Diacavolinia longirostris (De Blainville, 1821a)
Overview
This is a medium sized, uncoiled thecosomatous pteropod, 0.7 cm long. It has a flat dorsal side with moderately developed ribs. The ventral side is vaulted. The caudal spine is absent and a caudal spine mark is not left as the ventral and dorsal sides grow together. The lateral spines are well developed. Shell sculpture consists of faint growth lines and faint transverse striation. The dorsal lip has a notch and gutter but no constriction. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in warm waters of all oceans at shallow depths (Diacavolinia longirostris 1, Diacavolinia longirostris 6).
Taxonomic Description
The hyaline shell is triangular (Diacavolinia longirostris dorsal) and is brown, especially near the ventral lip border and the centre of the dorsal and ventral sides (Diacavolinia longirostris 2). The large dorsal lip (Diacavolinia longirostris shell details) has a notch and a gutter in the broad rostrum but no constriction (Diacavolinia longirostris 3, Diacavolinia longirostris swimming, Diacavolinia longirostris balancer). In some specimens this notch may be shallow when it is still growing. This phenomenon can be seen in all species that normally have a notch. The ventral lip is moderately sized and has a distinct median depression (Diacavolinia longirostris aperture). The sharp lateral spines are slightly bent, protruding laterally and slightly hooked; the gutter corners are large. The ventral side is globular. The 17 ventral ribs are line-shaped and thin being composed of small tubercles (Diacavolinia longirostris ribs, Diacavolinia longirostris base ventral rib). The lateral lines are slightly convex, the flanks are slightly developed. (Diacavolinia longirostris lateral spine). The dorsal side is slightly convex, with well-developed central rib, lateral ribs and lock ribs. The lock area is small with a blunt or sometimes sharp main tubercle and small link. The lunar and minor tubercle are typical. A second lock mechanism is present. The growth lines are strong, ventrally and dorsally. A hump is absent. Protoconch II area does not project far; it has a curved caudal joint, of about 1.28 mm. The caudal fold, about 1.60 mm long, is nearly straight, the left and right fold form an angle with each other. There is only a slight indication of lip flaps; the lip bellies and the lip shoulders are small. The aperture is large, the lip angle is about 135°, the lock angle is 77°, and the side angle is 66°. The centre of the wings is yellow as are the lips and visceral mass (Diacavolinia longirostris). When swimming the balancers float behind the shell as long ribbons (Diacavolinia longirostris active swimming, Diacavolinia longirostris in situ)
The shell length ranges from 7.28 to 5.20 mm and the width from 6.80 to 4.96 mm.
Juveniles
The juveniles are dorso-ventrally compressed, spoon-shaped in ventral view (Diacavolinia longirostris juv). The shell is straight except for the most posterior point that bends slightly dorsally, the aperture is oval, and forms two long lateral slits project posteriorly. In dorsal view the aperture rim forms half a circle. Protoconch I and II are both transversely striated. In older juveniles the caudal spine is thrown off (Diacavolinia longirostris juv2) and a rounded shell composed of two valves is left, first this shell is open caudally. The caudal opening is closed by fusion of the dorsal and ventral side as seen in the illustration of Diacavolinia longirostris protoconch II and more in detail in Diacavolinia longirostris closing fold. The animal is first smaller than the shell and a minute-stage is found (Diacavolinia longirostris line drawing). A comparison of the present juveniles with those of Cavolinia inflexa is given by Troost and Van der Spoel (1972). (See the illustrations:development Diacavolinia, development Cavolinia)
Reproduction
Knowers (1894) discovered in this species and for the first time in pteropods, separate male and female sexual apertures. The separation of male and female apertures in this species is much better developed than in Clio pyramidata forma sulcata. The gonad is large and asymmetrical, being larger at the left side. The youngest ova are found in the centre around the gonad lumen. The oldest ova are found closer to the periphery. Sperm and ova arise from the follicle septa. The gonoduct leaves the gonad at its anterior face, somewhat dorsally and far to the left. This gonoduct first forms a bursa before ending in the accessory sexual gland, called a muciparous gland by Knowers. A separate albumen gland was not found by him though it is present. Anteriorly the efferent duct leaves the accessory gland debauching to the left. The efferent duct and the gonoduct connect with the accessory sexual gland near to each other. The efferent duct forms, over its entire length, a closed tube and not an open groove (seminal groove) as in most other Euthecosomata. This closed seminal duct curves around the right side to reach the dorsal side of the body near the wing base, from there it runs anteriorly and opens at the aperture of the penis sac. On the left side of the accessory sexual gland a second aperture is found in the histological sections, it is a slit-like opening in a small papilla on the anterior left surface of the visceral mass. The ciliated epithelium of the lumen of the accessory sexual gland goes over without any interruption into the ciliated epithelium covering the body in this area. Quite near to the second opening of the accessory gland the receptaculum seminis is found. The second aperture, thus, must be considered the female aperture while the original one, in contact with the seminal tube, is the male aperture. Sperm transport is much better effected by the closed tube than by an open seminal groove, but the differentiation makes a new female aperture necessary. In some other Opisthobranchia double sexual apertures are also found e.g. in Lobiger and Acteon. The species is a protandric hermaphrodite.
Ecology
This species is phytophagous and epipelagic. It feeds, like all thecosomates, by using a large food web (Diacavolinia longirostris feeding).
Distribution
Diacavolinia longirostris has a wide distribution range in Atlantic and Indo-Pacific Oceans. It is not present in the NW-Atlantic, N-Indian Oceans, W-Pacific Oceans and in the Banda Sea, see the Diacavolinia longirostris map.
Geological Record
This form was found in the Pleistocene of the Mediterranean.
Others
The original description was unclear and the type material could not be found. This redescription restricts the species to the taxon that is certainly not described under any other name than "longirostris". Specimens with elongated spines, so far only found in Indo-Malayan waters, are not considered to represent a separate taxon as growth differences seems responsible for the difference. Morphologically related species are: Diacavolinia souleyeti, with a much more slender rostrum; Diacavolinia limbata, a larger species; and Diacavolinia deblainvillei which has more ventral ribs and shorter, more sharply hooked lateral spines. The ventral ribs in this species are always thin but in a few specimens transverse lines on the ribs make them comb-shaped.
Types
Hyalaea longirostris De Blainville, 1821: 81.
The type specimens could so far not be located.
Type locality: The type locality is not exact but is probably the Atlantic Ocean.