REISE, H., HUTCHINSON, J.M.C., SCHUNACK, S. & SCHLITT, B. 2011. Folia Malacologica 19: 201–223.

Deroceras panormitanum and congeners from Malta and Sicily, with a redescription of the widespread pest slug as Deroceras invadens n. sp.

In this paper we describe the anatomy and mating behaviour of three Deroceras species. One is D. invadens, formally described and named in the paper, but familiar right round the world as the tramp species previously known as D. panormitanum or D. caruanae. Another is the true D. panormitanum, known only from Sicily and Malta. The third is D. golcheri, the only other species known from Malta, which the mtDNA implies is a close relative of D. panormitanum. The abstract and links to download the full pdf are here.

Here we additionally provide the video recordings of copulations, the evidence that most clearly demonstrates the significance of the differences between these species.

Clicking on the pictures plays a video. The format is a choice of webm (with vp8 codec) or mp4 (with h264 codec). I have compromised quality somewhat so as not to demand too much downloading, so for higher quality versions, download and play the mp4 files. If you would like to examine these avi videos frame by frame, I recommend that you install the free VirtualDub2.


Mating of Deroceras invadens

Click on picture below to start/stop video (0.9 MB download) Larger, high-quality mp4 video download (3.4 MB).

Early courtship of D. invadens.

N.B. this video is not speeded up! Once both partners protrude their sarcobela, sometimes followed by a short period of peace, there is a spectacular period of lungeing towards the partner with jaws extended ready to bite. This recording starts 2 min after the second slug has protruded its sarcobelum. It is probably not incidental that the sarcobelum also contacts the partner during these lunges. Often the target is the partner’s tail, which leads to dramatic tail lashing. The flag-like configuration of the tip of the tail, is already present during precourtship. Following the partner’s tail leads to the formation of a circle that rotates, but this may be briefly disrupted by lunges towards the middle and head end of the partner. Slugs are from Cala di Luna, Sardinia, but this aspect of their mating is representative also of specimens from the type locality.

Mid-courtship of D. invadens.

These thumbnail videos (the mp4 file to download is twice the dimensions) show 3-4 min about half way through courtship. Aggressive lunges still occasionally occur, but mostly no reaction is apparent to the sarcobelum stroking the back and flanks. In other videos not shown the sarcobelum can be seen to transfer drops of a secretion. Unlike in the true D. panormitanum (see below), the position remains head to tail in a circle; this rotates only slowly and apparently incidentally. Note the flattened and raised tail. Slugs are laboratory bred from stock collected at the type locality.

Click on each picture below to start/stop video
REAL SPEED (1.2 MB download)
× 4 SPEED (0.9 MB)
If no video runs, or for better-quality images, download this 8.9 MB mp4 file and play with another program.

Click on picture to start/stop video (<2.4 MB) If no video runs, or for better-quality images, download this 4.9 MB mp4 file and play with another program.

Copulation of D. invadens.

Prior to copulation each mouth acts on the sarcobelum base of the partner, but mouth-to-mouth contact is lacking (in contrast with the true D. panormitanum shown below; nor are the heads raised so far off the ground). Each penial caecum everts quite slowly and deposits the sperm on the partner’s sarcobelum. After the caecae have retracted there is a pause, often longer than in this mating, when the penes remain pressed against each other pulsating. This lull is broken when the penial lobe of one partner everts, and then the long fingers of the penial gland evert to cover the other partner. In the mating shown here, the first penial lobe to evert is initially hard to spot, but the eversion is clearer with the second partner, which everts its penial lobe as the first partner is already everting its penial gland: lack of synchrony is typical at this stage. From the start of copulation until the second partner everts its tentacles again lasts over 5 minutes. Slugs were from the type locality.

Click on picture to start/stop video (<2.8 MB) If no video runs, or for better-quality images, download this 2.4 MB mp4 file and play with another program.

Copulation of D. invadens from Sardinia.

The main differences to typical copulations of slugs from the type locality are that the penial lobe starts to evert much earlier, even while the caecum is still extended, and the eversion of the penial glands does not follow directly after that of the caecum. The penial lobe also everts in a more posterior direction, in this respect more like the true D. panormitanum, as shown below. The same mating as Fig. 6r-t in the paper.


Mating of the true Deroceras panormitanum from Sicily and Malta

× 4 SPEED (0.8 MB)
Click on each picture above to start/stop video
If no video runs, or for better-quality images, download this 6.7 MB mp4 file and play with another program.

Early courtship of D. panormitanum.

These thumbnail videos (the version to download has twice the dimensions) show the early part of courtship (the videos start 18 min after the sarcobela protruded). The typical position is head to head, with each periodically lunging towards each other with their jaws barred. The speeded-up version of the video continues into the next phase of courtship when they slide past each other, but initially there is still some biting. Animals from Agnone Bagni, Sicily.

Mid-courtship of D. panormitanum.

These thumbnail videos below (the downloadable mp4 version is over twice the dimensions) show a 4 min sequence, 80 min into a 125 min courtship. Distinctive features are the sarcobelum-to-sarcobelum fondling, followed by the sliding of the bodies along each other, dragging the sarcobelum over the partner’s flank, and the sharp doubling back to re-engage when contact is lost with the partner’s tail. This pattern can be repeated multiple times during courtship. Animals from Agnone Bagni, Sicily.

Click on each picture below to start/stop video
REAL SPEED (1.1 MB download)
× 4 SPEED (1.1 MB)
If no video runs, or for better-quality images, download this 4.8 MB mp4 file and play with another program.

Copulation of D. panormitanum.

These two three-minute videos shows two views of a single copulation. Note the mouth-to-mouth and then mouth-to-sarcobelum-base contact immediately before copulation as the animals twist round each other and rear up. The eversion of the genitalia is very fast: the tentacle-like penial caecum reaches round the back of the partner’s sarcobelum whilst the prickley balloon-like penial lobe points backwards towards the animal’s own sarcobelum. Once the caecum retracts, the penial glands evert. Then the mantles start oscillating side-to-side, and the copious mucus production makes the animals appear to melt together as they push against each other and again rear up. The sarcobela retract at this stage and the animals eventually separate. Same mating as shown for the mid-courtship sequence.

Click on picture below to start/stop video (2.2 MB)
Click on picture below to start/stop video (1.4 MB)

Download mp4 file of both views both playing synchronously (5.1 MB).


Mating of Deroceras golcheri

Click on picture below to start/stop video (1.6 MB)


This 2 min video shows the copulation. When the swollen bases of the two sarcobela touch the heads entwine (causing the sarcobela to rotate like the hands of a clock) at the same time as the rest of the penis everts. The eversion is slower than in the other species and produces a broad flap. The flap of each partner overlaps that of the other and this is where sperm is transferred. Note the lobe-like extension of the flap that weakly hooks with that of the partner. The finger-like extension of the penis everting to the left produces the penial glands that evert over the partner. Once the penes disengage, the deposited sperm masses are clearly visible. At this stage the mantles oscillate side to side. The same mating as in Fig. 8 in the paper; animals were collected from the type locality (Il Maqluba, Malta).


Some notes on the video processing

It has taken me a while to find out about video processing. There is lots of advice on the WEB and some excellent free programs. I describe what currently works best for me. But it is a fiddly process, partly because the best trade-off between quality and file size is a value judgement, and it takes me much too much time. Anyhow, maybe these notes will help others with a similar job to do.

To capture the videos I mostly use an ISIS TFS-0406 digital video recording card. This can save clips as avi files with a resolution of 704×480 and at up to 25 fps. For older video on tape, I digitise using a Plextor ConvertX PX-M402U, which produces 720×576 25 fps avi files.

For the editing of these raw videos I write .avs text files that call Avisynth functions that are run using VirtualDub2. The filters and commands in VirtualDub2 are sufficient to do many of these steps without Avisynth, but Avisynth offers a few more possibilities and I find it useful that the .avs files document the processing steps fully. Nevetheless VirtualDub2 is useful for initial experimentation with selecting the best sequence, deciding where to crop, and adjusting contrast, etc. I also rely on it to generate the output files. An .avs script typically involves the following steps:

I then save the output using the vp8 (webm) and x.264 (mp4) codecs (both are offered in the html code) as recommended here as compatible with a wide variety of modern and not-so-modern browsers. It is fiddly work to select the compression level so as to minimise file size without noticeable loss of quality. Here I use VirtualDub2 again to compare different files alongside one another. Each parameter of each filter also ideally needs to be optimised for each video source, so that is why it takes so long to produce a nice looking video that is not too large.

John Hutchinson, revised 20.iv.2020

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