Hydrobiology

Varuna litterata is an estuarine crab species widely distributed across the Indo-Pacific region, commonly dwelling in brackish waters, mangrove forests, and tidal estuaries. In Bangladesh, while four Scylla species dominate the commercial crab market, the locally consumed V. litterata remains a biologically overlooked gem of the coastal waters. These crabs are frequently captured as a byproduct during shrimp fry collection from coastal estuaries. In this context, the current study investigates the reproductive biology, morphometric dynamics, and market potential of V. litterata collected from the Pasur River, a coastal mangrove forest-adjacent estuary of southern Bangladesh. A total of 75 individuals were collected from March to April 2023, comprising 35 males and 40 females, resulting in a sex ratio of 1:1.14 (♂:♀), with a predominance of females. A visual observation of ovary development revealed four distinct maturation stages, with Stage IV (fully mature) being the most prevalent (43%), indicating peak reproductive activity during the sampling period. Morphometric analysis revealed that the average carapace width (CW) was 31.2 ± 5.7 mm and 31.9 ± 5.8 mm and the mean carapace length (CL) was 29.3 ± 4.7 mm and 30.1 ± 4.9 mm in males and females, respectively. However, the mean body weight (BW) was 13.1 ± 4.3 g in males and 12.7 ± 3.8 g in female crabs. The dominant CW class ranges from 33 to 33.99 mm (males) and 28.99–29.99 mm (females), appear to be the most vulnerable to fishing pressure. BW-CW and CL-CW relationships demonstrated negative allometric growth, with high correlations in both sexes. Significant sexual dimorphism was observed, with males having larger cheliped dimensions, while females had broader abdomens, likely supporting reproductive functions that are essential to their conservation. The marketing of this crab remains largely informal, yet rising local demand and prices highlight its emerging commercial potential. Therefore, incorporation into aquaculture and coastal fishery development of this crab species could enhance food security, support livelihoods, and contribute to sustainable blue economy initiatives in Bangladesh.

Environmental DNA (eDNA) detection has emerged as a powerful, non-invasive tool for identifying aquatic organisms, particularly those that are rare, elusive, or invasive. Dreissena polymorpha (zebra mussel) is an invasive bivalve posing ecological and economic threats to North American freshwater systems. In April 2021, zebra mussels were discovered attached to a boat destined for Lake Sidney Lanier in North Georgia—a high-use recreational reservoir with no prior reports of infestation. To determine whether D. polymorpha had been introduced, we implemented a biomonitoring protocol leveraging eDNA collection and PCR-based detection. Sampling was conducted during summer 2022 across high-risk marina sites and potential habitats. Positive controls from the Tennessee River yielded expected results, while Lake Lanier samples showed no evidence of zebra mussel DNA. Our results validate using eDNA methodology for proactive biomonitoring and highlight the importance of molecular surveillance and community outreach to prevent the establishment of invasive species in vulnerable aquatic systems. This study demonstrates the utility of a scalable, replicable early detection framework that can be adopted by educational institutions, natural resource agencies, and community groups to mitigate the risk of biological invasions.

Conventional fish population sampling methods such as electrofishing and netting, pose risks to fish and are often restricted to small, shallow headwater streams—especially where endangered species may be present. Additionally, non-capture surveying (e.g., snorkelling) can disturb fish and make observation more challenging. This study evaluates the effectiveness and reproducibility of remote underwater video (RUV) surveys in a shallow (<0.5 m deep), freshwater stream. Additionally, fish response to disturbances (e.g., shadows, noise, surface disruptions) were characterized. Fish abundance was estimated by maxN (maximum number of individuals observed in a single frame) and used multiple cameras placed in the same habitat (pool). Findings indicated a high consistency in maxN when fish numbers were low (<5 individuals), with increasing variability at higher numbers (>15 individuals). This suggests that single camera setups can reliably detect minimum abundance. Fish responses to four disturbances (e.g., shadows, noise, surface disruptions, mink) were noted throughout. Typically, these responses were short-lived, with fish returning to pre-disturbance maxN values within minutes, with the most significant response to researcher-induced disturbance occurring immediately after RUV deployment. Overall, RUVs proved effective for passive, non-capture fish monitoring in shallow, sensitive habitats, producing replicable data with minimal impact caused by researcher disturbance. This technique can be added to our toolboxes for studying small-bodied fishes in challenging environments.