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 Author Chan, Ho-Yeung Institute of Marine Biology and Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, Taiwan Author Lee, Chen-Lu Institute of Marine Biology and Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, Taiwan & Biology Department, National Museum of Natural Science, Taichung, Taiwan Author Chen, Wei-Cheng Institute of Marine Biology and Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, Taiwan Author Chang, Chia-Hao 0000-0001-8566-3361 Department of Science Education, National Taipei University of Education, Taipei, Taiwan Author Shao, Yi-Ta Institute of Marine Biology and Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, Taiwan Author Pang, Ka-Lai Institute of Marine Biology and Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, Taiwan text ZooKeys 2026 2026-05-11 1279 269 284 journal article 315157 10.3897/zookeys.1279.184298 9ca93a43-332f-41d1-a7b7-2761391261a8 4155903B-128C-4525-A049-59C0D3C4BCB0 Thecacera sesama Chan & Lee sp. nov. Figs 1 , 2 Type material. Holotype . • ASIZM 0001725 , 2.02 mm preserved 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 m depth , on bryozoan on the reef , collected by Ho-Yeung Chan . Paratypes . • ASIZM 0001721 to ASIZM 0001724 and ASIZM 0001726 , 0.9–2.95 mm length, collected 04 May 2021 , 02 Aug 2021 , 09 Sep 2021 , 01 Jun 2025 , and 14 Jun 2025 , 82.5, 19–30 m depth , 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 Thecacera that 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 ). Living specimens of Thecacera sesama sp. nov. A . Ecological photos; B . ASIZM 0001722 ; C . ASIZM 0001721 . Scale bars: 1 mm. 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 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 types of 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 specimens and 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 sesama is 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 Thecacera species were observed to feed on only two species of bryozoan, and these two bryozoans have significantly different appearances. The COI gene of the prey of T. sesama was sequenced and had low sequence similarities with the top sequence matches in the NCBI database, including Watersipora edmondsoni Soule & Soule, 1968 (82.31 %) and Bugula neritina Linnaeus, 1758 (81.98 %) (Table 3 ). After a morphological comparison between the prey of T. sesama and 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 Thecacera sp. 3 ( Nakano 2018 ), but it was not identified. The species exhibits typical Thecacera feeding 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 ). Living specimens of bryozoan with Thecacera species. A . T. sesama sp. nov. feeding on a bryozoan; B . Bryozoan, ASIZ 0100005 ; C . T. pacifica feeding on a bryozoan; D . T. picta feeding on another bryozoan. Scale bar: 1 mm ( B ). 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 flabellata and Bugula neritina Thompson and Brown 1984 : 71; Mohamed Hatha 2017
T. picta Bryozoan This study
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 COI and 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 specimens were examined to ensure the preservation of the remaining specimens for long-term storage in museums. The COI sequences (658 bp) showed 88.27 % similarity to Thecacera picta (GenBank accession no. KP 871652 ), the closest match in GenBank databases. Intraspecific variations in COI sequences among the three specimens are minimal (0.2–0.4 %), while their interspecific divergence with T. picta is 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 COI sequences confirmed the taxonomic placement of Thecacera sesama sp. nov. within the genus Thecacera . The maximum-likelihood tree showed T. sesama forming a well-supported monophyletic clade (100 / 1.00, maximum-likelihood bootstrap ( BS ) / Bayesian posterior probability ( PP ) that is sister to T. picta with 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 COI tree, with T. sesama specimens forming a strongly supported clade within Thecacera (78 / 1.00) (Fig. 5 ). 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 clavigera was used as the outgroup. 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 clavigera was used as the outgroup.