Abstract
The reproductive and developmental characteristics of the nereidid polychaete, Neanthes glandicincta Southern, 1921, commonly recorded in tropical estuaries in the Indo-West Pacific, were examined from Malaysia (the mangrove area of Kuala Ibai, Terengganu) and Thailand (the Lower Songkhla Lagoon) on the east coast of the Malay Peninsula. Epitokous metamorphosis of fully mature males and females and their reproductive swimming behaviour were recorded based on ten Malaysian epitokous specimens, which were collected at night during spring tides in a period of January 2018 to March 2019. Six Thailand epitokes were obtained in February and March 2006 by the laboratory rearing of immature worms. Epitokous metamorphosis is characterised by the enlargement of eyes in both sexes, division of the body into three parts and modification of parapodia with additional lobes in the mid-body of males, and replacement of atokous chaetae in the mid-body by epitokous natatory chaetae, completely in males and incompletely in females. The diameter of coelomic unfertilised eggs in females was 100–140 µm. After fertilisation, each egg formed a jelly layer, inside which embryonic development progressed. Trochophores hatched out of the jelly layer, entering a short free-swimming larval phase followed by demersal life at the early stage of 3-chaetiger nectochaeta one day after fertilisation. Then, the larvae entered benthic life as juveniles, crawling on the bottom, at the late stage of 3-chaetiger nectochaeta two days after fertilisation. The results indicate that N. glandicincta has an annual life cycle, which is usually completed within an estuary with limited larval dispersal ability.
Highlights
Wu et al (1985) described the epitokous metamorphosis of this species as follows: eyes were enlarged, the body was divided into two parts, and the epitokous modification of parapodia occurred in the posterior body in a male, whereas eyes were enlarged, the body was divided into three parts, the inconspicuous epitokous modification of parapodia occurred in the middle body, and a cleft was present on the anterior margin of the prostomium in a female
The results revealed that the typical epitokous metamorphosis to heteronereid form occurred in only males, including the enlargement of eyes, marked modification of parapodia, and complete substitution of atokous chaetae by natatory paddle-like chaetae in the natatory region
Only a partial epitokous metamorphosis occurred in mature females, including the enlargement of eyes, and incomplete substitution of atokous chaetae by natatory paddle-like chaetae with no modification of parapodia in the middle body
Summary
The nereidid polychaete, Neanthes glandicincta Southern, 1921 (type locality near Calcutta, India) is commonly reported from Asian tropical estuaries in Indo-Western Pacific Oceans (Fauvel 1932, 1939, 1953; Wu 1967; Wu et al 1985; Lee and Glasby 2015; Ibrahim et al 2019). Lee and Glasby (2015) synonymised Ceratonereis burmen sis Monro, 1937 (type locality: Maungmagan, Burma, and off Bombay, India) with N. glandicincta, and described N. wilsonchani Lee & Glasby, 2015 from Singapore, which is similar to N. glandicincta but distinguishable by the number of paragnaths. Ibrahim et al (2019) established the Neanthes glandicincta species complex, which included these two species. Hsueh (2019) described N. kaomeiensis from Taiwan, which is similar to both N. glandicincta and N. wilsonchani but differs from them in the absence of a notopodial prechaetal lobe in posterior chaetigers, and it is regarded as the third species of the Neanthes glandicincta species complex. Lee and Glasby (2015), Sato (2017), and Ibrahim et al (2019) suggested that several more cryptic species belonging to this species complex may be distributed in the South China Sea and East China Sea coasts.Reproductive and developmental modes of nereidids are conspicuously variable even among morphologically similar congeneric species (Sato 1999, 2017), all nereidids are semelparous (breeding only once in a lifetime) (Olive 1983). Many nereidids show swarming behaviour (mass-swimming of sexually mature adults) to shed gametes freely into the water, accompanied by a common set of drastic morphological changes (known as epitokous metamorphosis) into epitokes of the ‘heteronereis form’ in both males and females or males only. These changes include marked enlargement of the eyes, division of the body into two or three parts, enlargement and/ or modification of the parapodial ligules and cirri with the addition of some lappets (most marked in the middle or posterior body), and replacement of atokous chaetae by paddle-like natatory chaetae (Clark 1961; Schroeder and Hermans 1975). These previous findings indicate that reproductive and developmental characteristics may be useful to distinguish morphologically similar but distinct species
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