Spiny-cheek crayfish Orconectes limosus (Rafinesque, 1817) on its way to the open coastal waters of the Baltic Sea

The overall length of the animals (TL) was measured over the maximally extended abdomen from the tip of the rostrum to the rear edge of the telson (Kossakowski 1962; Ďuriš et al. 2016; Buřič et al. 2010). The spiny-cheek crayfish occurring in brackish and fresh waters differ in size. The maximum size (121 mm in length) was recorded by Chybowski (2000). The crayfish characterized by large maximum total lengths (118 mm in length) was recorded in the Vistula Lagoon (Skrzecz & Szaniawska 2005) where the salinity is 2-3 PSU. Crayfish occurring in lakes of Warmia had maximum lengths of 110 mm (Kossakowski 1966), those from lakes in the East Suwałki Lake District had maximum lengths of 107 mm, and those from the Masurian lakes were 95 mm long at most (Krzywosz et al. 2014). The maximum total length of the crayfish caught in lakes of Western Pomerania was 105 mm (Śmietana 2013) (Table 1). However, the differences in the maximum size do not always imply the differences in the mean size. We do not have statistically confirmed differences between the maximum size of individuals from various water bodies. It is not possible to compare the impact of fresh and marine waters on the O. limosus body condition based on the maximum TL.

Table 1

Maximum total lengths of spiny-cheek crayfish in Polish and other waters

O. limosus is one of the smallest crayfish in Europe. In European waters, its body is no longer than 90-100 mm (Hamr 2002), although Leńkowa (1962) reported the maximum length of 120 mm. According to Krzywosz et al. (2014), the mean length of spiny-cheek crayfish caught in Polish lakes during the last 10 years (up to 2014) was 93 mm. Before that period, the value was 7 mm smaller and amounted to 86 mm (Krzywosz et al. 2014). In the Vistula Lagoon, males and females have roughly the same body dimensions and the range of particular parameters, however, females have broader abdomens (23.6 ± 2 mm) than males (20.3 ± 2 mm) (Skrzecz & Szaniawska 2005). In the Vistula Lagoon, the largest individuals reach the length of 118 mm at salinity 2-3 PSU. The Vistula Lagoon and the northern part of the Curonian Lagoon are brackish water bodies, in which this species has a stable population (Burba 2008; Kruk 2011). The occurrence of larger crayfish in the Vistula Lagoon and in the Curonian Lagoon may be due to water salinity that reduces the osmotic concentration gradient between the animal’s body fluids and its living environment. Another reason why these crayfish achieve larger sizes in newly colonized waters could be the lack of natural enemies. Krzywosz et al. (2014) believe that the larger sizes of crayfish are due to the superior food resources available. It is often believed that large individuals are characteristic of small populations living in recently colonized water bodies. This may well be the case in the parts of the Vistula Lagoon covered by our study.

The claws play a crucial role in the aggressive and defensive behavior of crayfish, in intra- and interspecific competitive mechanisms, in confrontations with individuals of the same or another species (Gherardi & Cioni 2004). The spiny-cheek crayfish has smaller claws than native crayfish or the signal crayfish. Individuals with larger claws may have greater chances of survival in confrontations with animals less generously endowed (Martin & Moore 2008).

In the experiments (Staszak & Szaniawska 2006), crayfish were acclimated for 7 days to laboratory conditions (T = 12°C, fresh water, S = 100% aeration, not fed). Each of the 21 animals was kept separately in a tank (21 × 20 × 20 cm). The water in all tanks was aerated. The crayfish were supplied with shelters made of PVC. Each crayfish was supplied with one of the following brackish waters’ food types: animal-based (cod or crayfish abdominal muscle), plant-based (green algae, Enteromorpha spp.) or granulate fodder. Prior to feeding, the wet weight of the animals was measured. In order to calculate the amount of food consumed, all uneaten food was removed from the experimental units after 24 h, weighed and dried at T = 55°C to obtain the dry weight. Food consumption rates (C = J ind.^-1 h^-1 d.w.) were calculated using the formula given by Klekowski & Fischer (1993). The favorite food of spiny-cheek crayfish was fodder (mean consumption rate was C2 = 308.27 J ind.^-1 h^-1 d.w., T = 12°C). Crayfish abdominal muscle was the second most favored food (C2 = 228.70 J ind.^-1 h^-1 d.w.), and plant food was third (C2 = 43.77 J ind.^-1 h^-1 d.w.). The least preferred food was cod (C2 = 15.02 J ind.^-1 h^-1 d.w.) (Fig. 2). Within the 80-112 mm length range of individuals, no significant differences were found between the crayfish size and the food consumption rate (Staszak & Szaniawska 2006), using statistical analysis: Mann-Whitney U test for p < 0.05.

Figure 2

Food resources are an important factor determining whether new areas can be colonized and how widespread could be a species in a given water body. Like other crayfish species, O. limosus is an omnivore that feeds on a wide range of foods, including macrophytes, algae, detritus and macroinvertebrates (Vojkovska et al. 2014). Being omnivorous throughout their life cycle, crayfish may prefer different types of food at different stages of their life, with juveniles feeding mainly on animal plankton and later on benthic invertebrates, while adults consuming mostly plants and detritus (Goddard 1988; Usio 2000). In large densities, the crayfish can resort to cannibalism (Goddard 1988). Under laboratory conditions, its preferred food was fodder (REP 497 Export, Aller Aqua), which is rich in proteins (53%) and lipids (14%), and has a high energy value (20.8 J mg^-1 d.w.) (Staszak & Szaniawska 2006). Fodder is used to feed many species that are bred for consumption by humans, and its composition is selected in such a way as to encourage animals to eat it and to ensure the largest possible biomass growth. Another preferred food was crayfish abdominal muscle, because it has a similar biochemical composition as the crayfish and is readily assimilable. It is rich in lipids (13% of d.w.) (Goddard 1988) and protein (80% of d.w.) (Holdich & Lowery 1988). The algae offered as food have the lowest energy value (10.1 J mg^-1 d.w.) (Haroon & Szaniawska 1995), and their consumption was relatively low. The size and dimensions of the offered food may also affect the food preferences. For example, the dietary preferences of Procambarus mexicanus (Erichson, 1846) were largely dependent on the ability to handle plant material rather than the plant chemistry itself (Hernández-Muňoz et al. 1999). Food preferences also depend on the age of individuals, season and time of the day (Whitledge & Rabeni 1996). Temperature is crucial for feeding of poikilothermic animals, although it was found that at 12 and 18°C there were only small, statistically insignificant differences in the amount of food ingested at the higher water temperature (Staszak & Szaniawska 2006).

The fact that fodder was the preferred food in the laboratory indicates that artificial food products are most suitable for breeding these animals. Even though in natural conditions cannibalism is not frequent, the abdominal muscle of O. limosus was often consumed under breeding conditions (our own observations).

Osmotic concentrations were determined microcryoscopically, based on the method used in many studies of osmoregulation (Dobrzycka & Szaniawska 1995; Dobrzycka-Krahel & Szaniawska 2005; 2007). A stereoscopic microscope (NIKON SMZ800) with a polarizing (C-POL) accessory was used to observe the melting of hemolymph crystals. In the experiments (Michałowska et al. 2002), the osmolality of hemolymph increased with salinity. There was a significant increase in the osmolality of hemolymph, from 333.5 ± 82.2 mOsm kg^-1 at 0 PSU to 879.5 ± 32.6 mOsm kg^-1 at 35 PSU. O. limosus is a hyper-regulator at 0 and 7 PSU and a hyporegulator at higher salinities > 13 PSU. The transition from hyper- to hyporegulation was found to occur at 385.0 mOsm kg^-1 (ca 13 PSU). The osmotic concentration was 333.53 ± 82.20 mOsm kg^-1 at 0 PSU, 340.08 ± 16.39 mOsm kg^-1 at 7 PSU, 409.33 ± 50.49 mOsm kg^-1 at 14 PSU, 503.86 ± 6.49 mOsm kg^-1 at 21 PSU, 601.389 ± 42.783 mOsm kg^-1 at 28 PSU and 879.5 ± 32.57 mOsm kg^-1 at 35 PSU (Fig. 3) (Michalowska et al. 2002).

Figure 3

Crayfish can adapt to a wide range of environmental factors (McMahon 1986). At the end of the Mesozoic era, they became independent of the marine environment (Hobbs 1988), becoming adapted to life in fresh waters, although some species preserved the ability to survive in brackish waters (Mantel & Farmer 1983). Andrews (1967) studied seasonal changes in the hemolymph composition of O. limosus from fresh waters with respect to temperature, sex and individual body size. Other studies were performed to investigate the effect of water salinity on osmotic body fluid concentrations in other crayfish species: Austropotamobius pallipes (Lereboullet, 1858), P. leptodactylus, P. leniusculus (Holdich et al. 1997, Kerley & Pritchard 1967, Wheatley & McMahon 1982). They showed that all these species are capable of osmoregulation over a wide range of salinity, just as O. limosus. The present study has shown that both fresh and saline waters are not osmotic barriers for O. limosus.

In the experiments (Jaszczołt & Szaniawska 2011), the animals were kept in aquaria (0.34 m², V = 117 dm³) with 10 cm long PCV tubes as shelters. The salinity was 3 ± 0.5 PSU and 7 ± 0.5 PSU (T = ca 16°C, Sat > 80%, measured with a WTW Ecoline LF 170 TetraCon 700 probe) and pH was 6.7-8.2 (measured with a WTW ph 197 Sen Tix 97 T probe). The water was filtered, aerated, and illuminated with a low intensity of light until the crayfish larvae hatched, after which a 12/12h photoperiod was applied. The animals were acclimated to the experimental salinity in steps of 1 PSU and 1.5 PSU every other day, starting from an initial salinity of 2 PSU. A recirculating system was used. Natural water with salinity of 7 PSU was pumped into aquaria directly from Puck Bay, while water with salinity of 3 PSU was prepared by diluting the 7 PSU water with tap water. Ten females with pleopodal eggs were kept at each salinity. The young crayfish were separated from their mothers after gaining independence and were individually weighed to the nearest mg one month after hatching. To assess the crayfish growth rate, 50 juveniles (10 groups of specimens from 5 females) from 3 PSU water and the same number from 7 PSU water were used. The growth rate was assessed as the mean increase in carapace length at molt. The young crayfish were weighed to the nearest 1 mg, and their total length (TL) and carapace length (CL) were measured to the nearest 0.5 mm on the basis of photographs, using the Corel Draw 11 program. The increase in carapace length of juvenile specimens was classified into four groups: < 1.0, < 1.5-2.0 >, < 2.5-3.0 > and < 3.5-4.0 > mm. The growth examination lasted 3 months. The young crayfish were fed twice a day with artificial fodder.

No loss or death of eggs were recorded in ovigerous females taken from the environment and kept at salinities of 3 and 7 PSU. Neither of the two salinities influenced the development of eggs or juvenile stages. Berried females survived the exposure to salinities of 3 and 7 PSU, while incubating their eggs and their mortality occurred only after molting. Eggs hatched into stage 1 juvenile, and all molted into stage 2 juvenile. The total number of crayfish hatchlings from 10 females was 1100 at 3 PSU and 827 at 7 PSU. The total mortality of stage 2 juvenile was 1.6% at 3 PSU and 2.5% at 7 PSU. The reduction in the number of juveniles was approximately 50% five weeks after hatching at both salinities.

One month after hatching, the young crayfish varied in length from 9.0 to 13.5 mm at 3 PSU and from 10.0 to 15.0 mm at 7 PSU. The carapace length ranged from 4.5 to 7.0 mm at 3 PSU and from 5.0 to 7.5 mm at 7 PSU. The wet weight was 18-59 mg at 3 PSU and 20-67 mg at 7 PSU. The growth (CL) was 0.5 mm greater in the crayfish at 7 PSU than at 3 PSU (Fig. 4).

Figure 4

During the experiment, 77 molts (among 50 tested crayfish) occurred in 3 PSU and 44 molts (among other 50 tested crayfish) in 7 PSU. Differences in the growth between 7 and 3 PSU were statistically significant (p < 0.05), tested with the Mann-Whitney U test.

Increases in carapace length in the class of length < 1.0 mm represented the highest percentage, 58.4% at 3 PSU and 34.1% at 7 PSU, respectively. It was similar in the class < 1.5-2.0 > mm in the same salinities – ca 32%. At salinity of 7 PSU in the classes < 2.5-3.0 > and < 3.5-4.0 > mm, it was 25% and 6.8%, respectively, and was greater than at 3 PSU (9.1% and 1.3%, respectively) (Fig. 5).

Figure 5

The results showed that the embryonic development, hatching and the development of juveniles were normal at both salinities (3 and 7 PSU), and the body size of individuals was greater at 7 than at 3 PSU.

The spiny-cheek crayfish is capable of reproducing at the age of 1 + and the body length of 5-6 cm (Crome 1955). Mating usually takes place in autumn (Van den Brink et al. 1988; Holdich et al. 2006). Sometimes it takes place in spring (Strużyński 2000). Females do not extrude their eggs after the autumn mating season, waiting for the spring season. After the mating season, crayfish are hidden during daytime and generally less active (Buřič et al. 2009). The female carries from 250 to 400 eggs, and about 100 of them produce hatchlings (Leńkowa 1962; Jaszczołt 2013). According to Holdich et al. (2006), O. limosus can produce over 400 eggs. It is positively correlated with the body size and ranges between 31 and 555 eggs (Pieplow 1938; Kozák et al. 2006). Linear relationships between the female size and ovarian fecundity, pleopodal fecundity and production of juveniles at the 3^rd fecundity stage is observed (Kozák 2009). The production of eggs by invasive females significantly increases at the active front of invasion. Invasive crayfish that carries the deadly crayfish plaque can reduce the population of indigenous crayfish. Invasive females can use the available resources to enhance their fecundity. O. limosus females closer to the invasion front produce significantly more eggs that are smaller in size (Pârvulescu et al. 2015). Laboratory studies performed in waters with salinities of 3 and 7 PSU indicate that the reproductive success of O. limosus at these salinities can be much more than 100 young (Jaszczołt & Szaniawska 2011). Unlike fresh water, saline water has an antiseptic effect, which can contribute to the survival of a greater number of offspring in brackish waters. In the latter, the reproductive success is often reduced by various kinds of fungi that attack the eggs developing beneath the female’s abdomen. It is not unlikely that crayfish in brackish waters are less exposed to pathogens, but it is also possible that others factors will appear, which can affect the overall condition of adults and/or developing eggs. The egg incubation period in O. limosus is 5-6 weeks (Krzywosz et al. 2014). According to Kozák et al. (2006), incubation of eggs lasts only 45 days, which gives a substantial advantage over the European native crayfish, whose incubation period is much longer (ca 8 months), as it sustains fewer losses during the embryonic development (Skurdal & Taugbøl 2002). During the mating season, the movement did not correlate with the water temperature and crayfish were active during daylight hours. The effect of water temperature on the movement was observed during the non-reproductive period (Buřič et al. 2009).

O. limosus success is due to its ability to reproduce by facultative parthenogenesis, in two mating seasons (autumn and spring). Females successfully reproduce using a long-term sperm storage. O. limosus spreads owing to its reproductive plasticity (Buřič et al. 2013).

Water salinity is one of the more important factors affecting the occurrence of aquatic species in newly colonized waters. Adults of the crustaceans Eriocheir sinensis H. Milne Edwards, 1853 and Carcinus maenas (Linnaeus, 1758) occur in the coastal waters of the Baltic Sea, but the low salinity < 20 PSU) prevents them from reproducing (Anger 1990; Anger et al. 1998; Panning 1952).

The salinities of 3 and 7 PSU are often quoted as being critical for many aquatic species. Kinne (1971) states that eggs, embryos and reproducing adults are particularly sensitive to salinities of 5-8 PSU. The fact that 100% of the reproducing females survived and that no eggs were deformed or died at 3 and 7 PSU indicates that O. limosus is capable of adapting to low salinities. At these salinities, early juvenile stages of the spiny-cheek crayfish molted into the next juvenile form. Holdich et al. (1997) reproduced P. leptodactylus and P. leniusculus at 7 PSU. The first juvenile stage developed into the second stage, but this survived for only two weeks. At higher salinities (14 and 21 PSU), the majority of eggs lost their color and their development ceased. The crayfish that did hatch died shortly thereafter. These results show that compared to the native European crayfish and to the North American signal crayfish, O. limosus is able to better adapt to life in brackish waters, already in the early developmental stage.

This study has demonstrated that at salinities of 3 and 7 PSU, ovigerous females can survive, embryos develop normally, the species has better reproductive success than that described in the literature, and juveniles undergo successive molts in a regular way. These results provide a broad insight into considerable adaptive capabilities of O. limosus in brackish waters, already in the early stage of ontogenesis. They may indicate that O. limosus can reproduce in brackish waters with salinity of up to 7 PSU or probably even higher. Wherever the salinity does not exceed 3-5 PSU, effective reproduction is possible and juveniles are able to develop. As an omnivorous species in a new environment, the search for food should not be a factor preventing its further expansion. The body size is another important feature testifying to an animal’s condition and adaptation to an environment (Guan & Wiles 1999). In the context of the reproduction rate of the spiny-cheek crayfish in fresh and brackish waters with salinity of 3 and 7 PSU, it is quite clear that young crayfish achieve the largest sizes at salinity of 7 PSU (y = 0.3893 × 2.55), somewhat smaller at 3 PSU (y = 0.1862 × 2.95) (Jaszczołt & Szaniawska 2011) and the smallest ones in fresh waters (y = 0.0346 × 3.20, R² = 0.999) (Orzechowski 1984). In the case of Procambarus clarkii (Girard, 1852), salinity of up to 12 PSU stimulates the growth (Sharstein & Charfin 1979). On the other hand, Loyacano (1968) states that salinities of 10 and 20 PSU reduce the growth of individuals of this species; likewise, salinities from 5 to 18 PSU also reduce the growth of P. leniusculus. Salinities above 6 PSU inhibits and salinities below this level stimulate the growth of Cherax destructor Clark, 1936 (Mills & Geddes 1980). Presumably, the evolutionary history and the course of osmoregulation will govern the growth rates of different crayfish species in brackish waters (Mills & Geddes 1980).

In adults, the change in salinity to 3 and 7 PSU increases the rate of waste matter excretion and significantly decreases the metabolic efficiency for food consumed at these salinities. On the other hand, there is no change in the rate at which food is consumed (Jaszczołt 2013). This study has shown that salinities of up to 7 PSU do not reduce the occurrence of this species.