Thecacera sesama sp. nov. (Nudibranchia, Polyceridae) from Taiwan, evident from morphology and phylogenetic analyses of the 16 S rDNA and cytochrome c oxidase I gene Chan, Ho-Yeung Lee, Chen-Lu Chen, Wei-Cheng Chang, Chia-Hao Shao, Yi-Ta Pang, Ka-Lai ZooKeys 2026 2026-05-11 1279 269 284 Chan & Lee Chan & Lee 2026 Gastropoda Polyceridae Thecacera Animalia Nudibranchia Mollusca species sesama sp. nov.   Figs 1, 2   Type material.    Holotype. • ASIZM 0001725, 2.02 mmpreserved length, collected  14 June 2025,  82.5 km, off Northern Coastal Highway, Ruifang District, New Taipei City, Taiwan(  25°12.09'N, 121°90.02'E, WGS 84),  23 mdepth, on bryozoan on the reef, collected by Ho-Yeung Chan.   Paratypes. • ASIZM 0001721to ASIZM 0001724and ASIZM 0001726, 0.9–2.95 mmlength, collected  04 May 2021, 02 Aug 2021, 09 Sep 2021, 01 Jun 2025, and 14 Jun 2025, 82.5,  19–30 mdepth, bryozoan on the reef, collected by Ho-Yeung Chan(Table 1).   Diagnosis.   Thecacera sesama sp. nov.is distinguished by a unique combination of the following external morphological characters: (1) maximum preserved length of specimens is 2.83 mm; (2) body colour is translucent whitish, allowing some internal organs to be faintly visible; (3) entire body, as well as the rhinophores, rhinophoral sheaths, gills, post-branchial appendages, propodial tentacles, and tail are covered with numerous, discrete, small, circular, black spots and large yellow spots, as well as many white, snowflake-shaped pigment patches scattered on the body; (4) rhinophores and rhinophoral sheaths are translucent whitish, with small black spots and large yellow spots; (5) rhinophoral lamellae number 9–12; (6) gills number 5 and are translucent whitish, and the branchial plumes are pinnate; (7) post-branchial appendages are translucent whitish; (8) the head is translucent whitish, with short, blunt propodial tentacles at the corners.   Description  (Figs 1, 2). Body and size. This is a small species of  Thecacerathat has the body covered with a snowflake-like pattern of pigments. Body length 0.9–2.95 mm( n= 6, mean = 2.34 mm), width 0.63–1.11 mm( n= 3, mean = 0.94 mm) and height between 0.47–1.89 mm( n= 5, mean = 1.27 mm) (Table 2).   Figure 1. Living specimens of  Thecacera sesama sp. nov. A. Ecological photos; B. ASIZM 0001722; C. ASIZM 0001721. Scale bars: 1 mm.   Figure 2. Details of appearance and morphological features, hand-drawn on a tablet PC by C-LL.  Thecacera sesama sp. nov.Abbreviations: R rhinophores; RS rhinophoral sheaths; EA extra-rhinophoral appendages; PT propodial tentacles; G gills; PBA post-branchial appendages; T tail. Scale bar: 1 mm   Table 2. Morphological data for the measured specimens in millimetres.     Voucher Body length Body width Body height Black spot Yellow spot   ASIZM 0001721 2.83 1.08 No record 0.05 0.07   ASIZM 0001722 2.60 1.11 1.80 0.05 0.11   ASIZM 0001723 2.77 No record 1.89 0.10 0.15   ASIZM 0001724 2.02 No record 1.24 0.07 0.09   ASIZM 0001725* 0.90 No record 0.47 0.03 0.07   ASIZM 0001726 2.95 0.63 0.98 0.05 0.13  Mean 2.34 0.94 1.27 0.06 0.10  * Holotype. All other specimens are paratypes.  Colouration. The ground colour of all entire specimens is uniformly translucent white. This translucency allows the pale yellowish-white visceral mass to be visible through the dorsal integument.  Spots. The most striking characteristic of this species is the dense coverage of discrete, circular, small, black spots between 0.03 and 0.10 mm( n= 6 and mean = 0.06 mm) in diameter and larger yellow spots between 0.07 and 0.15 mm( n= 6 and mean = 0.10 mm) in diameter. These two typesof spots are evenly distributed over the entire body. The spots do not coalesce (Table 2).  Head and propodial tentacles. The head is not clearly demarcated from the main body. At each anterolateral corner of the veil is a short, blunt, digitiform propodial tentacle.  Rhinophores. They emerge from cylindrical sheaths with smooth, slightly flared rims. The sheaths share the same translucent white ground colour and dense, small black and large yellow spotting as the body. The club bears 9–12 lamellae ( n= 6 specimensand mean lamellae = 10). The club is translucent white.  Extra-rhinophoral appendages. The inner surfaces of the extra-rhinophoral appendages processes are fimbriate.  Gills. The branchial apparatus is situated dorsally in the posterior half of the body surrounding the anterior and lateral sides of the anal papilla. The five gills are entirely translucent white, although some specimens present a few small black spots and larger yellow spots on the outer base of the gill rachis.  Post-branchial appendages. A single, large, unbranched digitiform appendage is located on each side of the dorsum, positioned posterolaterally to the gill arch. These appendages are prominent, often held erect or angled dorsolaterally. They are structurally simple, sharing the body’s ground colour and dense pattern of small black spots and big yellow spots. The distal tip of some specimens is marked with conspicuous, large black spots.  Foot. The foot is translucent white. It is approximately the same width as the body, with a posterior that tapers to a blunt point, extending slightly beyond the dorsum.   Etymology. The specific epithet sesamais derived from the Latin word for sesame seed, referring to the characteristic small, rounded, seed-like spots that cover the dorsal surface of this species, resembling scattered sesame seeds on the animal’s body.   Ecology and behavior.   Thecacera sesama sp. nov.is a specialised predator of bryozoans, specifically observed feeding on one species of bryozoan (Fig. 3). Based on eight years of diving experience at the collection site from 2017–2025,  T. sesama,  T. pacifica,  T. picta, and five undescribed  Thecaceraspecies were observed to feed on only two species of bryozoan, and these two bryozoans have significantly different appearances. The COIgene of the prey of  T. sesamawas sequenced and had low sequence similarities with the top sequence matches in the NCBIdatabase, including  Watersipora edmondsoniSoule & Soule, 1968(82.31 %) and  Bugula neritinaLinnaeus, 1758(81.98 %) (Table 3). After a morphological comparison between the prey of  T. sesamaand  W. edmondsoni/  B. neritina, significant morphological differences were observed between these species, and so the prey is referred as a bryozoan in this paper. The same bryozoan was only seen in the illustration of  Thecacerasp. 3( Nakano 2018), but it was not identified. The species exhibits typical  Thecacerafeeding behaviour, rasping the bryozoan colonies. Specimens are typically found in close association with their bryozoan prey, often positioned on or immediately adjacent to active colonies (Fig. 3, Table 4).   Figure 3. Living specimens of bryozoan with  Thecaceraspecies. A.  T. sesama sp. nov.feeding on a bryozoan; B. Bryozoan, ASIZ 0100005; C.  T. pacificafeeding on a bryozoan; D.  T. pictafeeding on another bryozoan. Scale bar: 1 mm ( B).   Table 3. Comparison of feeding habits among valid species of the genus  Thecacera.     Species Feeding habits Reference    T. sesama sp. nov. Bryozoan This study    T. darwini Beania magellanica  Marcus 1959: 59; Fischer et al. 2005    T. pacifica Bryozoan This study    T. pennigera  Bugula flabellataand  Bugula neritina  Thompson and Brown 1984: 71; Mohamed Hatha 2017    T. picta Bryozoan This study   Table 4. Comparative morphology between similar species of  Thecacera. a( Fischer et al. 2005); b( Nimbs et al. 2016); c( Baba 1960); d( Edmunds 2010); e( Jung et al. 2013); f( Yonow 1994); g( Kumar et al. 2019); h( Gosliner et al. 2008.).     -   Thecacera sesama sp. nov.   Thecacera darwini   Thecacera pacifica   Thecacera pennigera   Thecacera picta(striped)   Thecacera picta(spotted)   Thecacera vittata  Maximum body length (mm) 2.83 45 a 30 b 25 c 20 20 8.5 f  Body colour Translucent white Translucent white a Bright orange g Translucent white c Translucent cream to grey Translucent cream to grey Translucent cream with black, orange, and white markings f  Spots Large yellow interspersed with smaller black circular spots Black circular spots None Large orange interspersed with smaller black circular spots Small amount, streaks, and continuous lines Numerous and discrete circular spots Black spots form distinct lines f  Rhinophores Translucent white with large yellow and small black spots Light yellow Orange with black on the top h Whitish c Black with orange at the tips Black with orange at the tips Black with orange at the tips  Rhinophoral sheaths Translucent white with large yellow and small black spots Dark yellow Black, blue, and white tips Whitish with big orange and small black spots Black and orange tips Black and orange tips Black and orange tips  Lamellae 9–12 15–18 a No record 4–10 d No record No record 15–19 f  Gills colour Translucent white with large yellow and small black spots Whitish rachis and apices yellow a Orange with black on the top Whitish c Translucent often with black line Translucent often with black line Translucent with black line  Post-branchial appendages Translucent white with large yellow and small black spots Yellow with white on the top Black, blue, and white tips h Numerous big orange and small black spots scattered e Black, orange, and white tips Black, orange, and white tips Black, orange, and white tips  Tail colour Translucent white with large yellow and small black spots White and yellow tips Black, blue, and white tips h Numerous big orange and small black spots scattered e Black and orange tips Black and orange tips Bright orange  Distribution Indo-Pacific Chile Widespread in Pacific Cosmopolitan Widespread in Pacific Widespread in Pacific Maldives   Molecular characteristics. DNA sequences were successfully obtained for both COIand 16 S rRNA genes from three paratype specimens. The extremely small size of the specimens ( 0.9–2.95 mm) means that the whole body was used for DNA extraction. Therefore, only three paratype specimenswere examined to ensure the preservation of the remaining specimens for long-term storage in museums. The COIsequences (658 bp) showed 88.27 % similarity to  Thecacera picta(GenBank accession no.  KP 871652), the closest match in GenBank databases. Intraspecific variations in COIsequences among the three specimensare minimal (0.2–0.4 %), while their interspecific divergence with  T. pictais substantial (14.17 %). The 16 S rDNA sequences (387 bp) of the new species had a 98.44 % similarity to  T. picta(GenBank accession  KP 871701), and have intraspecific variations of 0.3–0.6 %, and interspecific divergence of 6.72 % with  T. picta.  Molecular phylogenetic analysis. The phylogenetic analysis based on COIsequences confirmed the taxonomic placement of  Thecacera sesama sp. nov.within the genus  Thecacera. The maximum-likelihood tree showed  T. sesamaforming a well-supported monophyletic clade (100 / 1.00, maximum-likelihood bootstrap ( BS) / Bayesian posterior probability ( PP) that is sister to  T. pictawith moderate support (63 / 0.97) (Fig. 4). Analysis of 16 S rRNA sequences provides additional support for the species-level distinction of  T. sesama. The 16 S phylogeny shows a similar topology to the COItree, with  T. sesamaspecimens forming a strongly supported clade within  Thecacera(78 / 1.00) (Fig. 5).   Figure 4. Maximum-likelihood ( ML) phylogenetic tree inferred from mitochondrial cytochrome c oxidase subunit I ( COI) gene sequences showing the relationship of  Thecacera sesama sp. nov.and related species within the family Polyceridae. Bootstrap support values (left) and Bayesian posterior probabilities (right) are indicated at each node. The newly species is highlighted in bold. The scale bar represents 0.10 substitutions per site.  Bathydoris clavigerawas used as the outgroup.   Figure 5. Maximum-likelihood ( ML) phylogenetic tree based on mitochondrial 16 S rRNA gene sequences showing the relationship of  Thecacera sesama sp. nov.and related taxa within the family Polyceridae. Bootstrap support values (left) and Bayesian posterior probabilities (right) are indicated at each node. The new species is highlighted in bold. The scale bar represents 0.10 substitutions per site.  Bathydoris clavigerawas used as the outgroup. 6257857301 on bryozoan on the reef 2025-06-14 ASIZM Ho-Yeung Chan Taiwan 23 25.2015 82.5 km, off Northern Coastal Highway Ruifang District ASIZM 0001725 1 New Taipei City holotype 6257858301 bryozoan on the reef 2021-05-04 2025-06-14 2021-05-04 ASIZM Ho-Yeung Chan Taiwan 25 25.2015 82.5 km, off Northern Coastal Highway Ruifang District ASIZM 0001721, ASIZM 0001724, ASIZM 0001726 1 New Taipei paratype