Apostichopus japonicus Selenka, 1867

Tolon, Mustafa Tolga & Gökçek, Emel Özcan, 2025, First record of Japanese sea cucumber Apostichopus japonicus (Selenka, 1867) (Aspidochirotida: Stichopodidae) from the Mediterranean Sea, Turkey, Zootaxa 5661 (1), pp. 95-107 : 98-104

Apostichopus japonicus Selenka, 1867

Habitat

The sampling site is on the coast of Çeşme District which is located in the westernmost part of Turkey, extends into the Aegean Sea, and stands out for its tourism, fishing, and maritime trade. This region is also an important area where local commercial sea cucumber species like Holothuria tubulosa Gmelin, 1790 , Holothuria poli Delle Chiaje, 1824 , Holothuria mammata Grube, 1840 , and Holothuria sanctori Delle Chiaje, 1823 are abundantly distributed. The seabed of the sampling zone predominantly comprised a stony-rocky substrate, with occasional observations of seagrass patches.

Remote sensing data from observation satellites for 2024 were used to evaluate the abiotic characteristics of the area where the non-native A. japonicus species had adapted and been sampled. The results indicated that the average bottom seawater temperature was 18.26 ± 0.65 °C (17.34-19.27 °C), mean salinity was 39.36± 0.09 ppt (39.25-39.52 ppt), mean pH was 8.08 ± 0.03 (8.02-8.12) ( Escudier et al. 2020), and the diatom concentration was 10.34 ± 4.84 mg /l (4.08-18.79 mg /l) ( Feudale et al. 2022).

Morphology

Morphologically, the sampled species were characterised by an elongated cylindrical body ranging from 72.5 to 160.0 mm in length and 21.79 to 88.30 g in weight. The body surface is soft but roughened by numerous papillae and conical projections that give it a tuberculated appearance ( Fig 2a ). Ventral side flattened. The dorsal side is typically darker, exhibiting shades of brown, olive-green, or reddish hues, often patterned with spots or bands, whereas the ventral side is lighter ( Fig 2 b-c). Tube feet arranged in 5–6 rows in the central ambulacral area and 4–5 rows in both lateral ambulacral areas ( Fig 2d ). The mouth is located ventrally and surrounded by a ring of small papillae ( Fig 2e ), and the anus is terminally positioned with respiratory trees extending into the cloaca for gas exchange. Cuvierian tubules are not observed. Creamy white and orange gonadal tubules are attached to the dorsal mesentery ( Fig 2 f-g). A single Polian vesicle is present in various sizes. The body wall is thick and contains ossicles, primarily perforated plates, and rods.

The mean length and gutted body weight of the sampled population were 110.34 ± 22.18 mm and 46.16 ± 18.38 g (n = 58), respectively. Young specimens between 72.50 and 112.50 mm in length were most abundant in the population ( Figure 3 ). However, nearly 75% of the remaining specimens (n=12), over 130 mm in length, had coloured mature gonad tubules in both sexes.

The estimated parameters for the Length-Weight Relationship (LWR) using the given data were a = 0.0085, b = 1.821, and R² = 0.877. This indicated a strong fit of the model (R² = 87.7%), and the growth pattern was slightly negative allometric (b <3), meaning that the weight increased more slowly than the cube of the length ( Figure 4 ).

Ossicles

Reduced tables with reduced pillars were absent in the dorsal and ventral body walls of the specimens. The table disk is oval to circular, featuring smooth edges and 4–22 irregular holes ( Fig 5 a-b). Papillae contain various ossicles including multiperforated plates, reduced tables and rods. The reduced tables were elongated and displayed two longitudinal rows with four to five perforations ( Fig 5c ). The multiperforated and reduced papillae plates have 5-7 uneven central holes that ranged from smooth to uneven, but not spiny ( Fig 5d ).

Molecular Identification and Phylogenetic Tree

In this study, partial gene regions of 692 bp for the COI gene and 570 bp for the 16S rRNA gene were amplified and sequenced from six randomly sampled sea cucumber individuals. Two haplotypes for the COI region and one haplotype for the 16S rRNA region were obtained, and the sequences were submitted to the NCBI GenBank database with accession numbers PQ873053, PQ873054, and PQ873886, respectively. BLAST analysis of the NCBI GenBank database showed that both COI haplotypes were more than 99% similar to the A. japonicus COI gene sequence with the accession number KP170616 which was sampled from Vladivostok, Russia. The 16S rRNA haplotype matched 100% with the A. japonicus 16S rRNA sequence from Oita, Japan (accession number GU557148 ).

The ML tree based on the T92 nucleotide substitution model for the mtCOI and 16S rRNA gene regions showed that more than 70% of the A. japonicus haplotypes sampled from the Aegean Sea and other A. japonicus samples from the Northeast Pacific Ocean belonged to the same clade. In contrast, A. parvimensis , Parastichopus regalis , Parathicopus californicus , and Holothuria poli samples showed separate divergence from A. japonicus samples ( Figure 6a, b ).

Figures

FIGURE 2. Morphological characteristics of Apostichopus japonicus Selenka, 1867 from the Mediterranean Sea. (a) Live specimen; (b) dorsal view; (c) ventral view; (d) tube feet in the central and lateral ambulacral areas; (e) ventrally located mouth and a ring of small papillae; (f) orange gonadal tubules of the female; (g) creamy gonadal tubules of the male attached to dorsal mesentery (Photos by M.T.Tolon).

FIGURE 3. Histogram of (a) total length (mm) distribution and (b) gutted body weight (g) for Apostichopus japonicus Selenka, 1867 sampled from the Mediterranean Sea.The distribution highlights the size structure of the population, with the most abundant size class observed at 72.50-112.50 mm and 20.00-40.00 g. The curved line represents the normal population distribution.

FIGURE 4. Length-Weight Relationships (LWR) of Apostichopus japonicus individuals along the east Aegean coast of the Mediterranean Sea (Izmir, Turkey). The cross points represent the observed data, and the line indicates the fitted curve derived from the equation, where a = 0.0085 and b = 1.821. The high coefficient of determination (R2 = 0.877) demonstrates a strong model fit, suggesting a slightly negative allometric growth pattern.

FIGURE 5. Ossicles from A. japonicus (Selenka 1867) body wall. (a) disk of reduced table ossicles from the dorsal body wall; (b) ossicles from the ventral body wall with smooth rims and 4-12 perforations; (c) elongated ossicle from the papillae tip; (d) reduced tables from the papillae.

FIGURE 6. (a) the ML tree was reconstructed based on the partialmtDNA COI sequences representing 2 haplotypes (PQ873053, PQ873054) of the present study, other A. japonicus sequences, P. nigripunctatus (NC_013432), P. californicus (NC_026727) and P. pervimensis (KX874373) sequences gathered from the NCBI GenBank. (b) The ML tree was reconstructed based on the partial 16S rDNA sequences representing one haplotype (PQ873886) of the present study and included the sequences of the species A. parvimensis (KX856750), Paratichopus regalis (KM048323), and H. poli (KJ408308) reference sequences obtained from NCBI GenBank.