Monday, October 4, 2021

[Ichthyology • 2021] Danionella cerebrum • The Emerging Vertebrate Model Species for Neurophysiological Studies: A New Cyprinid Fish (Cyprinidae: Danioninae) from Myanmar

Danionella cerebrum
Britz, Conway & Rüber, 2021

The four described species of Danionella are tiny, transparent fishes that mature at sizes between 10–15 mm, and represent some of the most extreme cases of vertebrate progenesis known to date. The miniature adult size and larval appearance of Danionella, combined with a diverse behavioral repertoire linked to sound production by males, have established Danionella as an important model for neurophysiological studies. The external similarity between the different species of Danionella has offered an important challenge to taxonomic identification using traditional external characters, leading to confusion over the identity of the model species. Using combined morphological and molecular taxonomic approaches, we show here that the most extensively studied species of Danionella is not D. translucida, but represents an undescribed species, Danionella cerebrum n. sp. that is externally almost identical to D. translucida, but differs trenchantly in several internal characters. Molecular analyses confirm the distinctiveness of D. cerebrum and D. translucida and suggest that the two species are not even sister taxa. Analysis of the evolution of sexual dimorphisms associated with the Weberian apparatus reveals significant increases in complexity from the simpler condition found in D. dracula, to most complex conditions in D. cerebrum, D. mirifica and D. translucida.

Danionella cerebrum. (a) male (ca. 10 mm SL) and (b) female (ca. 12 mm SL) in life, not preserved; note yellowish chromatophores dorsally on head, melanophores scattered in rows on body in both sexes, and eggs covered by large melanophores in female;

(c) MTD 39985, paratype, 10.4 mm SL, male and (d) BMNH 2021.8.30.1, holotype, 12.6 mm SL, female (below), white arrows mark position of vent, which is shifted anteriorly to the pelvic fins in males;

 (e) Weberian apparatus in male, MTD 39992, paratype, 11.7 mm SL and (f) female, MTD 39992, paratype, 11.8 mm SL, in lateral view; the same in male (g) and female (h) in frontal view; (e) and (g) black arrowhead marks connection between lateral process and outer arm of os suspensorium, star marks connecting flanges between inner and outer arms of os suspensorium and red arrow marks posterior extension of inner arm of os suspensorium covering swimbladder dorsally. Abbreviations: cl, claustrum; dc, drumming cartilage; ios, inner arm of os suspensorium; nc, neural complex; oos, outer arm of os suspensorium; r, rib; sc, scaphium; sw, swimbladder.
Above: Photo of a live female of the newly described species Danionella cerebrum.
Below: A cleared and stained female Danionella cerebrum illustrating its heavily reduced skeleton . Bones are stained red and cartilage is stained blue.
photos: Ralf Britz/Senckenberg Naturhistorische Sammlungen Dresden

Danionella cerebrum new species

Diagnosis: Danionella cerebrum is distinguished from D. translucida, D. dracula, and D. priapus by the number of anal-fin rays (15–18 vs. 12–15 in D. translucida, 12–14 in D. dracula, 20–21 in D. priapus, Table 1). It is further distinguished from D. mirifica, D. dracula, and D. priapus by fewer vertebrae (33–35 vs. 36–38, Table 1), from D. priapus and D. dracula by fewer pectoral-fin rays (6 vs. 8 in D. priapus and 7 in D. dracula, Table 1), from D. translucida and D. dracula by the presence of a ventromedially directed cartilage flange from the taenia marginalis anterior that approaches the trabecula communis (vs. absence, Fig. 2c,d), and from D. dracula by the presence in the male of bony flanges on the outer arm of the os suspensorium and a connection of these to the lateral process of vertebra 2 (vs. absence of flanges and of connection to second lateral process), the presence of a maxillo-mandibular cartilage (vs. absence), the absence of odontoid processes in the male (vs. presence), more anal-fin pterygiophores (14–17 vs. 11–13, Table 1), more principal caudal fin rays (9 + 9 vs. 8 + 8, Table 1) and fewer pelvic-fin rays (5 vs. 6, Table 1). Danionella cerebrum can be further distinguished from the similar syntopically living D. translucida, by the last dorsal-fin ray inserted opposite to the last anal-fin ray (vs. last dorsal-fin ray inserted posterior to last anal-fin ray, Fig. 2a,b), by the last anal-fin pterygiophore inserted in front of haemal spine of vertebra 22–24 (vs. 19–21), by the lateral process of the second vertebra blade-like (vs. axe shaped, Fig. 2g,h), and by the distal tip of the fused inner arms of the ossa suspensoria bifurcated (vs. single, Fig. 2e,f) and not reaching the middle of the anterior swimbladder (vs. curving around and reaching middle of anterior swimbladder, Fig. 2e,f).

Etymology: The species name cerebrum, Latin for brain, a noun in apposition, makes reference to the fact that this fish with one of the smallest adult brains among vertebrates has become a promising new model species for neurophysiological studies.

Distribution: Danionella cerebrum is known from a number of streams on the southern and eastern slopes of the Bago Yoma mountain range (Fig. 3) of Myanmar: Thandabin Chaung and Bala Chaung in Yangon Division, and from Daikme Chaung (type locality of Danionella translucida) and an unnamed stream northwest of Daikme Chaung in Bago Division.

Raxml tree for the coxI gene of the five species of Danionella (upper left) from different sampling locations (data set 1) and map (upper right) showing type localities (large circles) and locations of additional samples (small circles). Note that both species, D. cerebrum and D. translucida, co-occur at each other’s type locality. Roadside canal at Hmawbi (lower left), type locality of D. cerebrum, and Daikme Chaung (lower right), type locality of D. translucida, illustrating the typical turbid streams in which these two species occur. Map created with QGIS version 3.8.3-Zanzibar (

Ralf Britz, Kevin W. Conway and Lukas Rüber. 2021. The Emerging Vertebrate Model Species for Neurophysiological Studies is Danionella cerebrum, new species (Teleostei: Cyprinidae). Scientific Reports. 11: 18942. DOI: 10.1038/s41598-021-97600-0

The fish that provide a window into their brains: New fish species discovered in Myanmar