Filling knowledge gaps on vulnerable electric ray Narcine timlei (Elasmobranchii: Torpediniformes: Narcinidae) through ecological and biological studies

Abstract

Narcine timlei (Bloch & Schneider, 1801), commonly known as the brown numbfish, recently moved from Data Deficient to Vulnerable in IUCN Red List. However, the biological information on the species is still lacking. In the present study, we provide an integrative assessment of populational dynamics on N. timlei. Morphological and molecular analyses ascertain the species identity as N. timlei. A total of 331 individuals were examined, comprising 156 males and 175 females, including 13 gravid, bringing an overall sex ratio of 1:1.21. We observed the highest specimens during the post-monsoon season (33.5%), and minimum during summer (18%). Length-weight relationships indicated negative allometric growth (b=2.78). Most specimens fall under the 17-20 cm size class, and a few exceeded 23 cm. Pregnant females carried 2 to 9 embryos. A total of 69 embryos from 13 females were obtained. Embryo length and weight were 5.18 {+/-} 0.22 cm and 2.02 {+/-} 0.36 g, respectively. No correlation was observed between maternal length and fecundity (R2 = 0.0036). Male to female embryo ratio was skewed to 1:1.3. The average length of electric organs in adults was 4.30 {+/-} 0.47 cm. Mean electrocyte count for adults was 117.55 {+/-} 14.48, while in embryos it was 123.36 {+/-} 15.9, indicating early developmental establishment of electrocyte structure in N. timlei. We strongly recommend long term species-specific monitoring and management measures to ensure conservation of this ecologically significant but often overlooked batoid.