Author: Undescribed

Order: Nudibranchia  Family: Facelinidae

Maximum Size: 25 mm

Sightings: Sunshine Coast

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Cratena sp. 07 Undescribed

Similar in colouration to Cratena pawarshindeorum Bharate et al, 2020 – see Note on name, below.

The body is almost entirely translucent (except as mentioned below) and the yellow acini (gonads) are readily discernable internally giving an impression of a yellow dorsum from a distance. The acini are visible, both dorsally and ventrally, from the midway mark of the inter-hepatic space to the commencement of the tail. The head region too, is translucent however, anterior to the rhinophores are two bright orange patches the diffused edges meeting on the centreline. 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 propodial tentacles – the well-developed anterior foot corners, are also opaque white dorsally. The rhinophores are smooth, translucent white basally, a dark orange for the majority of their length with a subapical white band and a translucent tip. An eyespot is conspicuous laterally on each rhinophore base. 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.

Specimens were observed to to be feeding on hydroid polyps. Their cerata did not bristle aggressively as a reaction to stimuli.

Features that assisted in identification are discussed below.

Cratena sp. 07 was first discovered in the Mooloolah River on the Sunshine Coast of Queensland, Australia. There was initially some uncertainty as to which genus it should be placed. Some microscope images revealing particular features and their location helped to classify it.

The first ceratal group is in two rows forming a narrow arch, or upside down v-shape. The second and subsequent, posterior to the inter-hepatic space, are just simple slanting rows.

The anus exits via a raised papilla located on the right side posterior to the second row of cerata and in close proximity thereto. At the moment of taking the image, faecal matter was being ejected, confirming its function.

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. Confirmation of existence of the actual opening is only revealed when the animal moves with lateral flexions so that when the right side is being compressed it is squeezed open. Note that it is surrounded by an irregular speckling of red pigment.

In Edmunds, M., 1970: the redescription of Cratena lineata and his description of the Cratena simba (species nova), are identical for the above morphological features – the arrangement of cerata and the positions of both the anus and gonopore – provide strong evidence for placement of this example in the Cratena genus.

Additionally, in delineating this animal from Sakuraeolis, – Rudman, W.B., 1980 states: “In all species of Sakuraeolis the genital aperture is in the interhepatic space behind the first ceratal arch.” Review the images of Sakuraeolis nungunoides, on this website, some of which show the penis protruding from the genital aperture (or gonopore) serving to illustrate that location. Many of my Sakuraeolis images show the penis protruding.

Interestingly, Bernard Picton has kindly pointed out that the type species of Cratena – Cratena peregrina is very similar to this example in external presentation. (See OPK Opistobranquis website)

Note on name:

This species is similar in external colouration to a species recently described from the Maharashtra State (Uran, near Mumbai) on the west coast of India – Cratena pawarshindeorum Bharate et al, 2020.

It is entirely plausible that a species found off the coast of west India could turn up off the east Australia coast of Queensland. To simplify, the ballast water from Indian coal ships loading in central Queensland, although mandatory to be changed 200 nautical miles offshore, is only required to be 95% replaced (to keep the sediment retained). The fresh ballast water mixed with the residual is released closer inshore before coal loading.  Any released sea slug larvae in the ballast water could drift south in the prevailing current. Also, related species such as Cratena simba Edmunds, 1970 and Cratena lineata (Eliot, 1905) for example, have a wide Indo-Pacific distribution from East Africa to the western Pacific.

However, close examination of this species – Cratena sp. 07 – reveals a significant difference with its Indian doppelgänger. The anus, the location of which is a key and important feature in differentiating aeolid species, is situated posterior to the second group of cerata in Cratena sp. 07 rather than anterior to the second group of cerata as described for Cratena pawarshindeorum.

Tempting as it is to assign it an existing name, we cannot ignore the discrepancy and will list it as Cratena sp. 07 on this website until such time as future molecular sequencing decides the issue unequivocally.

David A. Mullins – May 2021

References:

– Edmunds, M. (1970). Opisthobranchiate Mollusca from Tanzania. II. Eolidacea (Cuthonidae, Piseinotecidae and Facelinidae). Proceedings of the Malacological Society of London 39: 15-57.

– Rudman, W. B. (1980). Aeolid opisthobranch molluscs (Glaucidae) from the Indian Ocean and the south-west Pacific. Zoological Journal of the Linnean Society, 68: 139-172.

– 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.

– Ballesteros, Manuel, Enric Madrenas, Miquel Pontes (2021) “Cratena peregrina” in OPK-Opistobranquis. Published: 17/05/2012. Accessed: 03/05/2021. Available at (https://opistobranquis.info/en/iNl4a)