Nereidid polychaetes (Annelida) inhabiting the inside of decaying fronds of the mangrove palm Nypa fruticans in a tropical estuary in Malaysia, with special reference to the life history of the dominant species, Namalycastis sp

Abstract

ABSTRACT Since relatively dense assemblages of nereidid worms were discovered from a unique habitat within the decaying fronds of the mangrove palm Nypa fruticans, we conducted a one-year survey from September 2015 to September 2016 at six sites in Setiu Wetlands, Malaysia, to clarify the species composition of the nereidid assemblages and the life history of the dominant species. The majority of specimens collected from the plant body (507 out of a total 678 specimens) belonged to Namalycastis sp. with remaining specimens belonging to Namanereis sp. (n=168), Namalycastis rhodochorde (n=1), and Perinereis aibuhitensis (n=2). Specimens of P. aibuhitensis (n=2) and Dendronereis sp. (n=2) were also collected from sediments around the palm trees. The taxonomic accounts of two Namalycastis spp. and Perinereis aibuhitensis are provided here. The life history of the dominant species Namalycastis sp. is evaluated based on the population characteristics. This species is gonochoric, with the sex ratio close to 1:1. The ovigerous females were found throughout the year, with their body width (BW) ranging from 1.3 to 4.8 mm. The ovigerous rate (no. of ovigerous females/no. of worms larger than the minimum ovigerous female) was lowest in February and March 2016 (8–9%) and highest from July to November 2016 (23–29%), when the ovigerous females had the largest coelomic oocytes of 100–120µm in diameter. Probable epitokes with additional neuropodial sesquigomph spinigers occurred most abundantly in November 2015 (21% of adults), which was also the only month when a mature male with mature spermatozoa appeared. Few juveniles with BW of less than 1.3 mm occurred from February to July, whereas large adults with BW of more than 3.5 mm occurred from May to November. These results suggest semelparous reproduction with peak spawning in November, coinciding with low salinity during the north-east monsoon.