Namalycastis sp.

Ibrahim, Nur Fazne, Ibrahim, Yusof Shuaib, Kan, Kotaro & Sato, Masanori, 2024, 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., Journal of Natural History 58 (45 - 48), pp. 2043-2074 : 2064-2068

Life-history characteristics of Namalycastis sp. inhabiting the decaying fronds of Nypa fruticans

Sex ratio and body size of females and males

Coelomic contents of a total of 507 individuals of Namalycastis sp. were examined. Sex could be determined for 209 individuals of them (43%), as 114 females with spherical oocytes (22%, Figure 3 (a)) and 99 males with sperm or clusters of spermatogonia or spermatocytes (21%, Figure 3 (b)), whereas the sex of 225 individuals without sexual products could not be determined (57%) ( Figure 15 ). The sex ratio (no. of females/no. of males) was 1.15. We could not find any hermaphrodite individuals with both female and male products.

BW of females (average ± standard deviation (SD), range: 2.97± 0.66 mm, 1.3–5.4 mm) and males (2.99± 0.71 mm, 1.5–5.2 mm) were not significantly different (Wilcoxon–Mann–Whitney test, p =0.83), and both of them were significantly larger than individuals with sex undetermined (1.94± 0.42 mm, 1.1–3.0 mm) (p <0.0001) ( Figure 15 ). BL of complete specimens (range: 20–340 mm) was positively correlated with BW according to the following regression formula ( Figure 16 ): BL=5.8 BW - 4.4 (r=0.80, p <0.05, n =277).

Seasonal variation of body-size distribution

Profiles of size–frequency histograms of seven months were relatively similar throughout the year from September 2015 to September 2016, with a mode ranging from 1.5 to 2.5 mm BW ( Figure 17 ). Females and males with coelomic sexual products occurred throughout the year, whereas juveniles with BW less than 1.3 mm occurred only in the period from February to July 2016, with the number of juveniles less than 10% of each population ( Figure 18 ).

Average BWs of both males and females were largest in November 2015 (3.1 mm and 3.3 mm, respectively), and smallest in May 2016 (2.4 mm and 2.7 mm, respectively) ( Figure 19 ). Average BW of individuals with sex unknown was largest in September 2015 (2.4 mm), and smallest in March 2016 (1.8 mm).

Seasonality of reproduction

Ovigerous females occurred throughout the year, with ovigerous rate (number of ovigerous females/number of adults) relatively high (20–28%) in September and November in 2015 and from May to September in 2016, and low (8–9%) in February and March in 2016 ( Figure 20 ).

The mean oocyte diameter ( OD) in ovigerous females was highest in November 2015 (83µm) and September 2016 (87µm), and lowest in February 2016 (35µm), increasing from March to September in 2016 in a similar pattern to the ovigerous rate ( Figure 20 ) . The largest value of OD (140µm) was recorded from a female collected from Sta. 1 in November 2015. Ovigerous females containing large oocytes with a mean diameter larger than 100µm occurred only in November 2015, and July and September 2016 ( Figure 21 ). The mean OD (µm) was positively correlated with the body width of the ovigerous females (BW mm) according to the following regression formula ( Figure 22 ): OD=22.1BW+9.5 (r=0.69, p <0.01, n =110).

Only a single male containing mature sperm was found, in November 2015.

Fecundity

The numbers of coelomic oocytes per 10 chaetigers in ovigerous females (NO) were in a range from 380 to 21,000, and positively correlated with the body width of the ovigerous females (BW mm) according to the following regression formula ( Figure 23 ): NO=349 e0.85BW (r=0.69, p <0.01, n =110).

Seasonal variation in salinity and temperature in the habitat

The mean salinity of the surface water in the six sampling sites was highly variable during our sampling period from September 2015 to September 2016, ranging from 7 psu (range: 3.5–8.4 psu) in November 2015 to 32 psu (27.5–33.9 psu) in May 2016, whereas the mean temperature was almost consistent in the same period, ranging from 28°C (26.4–29.4°C) in November 2015 to 31°C (30.1–33.2°C) in September 2015 ( Figure 24 (a)).

During our sampling period, the highest monthly rainfall amount (514 mm) was recorded in November 2015 in the monsoon season, and the lowest monthly rainfall amount (21 mm) was recorded in March 2016 in the dry season, according to the data from Climate Hazards Group InfraRed Precipitation with Station Data ( CHIRPS) ( Figure 24 (b)) .

Figures

Figure 3. Coelomic cells in gametogenesis of Namalycastis sp. (a) Oocyte of a female individual collected from Station 6 on 27 November 2015. (b) Round cluster of spermatogonia or spermatocytes of a male individual collected from Station 4 on 28 September 2016. Scale bars: 0.05 mm.

Figure 15. Size–frequency histograms of females (a), males (b), and individuals with sex undetermined (c) of Namalycastis sp. based on pooled data from all seven collections from September 2015 to September 2016 at six sites in Setiu Wetlands, Terengganu, in Malaysia (Table 1). n: total number of samples.

Figure 16. Relationship between body width and body length of 277 complete specimens of Namalycastis sp. collected from September 2015 to September 2016 at six sites in Setiu Wetlands, Terengganu in Malaysia (Table 1).

Figure 17. Seasonal change in size–frequency histogram of pooled samples of Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu, in Malaysia from seven samplings in the period from September 2015 to September 2016.

Figure 18. Seasonal change in ratio of females, males, adults with sex undetermined and juveniles in pooled samples of Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu, in Malaysia via seven samplings in the period from September 2015 to September 2016. The number at the top of each bar is the number of specimens examined.

Figure 19. Seasonal change in mean body width of females, males, and individuals with sex unknown in pooled samples of Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu, in Malaysia via seven samplings in the period from September 2015 to September 2016.

Figure 20. Seasonal change in ovigerous rate (%, open circle and solid line) and mean diameter of coelomic oocytes (μm, solid circle with SD bar, and dotted line) in females of Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu, in Malaysia during seven samplings in the period from September 2015 to September 2016.

Figure 21. Seasonal change in size–frequency histogram of mean diameters of coelomic oocytes of Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu in Malaysia via seven samplings in the period from September 2015 to September 2016. n: number of females examined.

Figure 22. Relationship between the body width of ovigerous females and the mean oocyte diameter in Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu, in Malaysia via seven samplings in the period from September 2015 to September 2016.

Figure 23. Relationship between the body width of ovigerous females and the number of oocytes per 10 chaetigers in Namalycastis sp. collected from six sites in Setiu Wetlands, Terengganu, in Malaysia via seven samplings in the period from September 2015 to September 2016.

Figure 24. Seasonal change in average salinity and temperature at six sites in Setiu Wetlands, Terengganu, in Malaysia (a), rainfall amount recorded at Bukit Berangan, Setiu, based on data from CHIRPS (Funk et al. 2019) (b), during the sampling period from September 2015 to September 2016.