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Copeia, 2002(1), pp. 213–219

Observations on Rostral Canal Bones of Two Species of Acipenser (, )

ERIC J. HILTON

Comparative osteological studies of the Acipenser are rare. In this paper, the rostral canal bones of Acipenser brevirostrum and oxy- rinchus are compared and three new osteological features that distinguish these spe- cies are described. In lateral view, the jugal bone of adult A. o. oxyrinchus possesses distinct vertical and horizontal arms, whereas that of A. brevirostrum is roughly tri- angular in shape. The posterior rostral canal bones in A. o. oxyrinchus possess larger and more pointed medial expansions relative to those found in A. brevirostrum. The rostral canal bones immediately lateral to the commissure in adult A. o. oxyrinchus are ornamented with thorn-shaped processes, which differ from the simple Y-shaped tubes found in A. brevirostrum and other taxa examined.

PECIES of sturgeon (Acipenseridae) can be geons are comprised of isolated elements (most- S notoriously difficult to distinguish (Birstein ly dermal scutes and pectoral fin spines). The and Bemis, 1997). This may be a result of sev- detailed knowledge of the skeleton of living taxa eral factors, including natural hybridization be- is, therefore, necessary to correctly and more tween taxa [e.g., Acipenser gueldenstaedtii Brandt specifically identify and interpret such material. and Ratzeberg and huso (Linnaeus), Bir- Acipenser brevirostrum and A. o. oxyrinchus are stein et al., 1997], high level of individual mor- found in coastal rivers and near-shore marine phological variation within a taxon (e.g., Acipen- habitats of eastern North America (Vladykov ser brevirostrum LeSueur, Hilton, and Bemis, and Greeley, 1963; Bain, 1997; Kynard et al., 1999), and the fact that some taxa are not well 2000). These are the only sturgeon species pre- differentiated morphologically [e.g., Scaphirhyn- sent in what Bemis and Kynard (1997) term the chus suttkusi Williams and Clemmer and Scaphir- North Western Atlantic biogeographic province. hynchus platorhynchus (Rafinesque), Mayden and A subspecies of (Acipenser ox- Kuhajda, 1996]. Acipenser, the largest genus in yrinchus desotoi Vladykov) differs from A. o. oxy- the family, has no known osteological synapo- rinchus in shape of the dorsal scutes and pro- morphies (Bemis et al., 1997; Grande and Be- portions of the head, pectoral fins and the mis, 1996), and molecular-based phylogenetic spleen (but see Wooley, 1985; Smith and Clugs- studies (e.g., Birstein et al., 1997; Birstein and ton, 1997) and is restricted to the Gulf of Mex- DeSalle, 1998) do not support monophyly of ico, the northern coast of South America and the genus. Despite such lingering systematic the waters surrounding Bermuda (Vladykov and questions, the five recognized North American Greeley, 1963). According to current morpho- species of the genus Acipenser are considered val- logical (e.g., Artyukhin, 1995, reproduced by id taxonomic units with distinctive life histories, Bemis et al., 1997:fig. 14) and molecular-based biogeographic ranges, aggregate morphometric (Birstein and DeSalle, 1998) phylogenies, A. bre- and meristic qualities, and genetic characteris- virostrum and A. oxyrinchus are widely separated tics (e.g., Vladykov and Greeley, 1963; Birstein phylogenetically. Dadswell et al. (1984) com- and Bemis, 1997). mented on their morphological similarity, This paper documents the structure and on- which is particularly evident in small specimens. togeny of the rostral canal bones in shortnose They regarded the relative width of the mouth sturgeon (A. brevirostrum LeSueur) and Atlantic as the most useful character to distinguish these sturgeon (Acipenser oxyrinchus oxyrinchus Mitch- two species at all sizes. According to their study, ill) and provides the first documented cranial mouth width of A. brevirostrum is greater than osteological differences between these two spe- 62% of interorbital width, whereas in A. oxyrin- cies. This is important because much skeletal chus mouth width is usually less than 55% of material, particularly dried specimens, can be interorbital width. However, they report an difficult, if not impossible, to correctly identify overlap in the ranges of these percentages (43– (e.g., many of the distinguishing characteristics 66% for A. brevirostrum and 63–81% for A. oxy- are not adequately preserved, if at all). Similar- rinchus). Other characters, such as dorsal and ly, much of the known record and subfos- anal fin ray counts, postpelvic scute arrange- sil remains (e.g., in archaeological sites) of stur- ment, and color of viscera, are potentially useful

᭧ 2002 by the American Society of Ichthyologists and Herpetologists 214 COPEIA, 2002, NO. 1 to distinguish these species (e.g., Eddy, 1969; RESULTS Robins et al., 1986). However, no discrete cra- nial osteological differences have thus far been The rostral sensory canal of Acipenseriformes reported. is defined as the continuation of the infraorbital sensory canal anterior to its exit from the jugal MATERIALS AND METHODS bone (Grande and Bemis, 1991; Findeis, 1993). Rostral canal bones form as a series of ossifica- Clearing and staining followed Dingerkus tions surrounding the rostral sensory canal and Uhler (1977), and dry skeletons were pre- along the length of the ventral surface of the pared by either dermestid beetles or water mac- snout. A medial flange of the jugal marks the eration. Specimens were examined under a exit point of the infraorbital canal from this Wild M5 dissecting microscope equipped with a bone (Figs. 1–2). In its anterior course, the ros- camera lucida and fiber optic lights. Twenty-sev- tral canal curves medially around the outer bar- en specimens of A. brevirostrum and 16 speci- bel (Figs. 1–2). Anterior to this medial curve, mens of A. o. oxyrinchus were examined; no spec- the rostral canal runs parallel to the ventral ros- imens of A. o. desotoi were available for study. tral bones, which develop in the ventral midline Specimens consist of both wild and laboratory- of the snout (Fig. 1). The left and right rostral reared individuals. While a comprehensive canals meet at a commissure at the tip of the study of the ontogeny of the rostral canal bones snout. Having the rostral/infraorbital canal en- is impossible given the available specimens, a closed in tubular bony ossicles (vs the plesiom- range of ontogenetic stages was examined (i.e., orphic condition of having these canals en- small juveniles to fully mature adults). Institu- closed in laminar bones) was found to be a syn- tional abbreviations follow Leviton et al. (1985) apomorphy for Acipenseriformes by Jin (1999: with the addition of UMA for the University of fig. 16, character 7; also see Jin et al., 1995). Massachusetts Amherst Museum of Natural His- This character was coded as ‘‘?’’ in †Chondrosteus tory. and †Strongylosteus ( Jin, 1999:table 12), and, The following specimens were examined and therefore, alternatively could be interpreted as are indicated as alcohol-preserved (a), cleared- a synapomorphy of Acipenseristomi (ϭ Acipen- and-stained (c&s), or dry skeletal (ds) speci- seriformes minus †, Grande mens; measurements are given in standard and Bemis, 1996). However, Grande and Bemis length (SL) or estimated total length (est. TL). (1996:character B7) and Grande et al. (In Press: Acipenser brevirostrum, UMA F10011—UMA character 14) code this character as derived in F10023 (13 ds; 600–800 mm SL), UMA F10362 †Chondrosteus, supporting Jin’s (1999) conclu- (1 c&s; 170 mm SL), UMA F10363 (1 c&s; 93 sion. The medial curve around the outer barbel mm SL), UMA F10634 (9 c&s; 95–154 mm SL), is a synapomorphy of Acipenseridae (Findeis, UMA F10377 (1 ds; 810 mm SL), UMA F10381 1997). (1 c&s; 410 mm SL), UMA F10521 (1 ds; est. The jugal bone develops around the infraor- 550 mm TL). Acipenser oxyrinchus oxyrinchus, bital canal and is the ventral most of the post- UMA F10360 (1 ds; est. 925 mm TL), UMA orbital bones. I observed small individuals of A. F10361 (1 ds; est. 925ϩ mm TL), UMA F10365 o. oxyrinchus (e.g., 56 mm SL, Fig. 1D) in which (1 ds; 525 mm SL), UMA F10366 (2 a; 58, 79 the infraorbital sensory canal is open along the mm SL), UMA F10367 (2 c&s; 60, 76 mm SL), posterior of the jugal (i.e., the canal is not yet UMA F10368 (1 c&s; 58 mm SL), UMA F10373 completely enclosed by bone), but soon during (3 c&s; 70–80 mm SL), UMA F10374—UMA development this canal is surrounded by bone F10376 (3 c&s; 53–335 mm SL), UMA F10382 (e.g., 58 mm SL, Fig. 1E). The vertical arm of (1 c&s; 290 mm SL), UMA F10684 (1 ds; est. the jugal is tightly sutured to the other dermal 2400 mm TL). Acipenser ruthenus Linnaeus, postorbital bones (not shown) and the horizon- UMA F10369 (4 c&s; est. 25–120 mm TL). Aci- tal arm, regarded as a synapomorphy of Acipen- penser stellatus Pallas, FMNH 63706 (2 c&s; seridae (Findeis, 1997:character 3), is directed 120ϩ, 135 mm SL). Acipenser transmontanus anteriorly and is continuous with the border Richardson, UMA F10372 (5 c&s; est. 100–120 rostral bones (e.g., Fig. 1B). In small individuals mm TL). Acipenser sp., MCZ 153683 (1 ds; est. of both species, the vertical and horizontal arms 500 mm TL). albus (Forbes and of the jugal bones are distinct in lateral view, Richardson), UMA F10370 (1 c&s; 135 mm SL); and in specimens of A. brevirostrum up to 380 UMA F10371 (1 c&s; 139 mm SL). Scaphirhyn- mm SL, the two arms remain separate (e.g., chus platorhynchus, UMA F10624 (1 ds; est. 640 UMA F10381). However, the jugal bone of larg- mm TL). kauffmani (Bog- er A. brevirostrum is roughly triangular in shape danow), MCZ 27653 (1 c&s; est. 200 mm TL). (Fig. 2A; this condition is well developed by ap- HILTON—ROSTRAL CANAL BONES OF 215

Fig. 1. Rostral canal bones at three stages of development in (A–C) Acipenser brevirostrum and (D–F) Acipenser oxyrinchus oxyrinchus. Specimens in ventral view; anterior facing left. (A) UMA F10363, 93 mm SL; (B) UMA F10362, 170 mm SL; (C) UMA F10381, 380 mm SL; (D) UMA F10374, 56 mm SL; (E) UMA F10368, 58 mm SL; (F) UMA F10376, 335 mm SL. Abbreviations: brb, border rostral bones; cb, commissural bone(s); haj, horizontal arm of the jugal; iba, inner barbel; ioc, infraorbital canal; mfj, medial flange of the jugal; oba, outer barbel; rcb, rostral canal bones; vrb, ventral rostral bones. proximately 500 mm SL), whereas that of A. o. medial jugal flange. These elements are ex- oxyrinchus retains two distinct arms (Fig. 2D); panded medially and are less exaggerated and this condition was also found in large adult A. often more rounded in A. brevirostrum than in o. oxyrinchus (e.g., UMA F10684; est. 2400 mm A. o. oxyrinchus (Fig. 2B, E). These medial ex- TL). A medial jugal flange (Findeis, 1997), al- pansions begin to develop later during ontoge- though well developed in A. brevirostrum (Fig. ny in A. brevirostrum than in A. o. oxyrinchus. For 2B), is less pronounced than the bladelike example, these expansions have already begun flange of A. o. oxyrinchus (Fig. 2E). to develop in a 58 mm SL specimen of A. o. Ossification of the rostral canal bones is well oxyrinchus (Fig. 1E), whereas they are not yet underway in all specimens examined here. In found in a 170 mm SL specimen of A. breviros- both species, a variable number of flattened ros- trum (Fig. 1B). The rostral canal bones anterior tral canal bones lies immediately anterior to the to the barbels completely surround the rostral 216 COPEIA, 2002, NO. 1

Fig. 2. Isolated jugal and rostral canal bones from adult (A–C) Acipenser brevirostrum (UMA F10377, 995 mm TL) and (D–F) Acipenser oxyrinchus oxyrinchus (UMA F10360, est. 925 mm TL). White lines indicate ap- proximate outline of head, orbit, nares and barbels. Note slight bilateral asymmetry in specimen of A. brevi- rostrum. (C and F) illustrate difference in form of the lateral commissural bones between the two taxa. Anterior facing left in all. Abbreviations: cb, commissural bone(s); en, excurrent nares; haj, horizontal arm of the jugal; iba, inner barbel; in, incurrent nares; lcb, lateral rostral commissural bone; mfj, medial flange of the jugal; or, orbit; oba, outer barbel; rcb, rostral canal bones; vaj, vertical arm of the jugal.

sensory canal and are tubelike in adults of both open in a trough in the elements posterior to species. In small specimens of both species (i.e., the barbels (e.g., Fig. 1C, F). Ͻ 100 mm SL), the rostral canal bones are open The rostral canal bones that lie immediately and trough-like along the entire length of the lateral to the commissure can be readily distin- rostral canal. The rostral canal bones decrease guished from the others of the series. These lat- in diameter anteriorly and open ventrally by a eral rostral commissural bones initially form as variable number of pores, which may be either C-shaped elements but soon become Y-shaped. slitlike or circular. The rostral canal remains In the smallest specimens of A. brevirostrum and HILTON—ROSTRAL CANAL BONES OF STURGEONS 217

A. o. oxyrinchus examined here, the stem of the similarly shaped element in other Acipenseri- Y was not well developed (if present at all), sug- formes. However, although there likely is cor- gesting that it generally forms later than the rest respondence of the antorbital of other actin- of the element. In adult A. brevirostrum, the lat- opterygians with one of the elements anterior eral commissural bones have a simple tubular of the jugal in Acipenser and other Acipenseri- form (Fig. 2C), whereas in A. o. oxyrinchus, these formes, the designation of this correspondence elements are elaborately shaped, with a variable to any specific element in this chain is difficult. number and arrangement of thornlike process- This is particularly true in Acipenser because a es (Fig. 2F). In A. o. oxyrinchus, these processes one-to-one correspondence of all the rostral ca- are not present in small individuals, only weakly nal elements with bones of other actinoptery- developed in the largest available specimen of gians cannot be established (i.e., it is difficult to known length (UMA F10365, 525 mm SL) but draw a one-to-one correspondence between spe- are well developed in larger specimens (e.g., cific elements of the rostral sensory canal series UMA F10361, est. 925ϩ mm TL). In the largest in Acipenser and the rostral, antorbital, or any specimen of A. o. oxyrinchus available (UMA other specific bone of other taxa). Still, Allis’ F10684, est. 2400 mm TL), the processes on the (1905) statement of homology of this element element identified as a lateral commissural with the antorbital of other actinopterygians bone are more subtle than in the other large warrants future study because qualities of this specimens (this specimen, which was prepared bone (e.g., a triradiate shape) do suggest ho- and referred to by Findeis (1997), is completely mology with the antorbital, which often possess- disarticulated and incomplete); this character es a triradiate canal (e.g., as in Amia calva, should be confirmed in more adult specimens Grande and Bemis, 1998:fig. 17). of A. o. oxyrinchus. These processes were never A variable number of commissural canal observed in A. brevirostrum, even in the largest bones was found in specimens of both A. brevi- specimens studied (Ͼ1000 mm TL). Pehrson rostrum and A. o. oxyrinchus. No bony elements (1944) found these Y-shaped bones in a 15 mm have formed in the commissure in the smallest TL specimen of A. ruthenus (see also Jollie, stages of both species examined here (Fig. 1A, 1980), and Jollie (1980:fig. 4) illustrated a Y- D), although number and presence of commis- shaped bone in a 24-day-old specimen of A. ful- sural canal bones could not be correlated strict- vescens Rafinesque (length not recorded). ly with the size of the specimen (e.g., some adult-sized specimens have no commissural ca- DISCUSSION nal bones). When present, these elements range from well-developed tubes to small nod- Findeis (1997:character 4; also Grande and ules of bone that are only weakly associated with Bemis, 1996:character R3) cited the presence of an antorbital as a synapomorphy of Acipenseri- the canal. It is often reported that there are no dae. However, the bone that he termed an ‘‘an- bony elements associated with the commissure torbital’’ is a platelike dermal element posi- in Acipenser (e.g., Allis, 1905; Sewertzoff, 1926; tioned between the orbit and the nasal capsule Pehrson, 1944), although Jarvik (1948) de- and does not carry a sensory canal (also see Hil- scribed a pair of elements in the commissure of ton and Bemis, 1999:fig. 5). Therefore, this el- a single specimen of A. sturio and considered ement, better termed a supraorbital (Grande them homologous to the rostral bone of other and Bemis, 1996), does not correspond to the actinopterygians. This was in contrast to the bone discussed here or to the antorbital of oth- usual interpretation of the rostral bone, which er actinopterygians, which is an anterior ossifi- is generally regarded as a single median ele- cation of the infraorbital sensory canal (e.g., ment (e.g., Gardiner, 1984:267–270; Grande Grande and Bemis, 1998). Allis (1905) consid- and Bemis, 1998:fig. 11; although Rojo, 1991, ered the Y-shaped lateral commissural bones in defined the rostral as a paired bone but gives Acipenser to be homologous with the antorbital no citation or further explanation). Given the bones of other actinopterygians, although Jollie variation of commissural canal bones in Acipen- (1980) questioned this based on their far ante- ser, their correspondence with the rostral bone rior position. Zhou (1992:fig. 2) and Grande of other actinopterygians remains unclear. How- and Bemis (1996:fig. 8) identified a Y-shaped ever, as the skeletal anatomy of sturgeons be- antorbital in †Peipiaosteus pani Liu and Zhou, comes better known, such homology issues will and Watson (1925:fig. 8) illustrated a similarly be able to be better addressed, and additional shaped antorbital in †Chondrosteus acipenseroides discrete osteological features that reliably differ- Agassiz. Because of the triradiate shape of this entiate sturgeon species and diagnose mono- element in Acipenser, it surely corresponds to the phyletic groups will likely be discovered. 218 COPEIA, 2002, NO. 1

ACKNOWLEDGMENTS of Western Australia. Bull. Brit. Mus. Nat. Hist. (Geol.) 37:173–428. I am grateful to B. Kynard, M. Kieffer, and E. GRANDE, L., AND W. E. BEMIS. 1991. Osteology and Henyey for donating to UMA many of the spec- phylogenetic relationships of fossil and Recent pad- imens used in this study and to W. E. Bemis dlefishes (Polyodontidae) with comments on the (UMA), Barry Chernoff and M. A. Rogers interrelationships of Acipenseriformes. J. Vert. Pa- leo., Mem. 1, vol. 11, Suppl. 1:i–v,1–121. (FMNH), and K. E. Hartel (MCZ) for making ———, AND ———. 1996. Interrelationships of Aci- available specimens. Comments and suggestions penseriformes, with comments on ‘‘’’, of W. E. Bemis and L. Grande greatly improved p. 85–115. In: Interrelationships of fishes. M. L. J. the manuscript. Support for this project came Stiassny, L. R. Parenti, and G. D. Johnson (eds.). from the Graduate Program in Organismic and Academic Press, San Diego, CA. Evolutionary Biology at UMass Amherst, a Na- ———, AND ———. 1998. A comprehensive phylo- tional Science Foundation Doctoral Dissertation genetic study of amiid fishes (Amiidae) based on Improvement Grant (DEB-0073066), and the comparative skeletal anatomy. An empirical search Lester Armour and William A. and Stella Row- for interconnected patterns of natural history. J. ley Graduate Fellowship (FMNH). Vert. Paleo., Mem. 4, vol. 18, Suppl. 1:i–x,1–690. ———, F. JIN,Y.YABUMOTO, AND W. E. BEMIS. In Press. †Protopsephurus liui, a well-preserved primitive pad- LITERATURE CITED dlefish (Acipenseriformes: Polyodontidae) from the Early of China. J. Vert. Paleo. ALLIS, E. P. 1905. The laterosensory canals and related HILTON, E. J., AND W. E. BEMIS. 1999. Skeletal varia- bones in fishes. Internat. Monatsschr. Anat. Physiol. tion in (Acipenser brevirostrum) 21:401–499. from the Connecticut River: Implications for com- ARTYUKHIN, E. N. 1995. 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Massachusetts Rivers, with notes on estuarine Atlan- DINGERKUS, G., AND L. D. UHLER. 1977. Enzyme clear- tic sturgeon: a hierarchical approach. Trans. Am. ing of alcian blue stained whole small Fish. Soc. 129:247–503. for demonstration of cartilage. J. Stain Technol. 52: LEVITON, A. E., R. H. GIBBS JR. AND C. E. DAWSON. 229–232. 1985. Standards in herpetology and ichthyology. EDDY, S. 1967. How to know the freshwater fishes. Part I. Standard symbolic codes for institutional re- Wm. C. Brown Co. Publishers, Dubuque, IA. source collections in herpetology and ichthyology. FINDEIS, E. K. 1993. Skeletal anatomy of the North Copeia 1985:802–832. American Scaphirhynchus pla- MAYDEN,R.L.,AND B. R. KUHAJDA. 1996. Systematics, torynchus (Rafinesque 1820) with comparisons to taxonomy, and conservation status of the endan- other Acipenseriformes. Unpubl. Ph.D. diss. Univ. gered , Scaphirhynchus suttkusi of Massachusetts, Amherst. Williams and Clemmer (Actinopterygii, Acipenser- ———. 1997. 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