Results

Microstructure of setae on the furca

Microstructures of the setae on the furca were observed in the interstitial Neonesidea alyamanaensis (Figs. 2 and 3) and epiphytic N. sp. (Fig. 4) in the examined specimens. No significant differences were observed between the two species. The seven setae on the furca were categorised into three types. Setae R/L1, R/L2, and R/L3 had one or two serrated rows. The serrations were directed backward from the body. Setae R/L4 and R/L5 were entirely covered by short setulae. Setae R/L6 and R/L7 had openings on their tips. In N. sp., the setae had fine setulae in the middle.

Comparison of setae length in three interstitial and one epiphytic species

The measured setae values, averages, and standard deviations for males and females of each species are listed in Tables 1‒4. These results are summarised in Fig. 5.

Among all species, R/L2 was the longest, followed by R/L1. The order of R/L3‒R/L5 depended on the species. The R/L6 and R/L7 ratios were extremely low. No notable differences were observed between males and females of each species. For each species, a consistent trend was observed in the length ratio of each seta. In Neonesidea arenalocus, R/L2 was relatively short (approximately 1.4 times of L1) compared to other species. R/L3 and R/L5 were also short (approximately 0.6), and the difference in length was particularly small in males. In Neonesidea alwakasaensisi, R/L6 and R/L7 were relatively short (approximately 0.1) compared with the other species, and R/L5 was relatively short (less than 0.6) among R/L3‒R/L5. In Neonesidea alyamanai, R/L6 and R/L7 were relatively long (approximately 0.2) compared with the other species, and R/L4 was the longest in R/L3 and R/L5. In Neonesidea sp., R/L2 was relatively long (> 2.0) compared to the other species, followed by R/L3‒R/L5. However, no notable differences were observed between the interstitial and epiphytic species examined here, except for the absolute seta length, which depends on body size.

Ethological observation

Behavioural observations revealed that the furca moved back and forth within the carapace from the posteromedian to the posterodorsal direction. The tips of the setae extended slightly beyond the outer margin of the carapace; however, the tips did not touch the substrate, and the longest R/L2 was not involved in locomotion.