Tripedalia maipoensis: A new species of Box Jellyfish from Hong Kong.

Box Jellyfish, Cubozoa, are a small group of Medusozoans noted for their approximately cube-shaped bells and the extreme toxicity of their venom. The group is divided into two monophyletic orders, the Carybdeida which have a single tentacle emerging from each pedalium (tentacle stalk) and Chirodropida, which have multiple tentacles emerging from each pedalium. The Tripedaliidae are a small group of Carybdeid Box Jellyfish, divided into two genera, the Copula, which have a single pedalium at each corner of their box, and the Tripedalia, which posses two or three. The genius Tripedalia is currently divided into two species, Tripedalia binata, with two pedalia at each bell corner, and Tripedalia cystophora, with three. Both species are found in estuarine Mangroves, though Tripedalia cystophora apparently has a wide distribution, having been reported from the United States, Mexico, Singapore, India, and Japan, while Tripedalia binata has only been reported from Australia and Thailand.

Box Jellyfish are common in the coastal waters of China, but have not been extensively studied, with only three species of Carybdeids reported to date.

In a paper published in the journal Zoological Studies on 20 March 2023, Yanan Sun of the Department of Biology at Hong Kong Baptist University, Justin Hon Yin Tsui and Rachel Ting Huen Wong of the Faculty of Biology, Medicine and Health at the University of Manchester, Ringo Nga Ching Cheung and Murphy Kam Pui Ng of the Ocean Park Corporation, Carmen Or of  WWF-Hong Kong, and Jian-Wen Qiu, also of the Department of Biology at Hong Kong Baptist University, describe a new species of Tripedalia from intertidal Shrimp ponds within the Mangroves of the Mai Po Nature Reserve in Hong Kong.

The new species is given the name Tripedalia maipoensis, where 'maipoensis' means 'from Mai Po', in reference to the Mai Po Nature Reserve. This species has three pedalia per bell corner, with each pedalium having one tentacle, and a velarium  (flap of tissue at the base of the bell) with forked canals.

Morphological characteristics and phylogenetic position of Tripedalia maipoensis (A)–(B) holotype (TMBC030991), female, living specimen, lateral view showing the gross morphology (A), and oblique top view showing gastric phacellus (B); (C) Rhopalial niche, preserved specimen, paratype (TMBC030992); (D) Rhopalium, showing six eyes and a statolith; (E) Upside down view of the bell showing the velarial canals, preserved specimen, paratype (TMBC030992), male; (F) details of the velarial canals, preserved specimen, holotype (TMBC030991), female; (G) oblique top view, showing nematocyst warts and gastric phacellus, living specimen, paratype TMBC030992; (H) manubrium, preserved, paratype (TMBC030997); (I) Nematocyst batteries from tentacle, paratype (TMBC030997). Scale bars: (A), (B) 1 cm; (C), (F), (H) 1 mm; (D) 0.3 mm; (E), (G) 5 mm; (I) 10 µm. Abbreviations: cs, crystalline statolith; g, gonad; gp, gastric phacellae; le, lower lens eye; nw, nematocyst wart; pe, pit eye; rn, rhopalial niche; se, slit eye; ue, upper lens eye; vc, velarial canal. Sun et al. (2023).

The bell of Tripedalia maipoensis is transparent, and roughly cube-shaped, with rounded corners and a slightly arched roof. This has a scattering of nematocysts (stinging cells) of different sizes, Two gonads are present at each of the lower corners of the bell, and halfway between the corners is a ropalial niche, covered by two flaps, from which can emerge a rhopalium (sensory structure), bearing six eyes (two lateral pit eyes, two lateral slit eyes, a smaller upper lens eye, and a larger lower lens eye) as well as a large crystalline statolith (weighted organ used to tell up from down). The adults reach about 12.5 mm in bell height.

Tripedalia maipoensis has only been found within the Mai Po Nature Reserve on Hong Kong, though Sun et al. suspect it is highly likely to be found in other areas of the Pearl River Estuary, to which the ponds in which was found are connected. It was found in water channels flanked by Common Reed, Phragmites australis, or Mangroves of mainly Kandelia obovata mixed with the Golden Leather Fern, Acrostichum aureum. It has been observed between April and June every year since it was first noticed. 

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