data on variation within a given species are mentioned below. Some differences in skulls of the three species already were mentioned in the section "Osteological Characters." Because most fossil remains are those of the carapace and plastron, attention is here given to those structures.