Biology of pre- and post-hatching stages of Loligo vulgaris Lamarck, 1798 and Loligo forbesi Steenstrup, 1856 (Mollusca, Cephalopoda)
Loligo vulgaris and Loligo forbesi (Cephalopoda, Myopsida) are sympatric in geographical distribution. Samples of Loligo vulgaris and Loligo forbesi eggs were collected in the two geographical areas where each species occurs predominantly, southern coast of Portugal and northwestern coast of Scotland, respectively. The embryos and hatchlings were reared in the laboratory at several experimental temperatures. This study presents new information on various aspects of the early life stages of Loligo sp. The spawned egg clusters of both species had similar external appearance. The egg strings of Loligo vulgaris are smaller and contain more eggs of smaller volume than those of Loligo forbesi. Temperature appears to be the factor limiting the distribution of these species. The percentage of deformed embryos was higher in Loligo vulgaris acclimated to lower temperature levels (13°C). The duration of the embryonic development was shortened with increasing temperature, and at similar temperature level the difference in the embryonic phase of Loligo vulgaris and Loligo forbesi was approximately 200 day-degrees. The normal hatching period from one egg string was 3 to 4 hours for both species at all experimental temperature levels. Starved Loligo vulgaris hatchlings survived longer than starved Loligo forbesi hatchlings. Maximum survival rates in Loligo vulgaris fed ad libitum were associated to the presence of live zooplankton and lower hatchling density in rearing tanks. At late embryonic stages the activity of the chorionated embryos increases in response to mechanical disturbance and to changes of light intensity. In undisturbed embryos the rate of mantle contractions increases towards pre-hatching stages. In artificial rearing condition, Loligo forbesi and Loligo vulgaris hatchlings exhibit positive phototaxis, swim at 2-3 mantle contractions per second and at relative velocities varying between 20-130 dorsal mantle lengths per second. The cost of sustained swimming was estimated between 0.02-0.06 J h-1, and the cost of transport for 7 mg hatchling was 4-12 J g-1 m-1.