Author: Bharate, Padula, Apte & Shimpi, 2020

Order: Nudibranchia  Family: Facelinidae

Maximum Size: 25 mm

Sightings: Sunshine Coast

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Cratena poshitraensis Bharate, Padula, Apte & Shimpi, 2020

Cryptic species are a double-edged sword. On the one hand they create doubt and confusion but on the other they are an absolute delight when discovered.

Cratena poshitraensis is just such a cryptic species. It was described in 2020 along side its doppleganger Cratena pawarshindeorum from the western coast of India. At the same time both were also delineated from Cratena minor Padula et al, 2014 and especially Cratena peregrina (Gmelin, 1791) with which they had long been confused.

There are only a couple of external features that can be used to separate Cratena poshitraensis from Cratena pawarshindeorum and even from our Cratena sp. 07. Now these might be considered somewhat subjective features, precarious even, to use for identification purposes however, these are the features laid out by the authors and are the only way forward when relying upon photographic evidence. The photographic records therefore need to be taken with the utmost care, almost forensic in composition, in order to reveal the evidence of those defining characteristics.

The body is almost entirely translucent (except as mentioned below) and the ascini (gonads) are most usually discernable internally as cream or yellow, both dorsally and ventrally, depending on maturity. The head region too, is translucent, however anterior to the rhinophores are two bright orange patches separated medially by a distinct gap. The translucent region extends from in front of those orange patches onto the base of the oral tentacles, the remainder of which are covered in opaque white pigment. The rhinophores are smooth, translucent white basally, a watery-orange for the majority of their length and finishing with a translucent white tip. An eyespot is conspicuous laterally on each rhinophore base. The anterior foot corners are well-developed into propodial tentacles. The cerata are smooth, straight and translucent, the brown unbranched digestive gland readily visible internally. The brown turns maroon or dark purple subapically finishing with a tapering white tip. The cerata appear to graduate in size with the longest located medially and these are also of greater diameter although the internal digestive duct is not so. The anterior-most group is arranged as a narrow arch the rest are posterior to the inter-hepatic space and present as simple slanting rows up to seven in number, on both sides. The tail is long and covered in opaque white pigment dorsally. The foot is translucent and unmarked. The gonopore is located on the right side below the arch of the first group of cerata. It is difficult to discern, however there is a slight surface swelling. The anus exits via a raised papilla or swelling located on the right side just anterior to the second row of cerata and in close proximity thereto.

Notes on identification:
Our Cratena sp. 07 is separated from Cratena pawarshindeorum by the location of the anal papilla being posterior to the second row of cerata (anterior in C. pawarshindeorum) on the right side. Both have orange patches that merge medially, anterior to the rhinophores that are themselves dark orange in their midsection. Cratena sp. 07, is without doubt, anatomically different to Cratena pawarshindeorum Bharate et al, 2020 (as confirmed by the corresponding author of the paper and also Dave Behrens: “…the location of the anus is NOT VARIABLE and in fact is a KEY important variable in differentiating species.” [his emphasis]) (personal communications) and for which it has been often mistaken in the Mooloolah River.

Cratena poshitraensis is separated from our Cratena sp. 07 by having the anal papilla anterior to the second row of cerata on the right side, a distinctive gap between the orange patches and lighter coloured rhinophores.

Cratena poshitraensis is separated from Cratena pawarshindeorum by the presence of a distinctive gap between the orange patches and lighter coloured rhinophores. Both of theses species have the anal papilla anterior to the second row of cerata on the right side.

David A. Mullins – January 2022

References:
– Padula, V., Araújo, A. K., Matthews-Cascon, H. & Schrödl, M., (2014). Is the Mediterranean nudibranch Cratena peregrina (Gmelin, 1791) present on the Brazilian coast? Integrative species delimitation and description of Cratena minor n. sp. Journal of Molluscan Studies. 80: 575-584.

– Bharate, M., Padula, V., Apte, D. & Shimpi, G. G. (2020). Integrative description of two new Cratena species (Mollusca: Nudibranchia) from western India. Zootaxa 4729 (3): 359–370.