Intraspecific variation in embryonic and larval traits of the dorid nudibranch mollusc Adalaria proxima (Alder and Hancock) around the northern coasts of the British Isles

Intraspecific variation in reproductive traits (egg size, clutch size, embryonic development rate, larval hatching success) of the dorid nudibranch mollusc Adalaria proxima (Alder and Hancock) was investigated for six intertidal populations embracing this species' geographic range throughout the northern British Isles. Populations were sampled from northeast England (Robin Hood's Bay), east Scotland (Kinkell Braes), north Scotland (Loch Eriboll), west Scotland (Cuan Ferry), north Wales (Menai Bridge) and Northern Ireland (Portaferry). The objective was to quantify within and between population variation throughout individuals' reproductive periods and to assess the putative adaptive significance of egg size variation. Significant variation both in egg size and clutch size was found to occur between populations. Mean population egg diameters varied from 160 μm (Portaferry) to 181 μm (Menai Bridge). Clutch size did not correlate with egg size between populations, neither did the observed geographic differences in egg size and clutch size exhibit a discernible pattern or clinical structure. The mean embryonic developmental period (range 53-69 d) also differed significantly between populations, but there was no correlation between egg size and developmental period or larval hatching success. None the less, a significant relationship between egg size and larval shell size was established, although whether a larger larval shell reflects a larger larval soma remains. Significant variation in egg size and clutch size also were observed within populations over the spawning period. Individual adult Adalaria proxima may lay up to 11 spawn masses in the laboratory over the reproductive period. Significant decreases in both the number and size of eggs contained within successive spawns was found, with a consistent pattern of the first spawn mass comprising the largest proportion (26-46%) of individuals' total egg production. These egg and clutch size patterns did not differ qualitatively between populations, and may represent progressive compensation for the gradual diminution of energetic reserves available for reproduction. Hatching success of spawn masses also decreased significantly with increasing position in the laying sequence. Maximum fitness may well be conferred on progeny from the first laid spawn masses, which contain more and larger eggs, which display higher levels of hatching success and result in larger shelled larvae. Previous genetic studies have shown British populations of this species to be highly differentiated on scales of only a few kilometres: differentiation is believed to be due to random genetic drift attributable to restricted larval dispersal (and hence gene flow) and local habitat selection need not be invoked. The lack of correlation between egg size and rate of embryonic development to hatching of the veliger larva begs the question as to whether this variation is truly adaptive, or non-adaptive and merely also a reflection of genetic drift.