General characteristics of Danionella priapus:

  • Head and eye large, mouth supraterminal
  • Body laterally compressed, elongate
  • Dorsal fin short, situated opposite posterior half of long anal fin
  • Caudal fin furcate, with remnants of larval fin fold in front of its dorsal and ventral margins
  • Remnant of pre-anal larval fin fold present in females only
  • Anus and genital papilla of adult males located between pelvic fins, in females at normal position in front of anal fin 
  • Genital opening of adult males located at tip of short, conical projection at proximal end of funnel-like structure formed by pelvic fins being fused along their midline 
  • Pseudotypanum present in body musculature at lateral side of anterior gas-bladder chamber, rendering its pigmented surface visible 
  • Body muscles greatly thinned out at lateral side of posterior gas bladder chamber
  • Scales absent
  • Lateral-line canals and pores on head and body absent
Vertebrae and ray characteristics:
  • Total vertebrae: 37 or 38
  • Abdominal vertebrae: 15 or 16
  • Caudal vertebrae: 22 or 23
  • Ribs present on vertebrae 5 to 14
  • Dorsal-fin rays: ii,6,i or ii,7,i
  • First dorsal-fin ray pterygiophore inserted behind neural spine of vertebra 21 or 22
  • Last dorsal-fin ray pterygiophore inserted in front of neural spine of vertebra 25 or 26
  • Anal-fin rays: ii,17,i or ii,18,i
  • Number of anal-fin pterygiophores in front of first hemal spine: 0, 1 or 2
  • Last anal-fin pterygiphore inserted in front of neural spine of vertebra 27 or 28
  • Principal caudal-fin rays: 9+9
  • Dorsal procurrent rays: 7 or 8
  • Ventral procurrent rays: 6, 7 or 8
  • Pectoral-fin rays: 8
  • Pelvic-fin rays: 5
Head of Danionella priapus

Dorsal and ventral view of the colour pattern of Danionella priapus head and anterior body. Holotype: BMNH 2009.9.9.1, male, 14.4mm SL. (Images: Ralf Britz, Natural History Museum)

Pigmentation-related characteristics:

Pigmentation in alcohol specimens restricted to several rows of melanophores:

  • mid-lateral row along horizontal septum from shoulder girdle to hypural plate
  • ventral row from in front of and slightly above anal-fin base along ventral larval fin fold to end of hypural plate
  • row along anal-fin base
  • mid-dorsal row starting behind head with several enlarged melanophores forming small blotch and extending usually halfway between head and anterior base of dorsal fin, rarely to anterior dorsal-fin base
  • paired dorsal paramedian rows from head to base of caudal fin
  • dorsal-fin base row
  • abdominal mid-ventral row from ventral tip of cleithrum to anus
  • several melanophores on otic capsule
  • melanophores capping dorsal and dorso-lateral face of gasbladder chambers and their connecting duct and lining peritoneum covering gonads
  • streaks of melanophores lining anterior edge of lower cleithrum, ventral edge of opercle and preopercle, branchiostegal rays and fin rays in dorsal, anal and caudal fins and anterior 2 or 3 rays of pectoral fin
  • several larger melanophores near symphysis of lower jaw

Head with:

  • supraoccipital blotch formed by several large contiguous melanophores
  • numerous large scattered melanophores between supraoccipital blotch and epiphyseal bar
  • a preepiphyseal blotch
  • a few prenasal melanophores
Dead specimens of Danionella priapus

Lateral view of Danionella priapus. Upper image: holotype (BMNH 2009.9.9.1), male, 14.4mm SL. Lower image: paratype (BMNH 2009.9.9.2-37), female, 15.0mm SL. Note presence of eggs and larval preanal fin fold in female, and difference in size and shape of pelvic fins and in position of anus and genital papilla between male and female. (Images: Ralf Britz, Natural History Museum)

In life, body colourless and largely translucent, except for:

  • melanophore patterns described above
  • a thin yellowish-greenish line running along body at level of neural tube
  • golden chromatophores forming ring around pupil, capping swimbladder chambers and connecting duct
Live Danionella priapus

Colouration of live Danionella priapus male (top) and female (bottom). Note eggs in body cavity of female. (Images: Ralf Britz, Natural History Museum)

  • Head of Danionella priapus
    Diagnostic description

    Live individuals of D. priapus may be easily confused with the other species of the genus due to their miniature size and lack of easily visible external characters. Find out the diagnostic characters that enable an accurate identification to be made.

  • Male of Danionella priapus
    Morphology of Danionella priapus

    Learn about the morphology of Danionella priapus.

  • Dead specimens of Danionella priapus

    Danionella priapus exemplifies the link between the evolution of morphological novelties and progenetic paedomorphosis via heterochronic change in developmental timing. Learn more about the evolution of this fish species.