COMMENTS & OBSERVATIONS

Another day of great diving conditions. The Wall, at Mudjimba Island was considered as a destination, however, being a weekend in the school holidays it had the potential to be too crowded, so Bornella Reef to the south east of the island was substituted – and so it proved. There was a swell running but its short period created little surge on the bottom. On reaching the substrate, and checking the anchor, the first sighting was of a colony of Okenia hallucigenia devouring their preferred food, the bryozoan Pleurotoichus clathratus.   Goniobranchus splendidus are still in large numbers but pipped in the count by the aeolid Pteraeolidia semperi. The Verconia simplex look-a-like, Verconia decussata, was sighted. A Goniobranchus with a smooth opaque white mantle, mantle margin of red with yellow inside of that and deep-red gills and rhinophores with white edging was recorded. Previously this unusual presentation has caused us to lump it in with Goniobranchus verrieri. However, a recent paper, Soong et al, 2022, has identified, through molecular sequencing, two colour morphs of Goniobranchus rubrocornutus (Rudman, 1985) and this presentation matches “Morphotype B” in that paper, being different in external presentation to the original 1985 description of the species.

[Soong, G. Y., Bonomo, L. J., Reimer, J. D. & Gosliner, T. M. (2021). Battle of the bands: systematics and phylogeny of the white Goniobranchus nudibranchs with marginal bands (Nudibranchia, Chromodorididae). ZooKeys 1083: 169–210.]

Taxonomic Diversity:
All 21 of the sighted species were nudibranchs, with 13 being dorids comprising 3 x phanerobranchs (non-retractile gill), 3 x porostome (suctorial feeders  of the Phyllidiidae family), 7 x cryptobranchs (retractable gill – all belonging to the Chromodorididae family) and 8 x cladobranchs comprising 4 x aeolids, 3 x arminids and 1 x dendronotinid. If diet is considered then 11 of the 21 species sighted are spongivores with 8 of the others having a cnidarian diet.

Our Historical Data:
– This excursion brings the number of survey dives to 32 at this site since we commenced fully recording data here on 19/07/2010.
– 133 species have been recorded at this site.
– The most regularly recorded species at this site are: Goniobranchus splendidus (91%), Phyllidiella pustulosa (75%), Doriprismatica atromarginata (72%), and Phyllidia ocellata (69%), this last species not being recorded on these surveys.
– This is the first time we have recorded Coryphellina lotos, Goniobranchus rubrocornatus or Okenia hallucigenia at this site.
– We have only recorded Verconia decussata on one previous occasion at this site.
– There are no obvious seasonal patterns in the abundance of the species recorded during this survey.
– This site last surveyed on 27/01/2023.
(Historical data courtesy of Terry Farr, who extracts it from his comprehensive database.)

Interestingly when I asked Terry to interrogate his database on the sighting frequency of Bornella anguilla, the species after which this dive site, Bornella Reef, was named, it just manages to make it into the top ten species at the site. Its percentage of sightings after 32 surveys is 44% meaning it has been sighted on less than half of the surveys. I can remember the first time I dived there and seeing so many Bornella anguilla it seemed like a good idea at the time to name the site after them. Seasonal variation of population numbers too, can be an influence on the number crunching. This serves to highlight the difference between frequency of sighting across the surveys and population on a particular survey and timing of the survey.

One observation does not a fact make; data collection and collation over a long period is essential before any conclusions can be accurately drawn.

Key to symbols
+++ New Species
+ First time sighted at this site
# Most commonly sighted at this site with %

TOTAL SPECIES SIGHTED: 21

DIVE IMAGES GALLERY

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