Photolateralis, Sparks, John S. & Chakrabarty, Prosanta, 2015

Sparks, John S. & Chakrabarty, Prosanta, 2015, Description of a new genus of ponyfishes (Teleostei: Leiognathidae), with a review of the current generic-level composition of the family, Zootaxa 3947 (2), pp. 181-190 : 182-187

Photolateralis , new genus

( Figures 2–4 A,B)

Photoplagios in part: Sparks et al., 2005; Sparks, 2006b; Sparks and Chakrabarty, 2007 Equulites in part: Kimura et al., 2008a; Chakrabarty et al., 2011a, 2011b

Type species: Photolateralis stercorarius ( Evermann & Seale, 1907) Other included species: Photolateralis moretoniensis ( Ogilby, 1912) , P. antongil (Sparks, 2006)

Diagnosis. Members of Photolateralis are distinguished from all other leiognathids by the presence of a translucent flank stripe, which depending on the species, may be comprised of either a continuous mid-lateral stripe ( P. antongil ), or a composite stripe comprised of numerous independent translucent windows ( P. stercorarius and P.

moretoniensis ). In Photolateralis , the translucent lateral stripe is either lacking entirely in females, or is considerably less developed. Internally and externally members of Photolateralis can be distinguished from their sister genus, Equulites , by morphology of the light-organ system. Externally, male members of Equulites are characterized by an expansive translucent triangular, cornucopia-shaped, or trapezoidal patch on the flank, in contrast to a continuous or composite mid-lateral stripe comprised of numerous independent translucent windows in Photolateralis . Although both genera share lateral clearing of the guanine-lined internal gas bladder surface corresponding to the location of the translucent external patch or stripe, clearing in Equulites is more extensive, extending the full length of the gas bladder, whereas clearing in Photolateralis is restricted to the posterior region of the gas bladder. The light organ in male members of Equulites is greatly enlarged, with paired dorsolateral lobes extending posteriorly well into the gas bladder (less E. leuciscus ), in contrast to a moderately enlarged donutshaped light organ in male members of Photolateralis that extends at most only slightly posteriorly into the gas bladder.

Discussion and comparisons. Aside from features of the light-organ system (LOS), leiognathids are, in general, morphologically conservative and non-descript silvery fishes with an external appearance that superficially resembles mojarras ( Gerreidae ), with which they are often confused. However, all leiognathids possess a circumesophageal light organ that contains symbiotic luminescent bacteria ( Photobacterium ), whose luminescence the fish co-opt for photic communication and predator avoidance ( McFall-Ngai and Dunlap, 1983), whereas gerreids are not bioluminescent. In a majority of leiognathid species, comprising all members of the subfamily Gazzinae ( Fig. 1 ), the light organ and associated internal and external anatomical features (e.g., translucent windows/stripes and gas bladder clearing) are sexually dimorphic and both intra- and intergenerically variable in morphology ( Sparks et al., 2005: Figs. 4 –8). This integrated LOS provides a rich set of phylogenetically informative characters for diagnosing both genera and species ( Fig. 1 ). The unique anatomical features of the LOS facilitate the transmission of bacterially-generated luminescence ( Sparks et al., 2005; Chakrabarty et al., 2011a) via a species-specific translucent lateral patch (or patches) in males of most species that is located in the head region or on the flank ( Sparks et al., 2005).

All members of Photolateralis were formerly placed in Equulites ( Kimura et al., 2008; Chakrabarty et al., 2011a, b). Photolateralis shares the presence of translucent lateral flank regions with its sister genus Equulites ( Figs. 2–4 ). In Photolateralis , however, these translucent areas form a mid-lateral stripe, either continuous or comprised of discrete “windows” ( Figs 2 and 3 ), whereas in Equulites , comprising E. elongatus , E. rivulatus , E. leuciscus , E. klunzingeri , E. laterofenestra , and E. absconditus , males are characterized by the presence of a species-specific triangular, translucent lateral flank patch ( Fig. 4 ; Sparks and Chakrabarty, 2007: Fig. 1 ). Although Equula lineolata Valenciennes , in Cuvier and Valenciennes, 1835 is placed within Equulites ( Eschmeyer, 2015) , with the new placement attributed to Kimura et al. (2008a), in fact Kimura et al. (2008a) never mention Equula lineolata . Sparks (2006b) discussed the taxonomic status of Equula lineolata and concluded the name was a nomen dubium of uncertain placement beyond the family level.

In both Photolateralis and Equulites , the lateral gas bladder lining exhibits clearing of the silvery guanine layer that corresponds specifically to the external translucent region characteristic of that taxon ( Figs. 2 and 4 ; Sparks et al., 2005). Males of both genera also possess enlarged light organs (moderately so in Photolateralis vs. greatly enlarged in Equulites ) compared to conspecific females and in which the dorsal lobes extend posteriorly into the gas bladder ( Figs. 2 and 4 ). However, both the degree of enlargement and posterior extension of the light organ are considerably muted in Photolateralis ( Figs. 2 B and 4B) as compared to Equulites ( Fig. 4 D). The light organ in male members of Photolateralis is more or less donut shaped ( Figs. 2 B and 4B), lacking the greatly enlarged dorsolateral extensions characteristic of male members of Equulites ( Fig. 4 D).

In addition to Photolateralis , new genus, we currently recognize the following nine extant leiognathid genera, in chronological order of their description, whose phylogenetic relationships are shown in Figure 1 (after Chakrabarty et al., 2011b):

1) Leiognathus Lacepède, 1802 . Type species: Leiognathus argenteus Lacepède, 1802 , by monotypy. Equula Cuvier, 1815 was resurrected from synonymy with Leiognathus Lacepède, 1802 by Chakrabarty and Sparks (2008) to encompass E. fasciata and E. longispinis . Chakrabarty and Sparks (2008:5) treated Equula as valid, however, they incorrectly assigned the wrong type species to Leiognathus . The correct type species is Leiognathus argenteus Lacepède 1802 (see Sparks and Dunlap, 2004; Eschmeyer, 2015). As the same specimen was used to describe the type species of both Equula and Leiognathus , these genera are objective synonyms and identical. The genus Equula is therefore a synonym of Leiognathus (corrected in Chakrabarty et al., 2009). Leiognathus is recovered as the sister group to the remaining members of Leiognathidae less Aurigequula . Members of Leiognathus , comprising Leiognathus equula and L. robustus , plus a number of undescribed species ( Chakrabarty and Sparks, 2008; Chakrabarty et al., 2011b), are not sexually dimorphic with regard to features of the light-organ system.

2) Gazza Rüppell 1835 . Type species: Gazza equulaeformis Rüppell 1835 , by monotypy. Gazza is recovered as the sister taxon to Deveximentum and placed within the tribe Gazzini, subfamily Gazzinae.

3) Deveximentum Fowler, 1904 . Type species: Zeus insidiator Bloch 1787 , by original designation. The species now placed in this genus were formerly placed in Secutor Gistel, 1848 . Kottelat (2013) considered Secutor a replacement name for Equula Cuvier 1815 , which is itself a synonym of Leiognathus Lacepède, 1802 (see above for Leiognathus ). Deveximentum is recovered as the sister taxon to Gazza and placed within the tribe Gazzini, subfamily Gazzinae.

4) Equulites Fowler, 1904 . Type species: Leiognathus vermiculatus Fowler 1904 , by original designation.

Equulites was described as a subgenus of Leiognathus . Equulites was removed from synonymy with Leiognathus and elevated to generic rank by Chakrabarty and Sparks (2008). Equulites currently comprises E. leuciscus , E. klunzingeri , E. elongatus , E. rivulatus , E. absconditus , and E. laterofenestra . Photoplagios was described by Sparks et al. (2005), but is a synonym of Equulites Fowler 1904 , given that Equulites , an older name, was available for one of the species included in Photoplagios ( Kimura et al., 2008a; Chakrabarty and Sparks, 2008). Equulites is recovered as the sister taxon to Photolateralis , new genus, and placed within the tribe Equulitini, subfamily Gazzinae.

5) Eubleekeria Fowler, 1904 . Type species: Equula splendens Cuvier 1829 , by original designation. Eubleekeria was described as a subgenus of Leiognathus . Eubleekeria was removed from synonymy with Leiognathus and elevated to generic rank by Chakrabarty and Sparks (2008). Eubleekeria encompasses the ‘‘ Leiognathus ’’ splendens species complex ( E. splendens , E. jonesi , and E. kupanensis ; Kimura et al., 2005), and also includes multiple undescribed species. Eubleekeria is recovered as the sister group to Photopectoralis and placed within the tribe Eubleekerini, subfamily Gazzinae.

6) Aurigequula Fowler, 1918 . Type species: Clupea fasciata Lacepède, 1803 , by original designation. Aurigequula was described as a subgenus of Leiognathus . Aurigequula was mistakenly placed in Equula by Chakrabarty and Sparks, (2008). Aurigequula was resurrected from synonymy with Leiognathus Lacepède, 1802 by Chakrabarty et al. (2009) to replace a previously established invalid name, and comprises A. fasciata (type species), A. longispina , and A. striata , plus a number of undescribed species. For additional comments regarding generic status see heading for Leiognathus above and Chakrabarty et al. (2009). Aurigequula is recovered as the sister group to all other members of Leiognathidae . Like Leiognathus , members of Aurigequula do not appear to be sexually dimorphic with regard to features of the light-organ system ( Sparks and Dunlap, 2004).

7) Nuchequula Whitley, 1932 . Type species: Equula blochii Valenciennes , in Cuvier and Valenciennes, 1835, by original designation. Nuchequula was described as a subgenus of Eubleekeria . The subgenus Nuchequula was elevated to generic rank and revised by Chakrabarty and Sparks (2007). Nuchequula was also considered to warrant generic rank and was subsequently revised again the following year by Kimura et al. (2008b). Nuchequula is recovered as the sister taxon to Karalla and placed within the tribe Nuchequulini, subfamily Gazzinae.

8) Photopectoralis Sparks, Dunlap, and Smith, 2005 . Type species: Leiognathus aureus Abe and Haneda, 1972 , by original designation. Photopectoralis currently comprises P. aureus , P. bindus , P. h at a i i, and P. panayensis , plus at least two undescribed species. Photopectoralis is recovered as the sister group to Eubleekeria and placed within the tribe Eubleekerini, subfamily Gazzinae.

9) Karalla Chakrabarty and Sparks, 2008 . Type species: Equula daura Cuvier 1829 , by original designation. Karalla currently comprises K. daura and K. dussumieri . Karalla is recovered as the sister taxon to Nuchequula and placed within the tribe Nuchequulini, subfamily Gazzinae.

Figures

FIGURE 1. Generic level phylogeny for Leiognathidae after Chakrabarty et al. (2011 b) indicating placement of Photolateralis, new genus (in boldface type).

FIGURE 2. Internal and external features of light-organ system (LOS) in members of Photolateralis. (A) Male P. stercorarius external LOS anatomy. (B) Male P. stercorarius internal LOS anatomy. (C) Female P. stercorarius internal LOS anatomy. Figure abbreviations: es = esophagus, g = gut, gb = gas bladder, lo = circumesophageal light organ, mls = translucent midlateral stripe. Stippled region on posterolateral region of gas bladder corresponds to clearing of silvery guanine lining in region internal to translucent mid-lateral stripe. Yellow region on light organ corresponds to “ window ” that is directed and opens into the gas bladder.

FIGURE 3. Species of Photolateralis illustrating phenotypic variability of translucent mid-lateral stripe characteristic of genus. (A) P. stercorarius, USNM 55906, holotype, 83.8 mm SL, Philippines. (B) P. moretoniensis, AMS I. 22983001, non-type, 76.9 mm SL, Australia. (C) P. antongil, AMNH 236544, holotype, 75.0 mm SL, Madagascar.

FIGURE 4. Comparison of internal and external anatomy of LOS in sister genera Equulites and Photolateralis, new genus: (A) Translucent lateral flank stripe comprised of discrete “ windows ” in male Photolateralis stercorarius (SIO 96 - 96, 98.2 mm SL, Philippines). (B) Internal anatomy of LOS in male P. stercorarius (AMNH uncat., [VN- 2010 - 11], 73.1 mm SL, Vietnam). Note comparatively small donut shaped, spotted light organ and posterior lateral clearing of silvery gas bladder lining. (C) Expansive translucent, triangular lateral flank patch characteristic of male members of Equulites (E. elongatus, UMMZ uncat., [PVD 82 - 06 / 19 a LE], 76.4 mm SL, Japan). (D) Internal anatomy of LOS in male E. elongatus (UMMZ uncat., 68.0 mm SL, Japan). Note greatly enlarged dorsolateral light-organ lobes in Equulites that extend posteriorly well into gas bladder, and lateral clearing of silvery guanine layer running entire length of gas bladder. Light organs circled in yellow. Figure abbreviations: gb = gas bladder, mls = translucent mid-lateral stripe, tp = translucent, triangular lateral flank patch.