Sternoptychidae

Marranzino, Ashley N & Webb, Jacqueline F, 2018, Flow sensing in the deep sea: the lateral line system of stomiiform fishes, Zoological Journal of the Linnean Society 183 (4), pp. 945-965 : 949-950

Family Sternoptychidae

The cranial bones in the four species of Argyropelecus examined are quite thin and some appear to have a reticulate structure. They also have a prominent bony ridge running longitudinally along each frontal bone ( Fig. 1A, C ). The ridge starts rostral to the orbit, and becomes more medial, so that between the orbits, the left and right longitudinal ridges nearly touch. Caudal to the orbits, the ridges are more lateral at their posterior end.

The SO, PO and MD canals are present in four species of Argyropelecus ( A. hemigymnus , A. aculeatus Valenciennes, 1850 , A. affinis Garman, 1899 , A. lychnus Garman, 1899 ) as revealed in cleared and stained material, histology, µCT images and via examination of whole preserved specimens. Cleared and stained specimens and µCT images show no evidence of bony canal pores that would suggest the presence of completely ossified canals. However, bony troughs in the preoperculum and along the mandible indicate the presence of partially ossified PO and MD canals, respectively ( Fig. 1B, D, E ). The skin covering the PO and MD canals is very thin and often damaged, making the interpretation of canal morphology difficult, but histological analyses of three specimens of A. aculeatus confirmed the presence of incompletely ossified SO, PO and MD canals ( Table 1). Infraorbital bones and an infraorbital (IO) canal are both absent.

The SO canal is in close association with the longitudinal bony ridge in the frontal bone in all four species of Argyropelecus examined. This feature was studied in detail in A. aculeatus ( Figs 2 , 3B ). Each ridge starts rostral to the orbit, extends caudally and medially, so that the left and right longitudinal ridges nearly touch between the orbits. Caudal to the orbits, the ridges are situated laterally before terminating ( Fig. 2 ). The SO canal starts rostral to the orbit and medial to the bony ridge. The first SO neuromast is found in the skin overlying the frontal bone ( Fig. 2 ). The next SO canal neuromast is enclosed only by soft tissue. The left and right canals merge into a single canal medial to the orbits, between the ridges, and the left and right canal neuromasts are found within a single median canal. Caudal to the orbits, the two SO canals are separate and SO canal neuromasts are found within the left and right SO canals. Each SO canal extends laterally passing through the bony ridge as a short ossified canal containing a neuromast. The canal opens to the surface, and the next neuromast (a SO canal neuromast homologue, given its similarity in size to enclosed canal neuromasts) is found on the skin surface ( Fig. 2 ).

In A. aculeatus , the MD canal is only partially ossified, appearing as a trough containing two canal neuromasts covered by a thin epithelium. The PO canal originates just caudal to the posterior end of the MD canal and varies in morphology along its length. The first PO neuromast sits in a fully ossified canal below a preopercular spine, but the portion of the PO canal oriented dorso-ventrally is a narrow trough enclosed by a thin epithelium and contains three neuromasts.

Histological analysis showed that canal neuromasts are present in the SO, PO and MD canals of A. aculeatus . The neuromasts varied in size among and within canals and with body size, but all appear to be elliptical or diamond shaped with a major axis parallel to the axis of the canal ( Marranzino, 2016). In whole specimens, the canal neuromasts appear as opaque, white, oval structures sitting in bony troughs (visible only if the epithelium covering the trough was absent); neuromast, which is out of the plane of section. Scale bar = 100 µm. © President and Fellows of Harvard College. their locations were confirmed in histological material. Neuromasts are present in troughs in the preopercular and anguloarticular bones in whole preserved A. hemigymnus .

Figures

Figure 1. Cranial osteology of Argyropelecus and Cyclothone. A, B, cleared and stained A. lychnus (26-mm SL) in dorsolateral and lateral views showing longitudinal bony ridge extending from frontal bone (white dashed line). C–E, 3D µCT reconstructions of A. aculeatus (MCZ 137835, 41-mm SL) in dorsal, lateral and ventral views, respectively, showing elongated troughs in the frontal, dentary and preoperculum. F, C. acclinidens (20-mm SL). G, C. microdon (MCZ 89489, 50-mm SL). Ossified lateral line canals are absent in both species. fr, frontal, de, dentary, po, preoperculum. Scale bars = (A) 1 mm, (B) 5 mm, (F) 1 mm. C–F, © President and Fellows of Harvard College.

Figure 2. Supraorbital canal and canal neuromasts in Argyropelecus aculeatus. Line drawing (in dorsal view) based on a 3D µCT reconstruction (MCZ 137835, 41-mm SL). Canal neuromast and superficial neuromast (SN) distributions based on histological analysis (n = 3) and examination of whole specimens (n = 11). SNs (blue), canal neuromasts (red), canal neuromast homologues on the skin surface (blue circles with red outline), canal boundaries (black dotted lines) and canal pores (black open circles). Representative transverse histological sections (MCZ 159086, 39-mm SL) show supraorbital canal neuromasts (n) in rostro-caudal sequence (left-to-right images), with the canal extending laterally through the bony ridge (r). In left-most section, arrowhead indicates a SN. In third section from left, dotted line indicates location of the left canal

Figure 3. Longitudinal bony ridges on the dorsal surface of head in representative stomiiform genera. Ridges