Eggs - differences between species

PHOTO

Comparative series of photos:
LEFT: Facelina quadrilineata
RIGHT: Facelina bilineata.
Row 1: Showing 4 and 2 lines on head.
Row 2: Egg mass. Scale = 1mm.
Row 3: Egg and capsule. Scale = 100 microns.
Row 4: C, veliger about to hatch (15.3 days).
G, veliger just hatched (10.5 days).
Row 5: D, 1 day after hatching.
H, 2 days after hatching.
Row 6: Shells of newly hatched veligers.
Scale bar = 100 microns.
PHOTOS: Yoshi Hirano.

Often, the shape and nature of egg masses and larval development can give us important clues to the identity of a species. In some cases, very similarly coloured species can only be separated by subtle differences in the spawn and embryonic development.

This series of photos by Yoshi Hirano are a good example of the importance of these structures in species identity. The differences shown in the photos caused him to investigate the anatomy of apparent colour morphs of F. quadrilineata (Baba, 1930) further and show that there were two species, Facelina quadrilineata, and a new species, Facelina bilineata.

These two sympatric species have a slightly different colour pattern, their egg masses are slightly different in colour, and as can be seen in the photos, one has fewer and larger eggs in the egg mass. The time required for embryonic development in Facelina quadrilineata is consistently longer (about 15 days) than for Facelina bilineata (about 11.5 days), and the size and shape of the veliger shells is quite different. Both species have lecithotrophic (non-feeding) larvae which spend only a short time in the plankton before settling on to the substrate and metamorphosing into small slugs. See Yoshi Hirano's wonderful photos of veligers of Flabellina amabilis settling from the plankton and metamorphosing.

References:
• Ito, M. & Hirano, Y.J. (1996). The spawn and development of two colour morphs of the aeolid nudibranch Facelinella quadrilineata (Baba, 1930). Venus (Japanese Journal of Malacology), 55(2): 115-121.
• Hirano, Y.J. & Ito, M. (1998). 'Facelinella quadrilineata' (Baba, 1930), a pair of cryptic species of Facelina from Japan. Zoological Science, 15: 791-797.

Authorship details
Rudman, W.B., 1999 (June 26) Eggs - differences between species. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/facedev

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