Frontiers in Marine Science (Jun 2014)
Preliminary data on <i>Littorina littorea</i> development under rearing conditions
Abstract
The periwinkle Littorina littorea (L.) is a common prosobranch gastropod at the North Atlantic coasts, ranging from Greenland to New Jersey in America and, from Portugal to the White Sea in Europe (Johannesson, 1988). It presents a planktonic development; females release planktonic egg capsules that are pelagic, asymmetrically biconvex with a flat peripheral rim. The egg capsules contain a maximum of nine eggs (Linke, 1933); more commonly, they contain one to three (Cummins et al, 2002). Upon release, the egg capsules swell osmotically and burst after five to six days. Each egg hatches into a free swimming veliger larva and remains in this planktonic stage for six to seven weeks (Fish, 1979). At hatching larvae measure about 180 micrometers long and are competent to settle after spending 2-6 weeks in the plankton and reaching about 300-360 micrometers in length (Hohenlohe, 2002). Metamorphosis may be delayed if conditions are not suitable and it results in considerable variation of settlement times, with larvae settling on the shore throughout several months of the year. Adults of L. littorea species are ecologically similar, so this genus is a potential model system for testing the hypothesis that gene flow among populations should be higher in species with planktonic larvae because of their greater dispersal capacity (Jablonski and Lutz, 1983). L. littorea is of economic interest in Portugal where it is highly appreciated as shellfish food, being the peak season of consumption in summer. Since the local production is not sufficient to meet demand, it has been necessary to import stocks from abroad (mainly from Scotland and Ireland) in order to satisfy market needs. For this reason, the development of strategies to produce L. littorea in Portugal is crucial. This work aims to determine basic rearing conditions and obtain data on L. littorea larval development and growth rate. These results are important to establish the conditions for cultivating larvae of this species, as well as to access its feasibility since a lack of information was detected in literature. Egg capsules and larvae were obtained from an adult broodstock maintained at Estação Litoral da Aguda. Larvae were reared in 30 L white tanks, maintained at a density of 5 larvae per ml in seawater with aeration, at 22°C. They were fed a mixture of microalgae that included Isochrysis galbana, Tetraselmis suecica, Nannochlorospsis gaditana, Rhodomanas marina, Phaeodactylum tricornutum and Chaetoceros gracilis. Water was partially (30-60%) changed every 2-3 days. Egg capsules and eggs were measured on the day following their arrival at the laboratory (n = 68). External diameter of the capsule, eggs chamber and each egg in the chamber were measured. Larval shell length was measured every 6-8 days (n = 20) using a Leica DM6000 microscope coupled with a digital B/W - 2.0 MPix and colour camera. Microphotographs of each egg and larvae were recorded and analyzed by the LEICA LAS AF version1.8.0. Since larval shells were planispiral, shell length was established as the maximum dimension of the shell: the distance from the outer aperture lip to the opposite side of the last whorl. The mean external length of egg capsules was 852.1 μm. This value is smaller than 1 mm, previously described in the bibliography (Fisher, 1972), probably due to the conditions in which eggs were obtained, a broodstock maintained from November to February in a recirculation system at 16-18ºC. According to Fisher (1972), L. littorea egg capsules are described to have 1 mm and containing 2-9 eggs, being 1-2 eggs the most common (Fish, 1972). Accordingly, the more frequent number of eggs inside each capsule, in the present study, was 2 eggs per capsule (Fig. 1A). However, no more than 5 eggs were observed in any capsule. Regarding the relation between the number of eggs per capsule and the outer capsule diameter, only capsules with one egg were significantly different from the other, although an increasing pattern seems to be revealed (Fig. 1B). It is worth to mention that data are preliminary, only few egg capsules with 4 and 5 eggs were observed, which may have compromised the significance of the difference of other values. Larvae hatched at veliger stage with a mean shell diameter of 179.3 μm. Post-larvae were first observed at day 12 when they have settled to the bottom and the typical velum of veliger larvae disappeared (Fig. 2). The duration of the veliger stage (12-15 days) was much shorter than previously described (Fish, 1979; Cummins et al, 2002) and comparing with other species of Littorina studied in laboratory conditions (Hohenlohe, 2002). In his study, Hohenlohe detected that the whole planktonic period L. scutulata and L. plena, under laboratory conditions, ranged from 37-70 days and 62 days (since only one larvae of this specie metamorphosed) respectively. We suppose shorter larval period observed in the present study is due to the optimized rearing conditions, with a wide variety of algae supplied, at adequate densities, and to the higher rearing temperature, 22ºC, as compared to normal temperatures in the wild for this time of the year in Portugal (12-15ºC) and other Atlantic water. The improvement of rearing conditions can play a crucial role in boosting both pre- and post-settlement development, required for rapidly overcome the more sensitive and labor intensive stages. Mean shell size observed at metamorphosis was 325.2 μm. Comparing these results with other Littorina species such as L. scutulata, with had a mean shell size at metamorphosis of 355 μm (Hohenlohe, 2002), L. littorea are slightly smaller. By day 30 after hatching mean shell length was 387.3 μm long (Fig. 3). The duration of the pelagic phase is primarily temperature dependent, lasting from 2-6 weeks and allowing a dispersal potential exceeding 10 km (Fisher, 1972). In the present study and since temperature was increased to 22ºC, time to metamorphosis was reduced to 12-15 days. We highlight the importance global warming may have on the dispersal of this species and consequent effects on gene flow among populations. Since the main aim of this work is to set up the conditions for rearing L. littorea under controlled conditions, we have observed that higher temperature (22ºC) and an adequate food supply are very important parameters to increase developmental rate, reduce time to settlement and improve survival on this species. The information obtained will be also useful to schedule laboratory trials intended for basic research, e.g. the study of potential settlement cues on this species. The management system developed is, however, extremely labor-intensive. Larvae require very careful hatchery conditions since they are very fragile and easily disturbed. Survival rates, not mentioned in this work, need to be improved, particularly after metamorphosis. In conclusion, the results reported here lay down the first steps to cultivate L. littorea industrially, describing some optimized conditions and revealing hardships. Figures' Captions Figure 1 – Data of frequency of number of eggs of L. littorea observed per egg capsule (A) and mean outer diameter of egg capsules with different number of egg (B). Figure 2 – Images of Littorina littorea larvae at various days after hatching; A – eggs; B – 3 day old larvae, dorsal view; C – 3 day old larvae, lateral view view; D – 7 day old larvae, dorsal view; E – 12 day old larvae; F – 27 day old larvae. Figure 3 - Relation between shell length and age of L. littorea larvae. From hatching to day 7 and from day 15 onwards, growth curve adjust to an exponential equation. There is a stagnation of growth between day 8 and 15 coinciding with settling.
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