J. Moll. Stud. (1989), 55, 489-496
© The Malacological Society of London 1989
research-article |
ECOLOGY AND TAXONOMIC STATUS OF THE APLYSIOMORPH AKERA BULLATA IN THE BRITISH ISLES
1Zoology Department, University of Bristol; and 2Barn Court Hamlet, Chetnole, Sherborne, Dorset, U.K.
Dwarf forms of Akera bullata found in the Fleet (Dorset) and Balta Sound (Shetland) were given the subspecific name nana by Jeffreys (1867); large Irish specimens were called farrani by Norman (1890). Dwarf Fleet akerids were studied in order to decide whether (a) these geographical extremes ranked as valid species, or (b) there was an ontogenetic series without any significant discontinuity.
Monthly samples (1984–87) of A. bullata nana from Langton Hive Point (LHP) in the Fleet were analysed. These results were compared with local hydrographic data, and with samples from other Fleet localities, from Portland Harbour, Plymouth Sound, and Lough Ine.
Salinity at LHP varied from 6
. in winter to 35.5 in summer. Akerids were most at risk when a cold wind coincided with low water and rain. Akera was herbivorous in the summer, feeding on rafts of Enteromorpha, but during the remainder of the year it fed directly upon the sediment. There was no evidence that it attacked the leaves of Zostera or Ruppia. The biometric data showed an annual life cycle, each generation growing to maturity during the spring and early summer. Spawning took place at the time of the Enteromorpha bloom; the spent adults then died. Moribund adults were common at LHP from April.
Monthly ovotestis samples confirmed this interpretation of the annual life cycle, but it was disrupted catastrophically in February, 1986 when the entire LHP population was destroyed. Recolonisation took many months and involved sporadic immigration to LHP from adjacent, deeper parts of the Fleet. There was no evidence of recruitment from outside.
Akerids of the subspecies nana from LHP reached 15 mm in shell-length, lacked defensive purple and the posterior pallial tentacle, and swam only in the juvenile phase. By contrast, Lough Ine samples of farrani reached 40 mm or more in shell-length, produced purple when disturbed, exhibited an elongated posterior tentacle and engaged in swimming activity in the adult phase. Intermediate specimens made it certain, however, that no significant discontinuities existed in the cline; consequently all British akerids can be embraced by a single specific name, Akera bullata Müller, 1776.
Development-type and gamete mass were similar throughout the geographic range of British Akera bullata. Fecundity, however, was lower in the dwarf subspecies nana. Features of the gametes and the early development confirm the necessity to transfer the Akeridae from the Bullomorptaa to the Aplysiomorpha.
(Received 24 June 1988; accepted 24 November 1988)