Phyllorhynchus decurtatus

Cadle, John E., 2011, Hemipenial morphology in the North American snake genus Phyllorhynchus (Serpentes: Colubridae), with a review of and comparisons with natricid hemipenes, Zootaxa 3092 (1), pp. 1-25 : 2-4

Phyllorhynchus decurtatus

Everted (CAS 223608. Fig. 1 , right hemipenis). Total length of organ about 23 mm. Greatest width of apical region (across the lobes) about 11.5 mm. Major retractor muscle divided (the muscle was pulled through the base of the organ in order to sever both slips to effect full eversion of the lobes). Hemipenis with a narrow base ornamented with scattered spinules and a pair of irregular bulges. Distal region greatly expanded and bilobed.

Sulcus spermaticus centrolineal almost to the base of the lobes, at which point the sulcus lips diverge, each passing somewhat centripetally for a short distance on the medial surface of its respective lobe ( Fig. 1A ). The diverging lips do not go fully to the crotch, but instead remain close to the sulcate side. Thus, the sulcus spermaticus is neither simple nor divided in the way those terms are usually defined. The distal tip of the sulcus spermaticus simply broadens abruptly and its lips flare laterally. As a result, the expanded tip of the sulcus spermaticus has a definitive proximal (or lateral) border formed by the short extensions of the sulcus lips but no definitive distal (or medial) border. The sulcus groove is expanded terminally and merges into the broad nude apical area at the point of divergence of the lips. In typically forked sulci, thickened lip tissue forms a definitive margin to both sides of each sulcus branch distal to the point of sulcus division.

Most of the hemipenial body is ornamented with short stout spines, which are larger proximally, gradually decreasing in size distally. Spines are arrayed in a strip about four longitudinal rows across on each side of the sulcus spermaticus, and a similar strip on each side of a large triangular asulcate nude area described below. The sulcate and asulcate strips of spines are continuous across the base of the lateral sides of the organ by several rows of large spines. Both the sulcate and asulcate strips of spines occupy bulbous tissue which, when inflated, forms broad spinose ridges rising above the base level of the hemipenial surface. On the lateral sides of the organ, enclosed between the sulcate and asulcate strips is a broad oval area (depressed below the enclosing spinose ridges) occupied by much smaller spines; distally these small spines become more organized into definitive transverse rows and near the lateral tips of the lobes they grade into small, poorly developed calyces. The sulcate spinose ridges deflect laterally just proximal to the nude lobes, decreasing in prominence and joining the small calyces on the lateral distal surface of the lobes.

Calyces are larger proximally, smaller distally; have thick, fleshy walls; and sparsely ornamented with thick, robust spinules having a short stout point projecting from atop the calycular walls. Proximally the spinules grade into the spines on the sulcate and asulcate surfaces; they become less prominent on the distal calyces, a few of which have nude walls lacking spinules.

From the distal edge of the calyculate region, delicate, low ridges extend over the tips of the lobes ( Fig. 1B ), converging toward the points of internal attachment of the retractor muscles on the medial surface of the lobes near the sulcate side. These ridges have low, blunt spinules or papillae toward the outer surface of the lobes but these become lower toward the crotch (crotch ends of these ridges smooth).

Except for their lateral surfaces and the apical ridges, the lobes are nude. From the crotch broad nude areas extend proximally on both the sulcate and asulcate sides. These nude areas are roughly the shape of an isosceles triangle (broad at the crotch, narrowing proximally); on the asulcate side the triangle is delimited proximally by about five rows of spines, whereas on the sulcate side a narrow nude strip borders the entire length of the sulcus spermaticus, leaving the proximal end of the sulcate nude area without a definitive border.

Retracted (CAS 178969. Fig. 2A . left hemipenis). Hemipenis extends to proximal end of subcaudal 14 and is slightly bilobed (dorsal and ventral lobes). Proximal end of major retractor divided. As the sulcus spermaticus approaches the lobes, its lips diverge, with one lip extending into the dorsal lobe, and the other lip visible in the transected ventral lobe. Tissue between the divergent lips and on either side of the distal end of the sulcus is in longitudinal folds, which apparently expands upon eversion to form the nude apical tissue. The bands of spines on the everted hemipenis occupy thick cords of tissue in the retracted organ.

Remarks. The hemipenis of Phyllorhynchus decurtatus illustrated by Klauber (1935: Fig. 4 ) is an asulcate view and seems to be incompletely everted to the base of the lobes. I also reexamined the holotype of P. arenicola, CAS-SU 14013 (= P. decurtatus ). The right hemipenis is missing and the left organ had been slit more or less midventrally and severed distally within the apical tissue. The morphology of this organ is similar to that depicted in Fig. 2A .

Perhaps because the hemipenis he examined was incompletely everted, Klauber (1935: 14) failed to fully understand the unusual apical and sulcus morphology of Phyllorhynchus decurtatus . Similarly, without having examined everted hemipenes it is understandable how Savage & Cliff (1954) misinterpreted the strongly divergent sulcus lips in retracted hemipenes of P. decurtatus as indicating a bifurcate sulcus spermaticus ( Fig. 2A ). In retracted organs it is not necessarily obvious that the folded tissue between the sulcus lips expands into an extensive nude area. A sulcus morphology in which the lips diverge strongly and the sulcus groove opens into an apical nude region was not fully understood until later and is associated primarily with natricids ( Dowling & Savage 1960, McDowell 1961, Rossman & Eberle 1977, Zaher 1999). Indeed, the similarity of Phyllorhynchus apical morphology to some natricids is significant and is taken up below.

Figures

FIGURE 1. Everted hemipenis of Phyllorhynchus decurtatus (CAS 223608) in sulcate (left) and asulcate (right) views. A, detail of the divergent lips of the sulcus spermaticus, one lip passing to each lobe (CAS 223608); B, outer (lateral) surface of the right lobe showing the low ridges passing over the apical tip (CAS 178969).

FIGURE 2. Retracted hemipenes of A, Phyllorhynchus decurtatus (CAS 178969) and B, P. browni (CAS 160227); apical (distal) ends of organs that were slit midventrally and spread flat. R, retractor penis magnus. ss, sulcus spermaticus (lettering placed over the sulcus proximal to the divergent lips). Arrows indicate lips of the sulcus after the point of divergence.

FIGURE 4. Everted hemipenes of natricids in sulcate view. A, Afronatrix anoscopus (CAS 230205). B, Amphiesma bitaeniatum (CAS 215037) (detail of apex shown in Fig. 5A). C, Natrix tessellata (CAS 183115) (detail of apex shown in Fig. 5D).