The European lobster is found in temperate waters along the coast of the North-Eastern Atlantic, from Morocco and the Mediterranean to the colder waters of the Arctic Circle around Norway, with the centre of distribution in the British Isles, see Fig.1 (Richards & Wickins, 1979; Middlemiss et al. 2015; Norwegian Ministry of Trade, Industry and Fisheries, 2013).
1.2 Taxonomy
Lobsters are part of a larger class of invertebrate organisms called Crustacea, see Fig.2, distinguishable because they bear a flexible shell, encompassing species such as crabs, shrimps and copepods (Cowan, 2004). Most inhabit marine environments, although many are fresh-water and a few occupy terrestrial habitats (Richards & Wickins, 1979). Crustaceans are divided into many subclasses, Malacostraca containing the most commercially important of the species, including the European lobster Homarus gammarus and its close relative, the American lobster Homarus americanus (The National Lobster Hatchery, 2016; Richards & Wickins, 1979). They are placed in the order Decapoda, meaning ten feet in Latin, corresponding with their number of appendages (Cowan, 2004; Middlemiss et al., 2015).
1.3 Aquaculture and the fishery
European lobsters form valuable fisheries along the coast of the North-Eastern Atlantic (Schmalenbach et al., 2009). At the end of the 20th Century total annual landings for lobsters were around 1300 tonnes, 10 years later this figure more than doubled, reaching approximately 3000 tonnes, and a value of £31m, almost half the value of all annual shellfish landings (Middlemiss et al., 2015; Schmalenbach et al., 2009). This overexploitation induced a general population decline in the UK, and in certain European countries a complete collapse (Bermudes et al., 2008 a; Middlemiss et al., 2015; Schmalenbach et al., 2009). This global overfishing coupled with a high demand and value, make lobsters a popular candidate for aquaculture (Fitzgibbon & Battaglene, 2012; Bermudes et al., 2008 a). Several efforts currently exist for restocking and increasing the production of fisheries for the clawed lobster, Homarus sp. by releasing mass amounts of reared juveniles into the wild provided from ‘berried’ females brought into hatcheries by fishermen (Hache et al., 2015; Schmalenbach et al., 2009). The cost of culturing lobsters is high and time intensive (Schmalenbach et al., 2009). Hindered by low survival in hatcheries at planktonic larval stage, this inhibits the commercialisation of lobsters for farming to decrease impact on wild populations (Bermudes et al., 2008 b). Therefore optimal environmental conditions are required for larvae to reach maximum production at lowest costs for economic feasibility (Schmalenbach et al., 2009; Neal et al., 2010).
1.4 The Value of Research
European lobster hatcheries’ main aims are to improve production of their local fisheries, enhancing the natural population, and preventing a collapse (The National Lobster Hatchery, 2016; Schmalenbach et al., 2009). Aquaculture is notoriously difficult, as without optimal rearing conditions, many metabolic processes and natural behaviours are affected, causing problems with growth, survival, and activities when released into the wild (Bermudes et al., 2008 a). The high costs associated with indoor recirculating systems also mean maximum production rates are an important component (Neal et al., 2010). Research has led to aquaculture in lobsters providing correct temperatures, photoperiod, light intensity, feeding requirements and many other variants, for maximum yield, closely matching lobsters needs in captivity to their natural habitats.
2. Biology
2.1 Larval stages
Spawning occurs during the summer months after the ‘berried’ female has carried her eggs for approximately 9-12 months (Richards & Wickins, 1979; Cowan, 2004). The eggs hatch over several nights, the female releasing around 10,000-100,000 depending on her age, and the planktonic larvae will swim towards the moonlight, reaching the surface to prey on zooplankton (Cowan, 2004, The National Lobster Hatchery, 2016). Pelagic larvae will drift in the ocean currents at the surface anywhere between 15-35 days in the British Isles, during which mortality is highest, usually from predation (Cowan, 2004; Hache et al., 2015; Richards & Wickins, 1979). Before reaching the IV stage, after 3 moults, where there is a noticeable change in body-structure and habitat, as the young lobsters seek a suitable substrate to settle into for a benthic existence (Knudsen & Tveite, 1999; The National Lobster Hatchery, 2016). Once they seek shelter in sandy sediment to reduce vulnerability to predators, they remain burrowed into the seabed for approximately two years, hidden from daylight (Hache et al., 2015; Templeman, 1936; The National Lobster Hatchery, 2016). The juvenile lobster will feed off marine worms, small crabs, gastropods and fauna and plankton drawn into their burrows by pleopod fanning, as well as filter feeding on plankton (Barshaw, 1989; The National Lobster Hatchery, 2016). In the first year a lobster will moult approximately 10 times, intermoult period gradually increasing as the lobster grows, see Fig.3 for larval stages (The National Lobster Hatchery, 2016; Richards & Wickins, 1979).
2.2 Natural behaviour
H.gammarus are primarily nocturnal, seeking shelter during the day, only emerging as dark falls to forage for food, before returning when light levels increase. Studies have found they need the influence of daily light cycles, keeping a circadian rhythm to maintain timing relationships with locomotor functions (Bermudes et al., 2008 a; Dalley, 1980). Homarus sp. are not visual feeders, so need to encounter prey, Eagles et al. (1986), found larvae in continuous darkness grew larger and developed twice as fast. Lobsters are opportunistic carnivorous scavengers, eating a large variety of marine organisms including, crustaceans, molluscs and worms, but eating less at low temperatures (Bell et al., 2015). They are highly territorial, solitary animals; confined conditions and high densities can often result in fighting and cannibalism, as reported in studies (Gardner & Maguire, 1998; Schmalenbach et al., 2009), or if they come in contact with another lobster in the wild; soft, newly moulted individuals being most vulnerable (Bell et al., 2015; Richards & Wickins, 1979).
In order to grow, lobsters, as in all arthropods, need to shed their exoskeleton by moulting in a process known as ecdysis. They do this by absorbing excess amounts of water in their body tissues, causing swelling and thus rupturing their old shell. Once freed, a lobster will continue to swell, as the new shell begins to harden, a process taking anywhere between a few hours to several weeks, depending on size of lobster and availability of calcium. Intermoult periods are where new tissue growth replaces the water absorbed at moulting. This is a stressful process and a highly vulnerable period, resulting in high mortality. Occurring in summer months in the wild, at approximately once a year for adults, less frequently in older animals, and mating will occur directly after the female has moulted, while the animal is still soft (Bell et al., 2015). During July and August lobsters look for mates along the migration corridors, females breeding once every two years and hatching 10,000-100,000 eggs, with only two or three of these reaching adulthood in the wild (WAZA, 2016).
2.3 Natural environment
H.gammarus are found in waters anywhere from the lower shore up to a depth of 150m, although rarely deeper than 50m (Middlemiss et al., 2015). In warm summer waters they inhabit shallow areas, retreating to the depths in winter (Cowan et al., 2007). Juveniles prefer sand or gravel sediment to bury into, whereas adults are usually found on hard rocky or compressed mud substrates (The National Lobster Hatchery, 2016). They retreat into dark holes and crevices during the day, perfectly suited to their size, where they spend most of their time, seldom moving (Corson, 2005). Larval lobsters were found to grow best at temperatures of 20°C, although adults prefer a range between 13-16.5°C and a salinity of 28-32%, temperature increases making lobsters grow quicker (Crossin et al., 1998; Richards & Wickins, 1979; Templeman, 1936).
3. Previous research in aquaculture
3.1 Effects of light regimes on growth and survival
3.1.1 Photoperiod
Many studies have been undertaken on the effects of different light regimes on a variety of larval and juvenile marine species in aquaculture. There is no obvious interspecies consistency in their response to photoperiod, and current knowledge on the effects of photophase manipulation on growth and survival is somewhat confused. Photoperiod had no effect on survival and growth for species such as the hermit crab, Pagarus hirsutiusculus (Fitch & Lindgren, 1979), Hache et al. (2015) also reported no significant difference in H.gammarus, unless paired with light intensity, in contrast to Templeman (1936), Eagles et al. (1986), Aiken et al. (1981 & 1982), and Richards & Wickins (1979) all reporting shorter photoperiod to be more beneficial in studies of the same species. Certain studies reported affect on growth, but not on survival, such as the Australian giant crab, Pseudocarcinus gigas (Gardner & Maguire, 1998) and the spiny lobster, Jasus edwarsii (Bermudes & Ritar, 2008). However, photoperiod regimes have been known to affect survival and growth of such species as the spiny lobster, Sagmariasus verreauxi (Fitzgibbon & Battaglene, 2012), red frog crab, Ranina ranina (Minagawa, 1994), American lobster, Homarus americanus (Eagles et al., 1986), European lobster, Homarus gammarus (Richards & Wickins, 1979), Japanese spiny lobster, Panulirus japonicas (Matsuda et al., 2011), haddock, Melanogrammus aeglefinus (Trippel & Neil, 2003), Atlantic cod, Gadus morhua (Puvanendran & Brown, 2002), and the banana prawn, Penaeus merguiensis (Hoang et al., 2003).
For many of these animals, an influence of a daily light cycle is important in synchronising various behavioural and physiological processes. Organisms have evolved light sensitive circadian clocks to maintain rhythmic functions in accordance with the environment, keenly observed in the common shrimp, Crangon crangon (Dalley, 1980), such as feeding and breeding, larval lobsters for example being extruded in summer months, when ocean currents bring larvae onshore, to settle once reaching metamorphosis, and reducing predation. Species that are affected by photoperiod, may either be more inclined to living in dark areas, such as cracks and crevices, or at greater depths, as reported in H.americanus (Eagles et al., 1986), and H.gammarus (Richards & Wickins, 1979), both experiencing increased growth and survival in darker regimes. Clawed lobsters being nocturnal, and foraging at night, meant these dark periods were more favourable, as they could forage at any hour, therefore consuming more food, increasing growth. In contrast species such as P.japonicas (Matsuda et al., 2011), M.aeglefinus (Trippel & Neil, 2003), and G.morhua (Puvanendran & Brown, 2002), all of which show a preference to longer photoperiods. This trend can be for reasons such as, the species being a visual feeder, therefore catching more prey and growing faster in longer periods of light (Puvanendran & Brown, 2002), from better food conversion efficiency (Trippel & Neil, 2003), or a promotion of smoother, more regular metamorphosis (Matsuda et al., 2011).
3.1.2 Light Intensity Light intensity has also been commonly studied; again growth and survival appear to differ between species, and is an evident basic motivator in aquaculture. Most species studied, preferring a natural, or dim light intensity, such as J.edwarsii (Bermudes et al., 2008; Moss, 1999), whiteleg shrimp, Litopenaeus vannamei (Neal et al., 2010; Guo et al., 2013), P.gigas (Gardner & Maguire, 1998), M.aeglefinus (Trippel & Neil, 2003), and H.americanus (Eagles et al., 1986), although Hache et al. (2015) in fact recommends a light intensity of 1000lux (lx), which is reasonably bright for H.americanus larvae. Trippel & Neil (2003) state that a lesser swimming activity was observed under dim light, thus increasing metabolic savings and body mass, therefore reporting low light intensity (30lx) with continuous light was the better strategy for rearing lobsters, this may be because haddock are diurnal, therefore rest during dark periods. Moss (1999) concludes that high intensities cause an imbalance of energy uptake and consumption, possibly from an innate response of swimming towards light to reach the ocean surface, which aids in offshore dispersal, this response was accentuated in higher light intensities, contributing to greater energy consumption and decreased growth. The Australian giant crabs’ improved survival was reportedly down to less cannibalism in dark phases, as during light periods they experience passive sinking in response to high intensity lights, accumulating at the base of tanks en mass (Gardner & Maguire, 1998). J.edwarsiis’ improvement in low light intensity was seemingly a cause of metabolic feeding efficiency where their food intake to oxygen consumption ratio was high, showing that there was a positive return; in prey encounter, to extra energy expended, a similar conclusion to Moss (Bermudes et al., 2008).
Opposing these studies were the prawn, P.merguiensis (Hoang et al., 2003) and the cod, G.morhua (Puvanendran & Brown, 2002) both finding higher light intensities more favourable conditions. It was reported that first feeding larval cod require a threshold light intensity to initiate feeding, also prey encounter was greater, as locomotor activity increased with higher intensities (Puvanendran & Brown, 2002).
3.2 Other environmental factors in aquaculture
Other environmental factors are also important to consider in aquaculture such as pigmentation in lobsters. Tlutsy et al. (2009) discovered that UV light caused lobsters to become darker in colour, as pigment protects them from UV radiation, with an absence of UV, lobsters were found to match their background colour, white making them lighter, and black making them darker. This is a survival response helping them hide from predators, predation being a great threat to larval and juvenile lobsters. Juveniles exhibiting a wide range of colours corresponding with their settlement habitat, their colour controlled by the amount, location and form of carotenoid pigment astaxanthin, lobsters changing colours by the deposition of astaxanthin. Lobsters living at depth are commonly uniformly red, since red wavelengths are absorbed at the surface; this renders them invisible (Tlutsy et al., 2009).
Feeding every two days with a natural diet, proved to be more beneficial to growth rates of juvenile lobsters than artificial, also producing survival rates of 90-96%, as this reduced blockage of compartments, keeping water flow and water quality high. The addition of live isopods as food was beneficial, as this also contributed to keeping compartments clear, as isopods consumed remaining food (Schmalenbach et al., 2009).
Larger holding compartments in relation to the lobsters’ size represented sizable growth, at a higher rate, juvenile lobsters also showed a positive effect from growing at depth in the wild (Knudsen & Tveite, 1999). An area of 75cm2 is required to allow unrestricted growth of juvenile lobsters up to 3cm in total length, whereas 6cm will need an area four times that size (Schmalenbach et al., 2009).
4. Key Studies
There have been a number of studies carried out regarding the effects of photoperiod and light intensity on survival and growth of lobsters. For best analysis of the data collection method, the key studies referenced here are those concerning the clawed lobster species Homarus sp.
Hache et al. (2015) measured the effects of photoperiod and light intensity on growth, survival and behaviour of Homarus americanus from a postlarvae stage IV, the only study similar to the present study, using juvenile lobsters rather than planktonic larvae. They found no significant difference in their results of growth and survival across treatments, although 14 hours light and 10 hours of dark (14L:10D) gave best results for survival, and 8L:16D had a trend for best growth. This contrasted with previous studies by Aiken et al. (1981 & 1982) where they concluded that the shortest photoperiods were most advantageous for development and survival of larvae up to stage IV, however larvae at stage IV were reported as uninfluenced by either season or photoperiod. Similar results were found in Eagles et al. (1986) and Templeman (1936) with planktonic larvae reared in continuous darkness until stage IV. Eagles et al. (1986) found growth increasing in 0L:24D, yet survival decreasing, this study took place in enclosed cabinets with an artificial light source set at a intensity of 100 lx for continuous light and 12L:12D, and 1000 larvae held in 40 litre (L) pots. The study by Templeman (1936) concluded that growth was faster in constant darkness than in natural photoperiod, and survival was in fact greater, a result probably from cause of methods. Templeman used one single larvae per 15cm high x 8cm diameter jars, changing water every 1-3 days, without a recirculating system, presumably because of the date research was undertaken, he also used natural daylight, as in the present study, in contrast to artificial light used in Hache and Eagles. This rearing density evidently affected survival rates, Templeman would have no cases of cannibalism, and therefore larvae would only suffer mortalities in the event of moulting.
The present study is very similar in terms of method to Richards & Wickins (1979) as juvenile H. gammarus lobsters were researched in constant darkness, and compared to a constant light and 12L:12D photoperiod. They were reared in individual containers of 10x10cm and held in a 900L semi-enclosed recirculation system. Results concluded that constant darkness was most favourable in growth for the nocturnal lobsters, although questionably survival was poor at 55%, the method states individual containers, although does not state whether these are solid containers. Lobsters, being cannibalistic are quite adept at dismembering parts of other lobsters if they are able to reach. Activity was reported to be highest in a 12L:12D regime and lowest in constant light.
Preference to dark conditions has been noted in all studies, with exception to Hache et al. (2015) being a more recent study, methods of aquaculture may have been improved. Aiken and Eagles both used higher rearing densities of 50vs20, therefore reducing cannibalism in Hache, although separation is necessary in juveniles compared to larvae, as claws have developed and cannibalism is increasingly more adept. Feeding strategies between Hache and Eagles were different, Eagles fed during the light period in 12L:12D and 24L:0D and in dark in 0L:24D, reporting that larvae that fed only in the dark had a better survival, but poor growth, whereas Hache fed 1 hour after dark and in all light periods. Differences in feeding methods could be a major affect on results in all of these studies, as lobsters prefer to feed as darkness falls and just before first light, meaning any food deposited during light periods could be left, degrading water quality and maximising algal growth. Hache also did not study a continuous dark period with the juvenile lobsters, unlike Richards & Wickins, but used a 24L:1D period, perhaps to keep normal circadian rhythms in order for functional locomotor activity.
Water temperature is not always noted in each study, despite having an immense affect on growth rates of lobsters, as seen in Fig.4, which could have altered results, although comparisons of survival and growth within each study would still be conclusive.
Figure 4. Survival and growth rate of larval lobsters at two different temperature ranges, in a natural photoperiod and in complete darkness (Templeman, 1936).
Unfortunately most studies found of the effects of photoperiod and growth, were quite dated. Being quite a simple, straightforward study not a great deal of recent research has been undertaken, presumably all lobster hatcheries currently operate at a light level deemed optimal by these previous studies, or have discovered from their own experiences, without publishing work.
5. Aims and hypotheses
This study aims to investigate the effects different photoperiod regimes have on juvenile lobsters reared in recirculation tanks in a hatchery environment, it will determine whether continuous darkness is beneficial to the growth and survival of stage IV- stage VI larval lobsters. The research will aid in culturing larval lobsters at maximum viability, while keeping outlay low.
The hypothesis is that European lobsters cultured in dark conditions will have a higher growth rate, greater muscle mass and lower mortality rate than those cultured in light conditions, as previous studies such as Eagles et al. (1986); Aiken et al. (1981) and Richards & Wickins (1979) show.
6. Conclusion
It is apparent from research into previous studies that photoperiod regimes are certainly species specific in affecting growth and survival, with a trend of nocturnal larvae preferring dim light with shorter photoperiods, than diurnal species. Many of the studies on clawed lobsters, showed that long periods of darkness was more preferential, although this research was mainly on planktonic larvae rather than juvenile, of which prefer entirely different light regimes, results may also affect survival in the older experiments using single larvae to one jar.
Using information from previous studies, the present study will incorporate ideas such as, larger compartments for the lobsters, to maintain growth to the full capacity; they will be reared at a low density, so to avoid excessive cannibalism from lobsters in neighbouring compartments; the control study will be held under natural light as opposed to artificial; and each will receive frequent, equal amounts of food. This should provide more reliable results.