The feeding strategy, overlap and trophic interactions of four demersal catfishes inhabiting Cochin Estuary, Arius maculatus (n = 67), A. subrostratus (n = 63), A. arius (n = 21) and Mystus gulio (n = 69) were investigated through gut‐content analyses. The indices of relative importance revealed that the prey items in their stomachs were dominated by crustaceans except in A. arius where molluscs constituted the major prey item. The highest diet and niche breadth values (3.93, 0.36 respectively) were recorded in A. subrostratus, while the lowest was recorded in A. arius (2.64, 0.23 respectively). Pianka's overlap, calculated through null models constructed by Ecosim 7.0, revealed significant niche overlap between A. maculatus and A. subrostratus (O–0.91, P < 0.001), between A. maculatus and M. gulio (O–0.72, P < 0.05) and between A. subrostratus and M.gulio (O–0.64 P < 0.05). However, no overlap was recorded between A. arius and other species. The trophic niche breadth along with prey specific abundance confirmed that these fishes are highly specific feeders. The present results on trophic level status indicate that these catfishes are mid‐level carnivores that can be considered as moderate benthic mesopredators and are specialist feeders unlike most catfishes.

Genus Macrobrachium includes freshwater prawns which inhabit most diverse habitats ranging from low saline areas to inland hill streams and impounded water bodies. Being morphologically conserved, this genus has been exposed to severe disputes related to their taxonomy, systematics and phylogeny. Macrobrachium striatum and M. equidens represent two morphologically related congeneric species within this genus. Earlier, M. striatum was considered as a striped form of M. equidens. Though these species are now well-described morphologically and differentiated into two species, no molecular level investigation has been carried out in support of their speciation. We report a study on M. striatum and M. equidens with emphasis to their molecular data through mitochondrial markers (16S ribosomal RNA and cytochrome oxidase subunit I). Results obtained from developed molecular markers of the two species revealed considerable genetic differentiation between them. Phylogram generated using Minimum evolution and Neighbour joining analyses differentiated M. striatum and M. equidens as two independent species. Genetic distance data showed high interspecific divergence (ranging from 3.9% to 17.0% for 16S rRNA sequences and 13.8% to 21.0% for COI sequences) between M. striatum and M. equidens confirming the findings of phylogram. Hence, it could be delineated that M. striatum and M. equidens represent two distinct species within genus Macrobrachium with emphasis to their morphology and genetics.

Species specific identification of early larval stages of many decapod crustaceans sampled from plankton collections remains cumbersome owing to lack of distinguishable characteristics, where DNA based molecular methods provide accurate results without taxonomic ambiguities. In the present study, an attempt was made to detect temporal occurrence of early zoea of freshwater prawn Macrobrachium rosenbergii (de Man) using real-time PCR assays in polyhaline, mesohaline and oligohaline areas of a tropical positive estuary, the Vembanad lake (S. India). High caridean larval abundance could be recorded in polyhaline areas in all seasons while it could be recorded in monsoon season in mesohaline and oligohaline areas. 113 DNA isolations were successfully made from morphologically identified taxonomic units (MOTU) and SYBR Green based RT-PCR amplifications using designed primer for M. rosenbergii yielded positive detections in 38 samples (34%) representing all seasons in all three zones. Positive detections could be recorded in all months except May in mesohaline areas and differed significantly (F = 17.2 p < 0.01) with the same in polyhaline and oligohaline areas. The present results of molecular detection of M. rosenbergii larvae extend confirmation of its breeding ground in Vembanad lake where appropriate management strategies could be enforced for stock conservation of this species.