Providing nursery habitats to a number of marine fish larvae that recruit after prolonged pelagic larval duration, has been identified as one of ecosystem services rendered by estuaries and protected inshore water bodies like mangroves, mudflats, swamps, and marshes. Larval fish congregation and survival are largely dependent on abiotic and biotic potential of such systems and many migrant marine fishes are adapted to them. However, occurrences of larval forms of tropical reef-associated vagrant species which are known for extensive range adaptations generate considerable academic interest. The present study provides the first report of ontogenic habitat utilization of yellow fin surgeon fish, Acanthurus xanthopterus Valenciennes 1835 in a tropical microtidal positive estuary, the Vembanad lake, South India. Surface plankton collections from the downstream part of the estuary revealed considerable proportions of acanthurid larvae in post monsoon (mean 354 ± 180 numbers/100 m3) and pre monsoon (mean 217 ± 120 numbers/100 m3) while they were absent in monsoon season. These acronurus larval forms were morphologically identified and sorted before being subjected for DNA barcoding. Mitochondrial DNA COI sequences developed from morphologically characterized acronurus larvae exhibited genetic congruency to sequence of A. xanthopterus which was evident from phylogram (bootstrap support of 100) and genetic distance data (intraspecific distance of 0%). The study indicates that Acronurus larvae of A. xanthopterus, after extensive cross-habitat dispersal, utilize the estuarine habitat to promote potential growth.

The epibenthic euryhaline hydromedusa Vallentinia gabriellae Vannucci Mendes, 1948 is an olindiid species native to tropical Atlantic waters. Here, we describe the cryptic introduction of this species in an estuary along the coast of Kerala, in southwestern India. This study records the existence of V. gabriellae outside of Atlantic waters for the first time and documents its geographical range expansion. Our identification was based on a combined morphological and DNA barcoding assessment using the COI and 28S markers. Although we noted some morphological differences between our specimens and those from their native range, our findings were conclusive. We suggest that phenotypic plasticity may result from differences such as prey availability between the native and introduced habitats. V. gabriellae medusae are epibenthic and cling to a variety of hard and soft substrates, including bivalves. Our specimens were associated with the invasive fouling mussel Mytella strigata, and we suggest that V. gabriellae could have been transported along with these mussels to the Kerala coast.