The paper presents the first detailed study on systematics, otolith structure and biology of Diaphus coeruleus, from the south-eastern Arabian Sea. The closely related and co-inhabiting species D. coeruleus and D. watasei are distinguished on the basis of morphometric, meristic and otolith characters. Analysis of gut content revealed that these fishes mainly feed on deep sea squids and shrimps. Six maturation stages of gonads are identified in males (immature, quiescent, mature, ripe, spawning and spent) and five stages (immature, quiescent, mature, ripening and spawning stages) in females. Overall sex ratio deviated in favour of males (1 male: 0.2 female) and the mean fecundity was 2150 ± 353 eggs. The length-weight relationship shows an overall isometric growth in males with N = 76 and b = 3.13 and for females with N = 17 and suggests negative allometric growth (b = 1.719). Otolith length (LO) and otolith weight (WO) are positively correlated to standard body length (LS) and body weight (BW).

Records of long-finned gurnard Chelidonichthys obscurus in the western English Channel are rare. Prior to this study, reported incidences were largely restricted to the 19th Century and it appears to have been unrecorded in this well-studied ecosystem over much of the 20th Century. Data from a contemporary trawl survey in and around Lyme Bay (1989–2013) indicate that this species (N = 58) was only present during the period 2005–2013. These data and records from other surveys (N = 24) indicate a localized population may persist in an area just south of Start Point (64–77 m water depth), although it can occasionally occur in shallower inshore waters. A sub-sample of fish was collected to provide additional information on their morphometrics, meristics and diet. Meristic counts were generally in accord with earlier studies as were observations on their diet, which showed that they fed almost exclusively on crustaceans, especially mysids.