John Witton John Witton, Borneo
Info
Thalamita danae is a small crab with attractive coloring, but it is very widespread and has been observed by many divers.
In 2024, the blue-speckled swimming crab became a special subject of scientific study due to its preference for mussels.
Digression:
Biological invasion is considered one of the most important direct drivers of global biodiversity decline in recent decades (Kolar and Lodge 2001; Perrings 2001; Louette 2012). Invasive species can displace or even exterminate native species through niche displacement, hybridization, and exploitation (i.e., predation and parasitism), thereby altering community structure and ecological processes (Ruiz et al. 1997; Streftaris et al. 2005).
Some of these ecological impacts can further disrupt ecosystem services and have negative economic consequences (Perrings 2001). For example, the accidental introduction of the comb jellyfish Mnemiopsis leidyi A. Agassiz into the Black Sea in 1865 led to a dramatic decline in local fish populations, causing losses of around US$250 million to the fishing industry.
Scientists Ming Fung Franco Au, Tin Yan Hui, and Gray A. Williams asked themselves whether Thalamita danae could be a more effective predator than the native mussel against the invasive black dwarf mussel Xenostrobus securis (Lamarck 1819), which is spreading rapidly in Hong Kong.
To test this hypothesis, the crabs were exposed to different temperatures and salinity levels at various stages of development, and the two mussels were also exposed to different conditions.
In addition, however, some important observations and conclusions were made. Here are some interesting results:
Depending on their physical environment, marine predators use two general strategies: energy maximization and time minimization.
Energy-maximizing predators spend more time searching for food and consume prey with acceptable profitability to maximize their net energy gain, especially in favorable environments, while time-minimizing predators spend less time hunting and obtain less energy to minimize the time they are exposed to danger and environmental stressors (Seed and Hughes 1995).
In the intertidal zone, temperature and salinity are two important physical environmental factors that determine the performance, survival, and distribution of predators and prey (Kinne 1966; Garton and Stickle 1980). The metabolism of ectothermic predators generally increases with ambient temperature, leading to increased energy requirements and enabling them to attack, process, and consume prey more quickly (Wallace 1973; Bergman 1987; Barbeau and Scheibling 1994).
Similarly, changes in salinity also cause an increase in the metabolism of predators in order to maintain internal osmotic balance, leading to an increase in energy requirements and food consumption (Taylor et al. 1977; Curtis et al. 2010).
Severe environmental stress can also indirectly reduce predation rates by limiting mobility and thus reducing the encounter rate between predator and prey (Draper and Weissburg 2022).
Xenostrobus securis has an extremely wide temperature and osmotic tolerance range of 5 to 32 °C (Morton and Leung 2015; Astudillo et al. 2017) and 1–31 ‰ (Wilson 1968), which has allowed the population to spread rapidly across the hydrological gradient from the eastern (oceanic) to the western (estuarine) waters of Hong Kong (Astudillo 2015; Lau et al. 2018), that this species is displacing the native mussel Arcuatula senhousia (Benson 1842) and increasing the mortality of the farmed oyster Magallana hongkongensis (Lam & Morton 2003), resulting in an estimated loss of more than HKD 2.4 million (approx. USD 304,000) for local oyster production (Lau et al. 2018).
In tests without choice, no significant variation in consumption rates was found between native and invasive mussels (linear mixed model, χ22 = 2.32, P > 0.05, see Table S1 for results). However, the proportion of mussels consumed decreased significantly when the crabs were exposed to a salt concentration of 15 ‰ (χ22 = 18.30, P < 0.01). At salt concentrations of 25 and 35 ‰, the crabs consumed more than 55% of their mussel prey, while at a salt concentration of 15 ‰, only about 6% of the mussel prey was eaten (Fig. 4).
The break-out time of the crabs was significantly prolonged when they were exposed to a temperature of 28 °C (χ21 = 4.21, P < 0.05, Fig. 5a). The feeding time was significantly prolonged when the crabs were exposed to a concentration of 25 ‰ (χ21 = 18.53, P < 0.01, Fig. 5b).
However, significant interactions between crab size and the three fixed factors (species, temperature, and salinity) were found in feeding time, with larger crabs consuming the native mussels more quickly, especially at 28 °C or 35‰
The invasive mussel Xenostrobus securis, on the other hand, is euryhaline and occurs in both the eastern and western waters of Hong Kong (Astudillo 2015; Morton and Leung 2015; Lau et al. 2018). Given these environmental conditions, T. danae does not appear to be a suitable biological control agent for X. securis in estuarine areas due to its low survival rate.
Water temperature therefore appears to play a less important role than salinity in determining the predation pressure of crabs on mussels. Prey preference of Thalamita danae Susceptibility to predation by crabs was similar between invasive and native mussels, although native mussels took significantly longer to be consumed. The differences in processing time can be explained by a significant difference in shell morphology between the two species: the invasive mussel has a thinner, cylindrical shell (average shell index = 0.59 mm) with straight dorsal and ventral margins (Astudillo et al. 2018; Garci et al. 2007), while the native mussel has a thicker triangular shell (shell index = 0.72 mm) with a spherical and inflated posterior margin (Astudillo et al. 2018; Morton et al. 2020, see Fig. S1). The crab therefore requires greater pressure to break the shells of the native mussel (Boulding 1984; Dudas et al. 2005). The round shell shape also makes the native mussels difficult to handle, as they can easily slip out of the crab's claws (Dudas et al. 2005). As a result, the crab may switch from the usual method of crushing the shells to the more time-consuming method of nibbling off the edges when feeding on native mussels (Fig. S5, Elner and Hughes 1978; Hughes and Seed 1981; Seed and Hughes 1995), making the invasive mussel more profitable to hunt than the native mussel.
Previous studies have shown that crab size is another important factor in prey selection (Elner and Hughes 1978; Hughes and Seed 1981). The claws of larger crabs are bigger and stronger, enabling them to open more resistant prey in less time than smaller crabs (Hughes and Seed 1981; Seed and Hughes 1995). Therefore, the size of the mussels consumed should increase with the size of the crabs (Enderlein et al. 2003). However, crab size had no effect on mussel consumption or breakage time. The effects of crab size were only evident in feeding time, with smaller crabs spending more time consuming native mussels.
Result:
The invasive mussel Xenostrobus securis, on the other hand, is euryhaline and has been found in both the eastern and western waters of Hong Kong (Astudillo 2015; Morton and Leung 2015; Lau et al. 2018).
Given the different environmental conditions, Thalamita danae does not appear to be a suitable biological control agent for Xenostrobus securis in estuarine areas due to its low survival rate.
The predatory crab Thalamita danae exerts similar predation pressure on the invasive mussel Xenostrobus securis and the native mussel Brachidontes variabilis of the same size, suggesting that the crab could prevent the establishment of Xenostrobus securis in Hong Kong.
However, variations in the physical environment strongly influence the predation pressure of crabs on invasive mussels, with the survival and consumption rates of crabs decreasing significantly at low salinity.
Although the invasive mussels can be eaten by Thalamita danae in regions with salinity levels above 15 ‰, both this and previous studies show that local predators (swimming crabs and whelks) are unable to effectively hunt the invasive mussels in more estuarine environments (Astudillo et al. 2018).
Therefore, there is an urgent need to investigate the biological control potential of other predators in estuarine and freshwater habitats to control invasive mussel populations in these environments.
Those who take the time to read the English article will find much more information about Thalamita danae and its prey.
Our heartfelt thanks for the first photo to John Witton who was able to take the great photo of Thalamita danae around Chong Samet, Thailand!
Synonyms:
Thalamita prymna var. proxima Montgomery, 1931 · unaccepted > junior subjective synonym
Thalamita stimpsoni A. Milne-Edwards, 1861 · unaccepted > junior subjective synonym
Thranita danae (Stimpson, 1858) · unaccepted > superseded combination
In 2024, the blue-speckled swimming crab became a special subject of scientific study due to its preference for mussels.
Digression:
Biological invasion is considered one of the most important direct drivers of global biodiversity decline in recent decades (Kolar and Lodge 2001; Perrings 2001; Louette 2012). Invasive species can displace or even exterminate native species through niche displacement, hybridization, and exploitation (i.e., predation and parasitism), thereby altering community structure and ecological processes (Ruiz et al. 1997; Streftaris et al. 2005).
Some of these ecological impacts can further disrupt ecosystem services and have negative economic consequences (Perrings 2001). For example, the accidental introduction of the comb jellyfish Mnemiopsis leidyi A. Agassiz into the Black Sea in 1865 led to a dramatic decline in local fish populations, causing losses of around US$250 million to the fishing industry.
Scientists Ming Fung Franco Au, Tin Yan Hui, and Gray A. Williams asked themselves whether Thalamita danae could be a more effective predator than the native mussel against the invasive black dwarf mussel Xenostrobus securis (Lamarck 1819), which is spreading rapidly in Hong Kong.
To test this hypothesis, the crabs were exposed to different temperatures and salinity levels at various stages of development, and the two mussels were also exposed to different conditions.
In addition, however, some important observations and conclusions were made. Here are some interesting results:
Depending on their physical environment, marine predators use two general strategies: energy maximization and time minimization.
Energy-maximizing predators spend more time searching for food and consume prey with acceptable profitability to maximize their net energy gain, especially in favorable environments, while time-minimizing predators spend less time hunting and obtain less energy to minimize the time they are exposed to danger and environmental stressors (Seed and Hughes 1995).
In the intertidal zone, temperature and salinity are two important physical environmental factors that determine the performance, survival, and distribution of predators and prey (Kinne 1966; Garton and Stickle 1980). The metabolism of ectothermic predators generally increases with ambient temperature, leading to increased energy requirements and enabling them to attack, process, and consume prey more quickly (Wallace 1973; Bergman 1987; Barbeau and Scheibling 1994).
Similarly, changes in salinity also cause an increase in the metabolism of predators in order to maintain internal osmotic balance, leading to an increase in energy requirements and food consumption (Taylor et al. 1977; Curtis et al. 2010).
Severe environmental stress can also indirectly reduce predation rates by limiting mobility and thus reducing the encounter rate between predator and prey (Draper and Weissburg 2022).
Xenostrobus securis has an extremely wide temperature and osmotic tolerance range of 5 to 32 °C (Morton and Leung 2015; Astudillo et al. 2017) and 1–31 ‰ (Wilson 1968), which has allowed the population to spread rapidly across the hydrological gradient from the eastern (oceanic) to the western (estuarine) waters of Hong Kong (Astudillo 2015; Lau et al. 2018), that this species is displacing the native mussel Arcuatula senhousia (Benson 1842) and increasing the mortality of the farmed oyster Magallana hongkongensis (Lam & Morton 2003), resulting in an estimated loss of more than HKD 2.4 million (approx. USD 304,000) for local oyster production (Lau et al. 2018).
In tests without choice, no significant variation in consumption rates was found between native and invasive mussels (linear mixed model, χ22 = 2.32, P > 0.05, see Table S1 for results). However, the proportion of mussels consumed decreased significantly when the crabs were exposed to a salt concentration of 15 ‰ (χ22 = 18.30, P < 0.01). At salt concentrations of 25 and 35 ‰, the crabs consumed more than 55% of their mussel prey, while at a salt concentration of 15 ‰, only about 6% of the mussel prey was eaten (Fig. 4).
The break-out time of the crabs was significantly prolonged when they were exposed to a temperature of 28 °C (χ21 = 4.21, P < 0.05, Fig. 5a). The feeding time was significantly prolonged when the crabs were exposed to a concentration of 25 ‰ (χ21 = 18.53, P < 0.01, Fig. 5b).
However, significant interactions between crab size and the three fixed factors (species, temperature, and salinity) were found in feeding time, with larger crabs consuming the native mussels more quickly, especially at 28 °C or 35‰
The invasive mussel Xenostrobus securis, on the other hand, is euryhaline and occurs in both the eastern and western waters of Hong Kong (Astudillo 2015; Morton and Leung 2015; Lau et al. 2018). Given these environmental conditions, T. danae does not appear to be a suitable biological control agent for X. securis in estuarine areas due to its low survival rate.
Water temperature therefore appears to play a less important role than salinity in determining the predation pressure of crabs on mussels. Prey preference of Thalamita danae Susceptibility to predation by crabs was similar between invasive and native mussels, although native mussels took significantly longer to be consumed. The differences in processing time can be explained by a significant difference in shell morphology between the two species: the invasive mussel has a thinner, cylindrical shell (average shell index = 0.59 mm) with straight dorsal and ventral margins (Astudillo et al. 2018; Garci et al. 2007), while the native mussel has a thicker triangular shell (shell index = 0.72 mm) with a spherical and inflated posterior margin (Astudillo et al. 2018; Morton et al. 2020, see Fig. S1). The crab therefore requires greater pressure to break the shells of the native mussel (Boulding 1984; Dudas et al. 2005). The round shell shape also makes the native mussels difficult to handle, as they can easily slip out of the crab's claws (Dudas et al. 2005). As a result, the crab may switch from the usual method of crushing the shells to the more time-consuming method of nibbling off the edges when feeding on native mussels (Fig. S5, Elner and Hughes 1978; Hughes and Seed 1981; Seed and Hughes 1995), making the invasive mussel more profitable to hunt than the native mussel.
Previous studies have shown that crab size is another important factor in prey selection (Elner and Hughes 1978; Hughes and Seed 1981). The claws of larger crabs are bigger and stronger, enabling them to open more resistant prey in less time than smaller crabs (Hughes and Seed 1981; Seed and Hughes 1995). Therefore, the size of the mussels consumed should increase with the size of the crabs (Enderlein et al. 2003). However, crab size had no effect on mussel consumption or breakage time. The effects of crab size were only evident in feeding time, with smaller crabs spending more time consuming native mussels.
Result:
The invasive mussel Xenostrobus securis, on the other hand, is euryhaline and has been found in both the eastern and western waters of Hong Kong (Astudillo 2015; Morton and Leung 2015; Lau et al. 2018).
Given the different environmental conditions, Thalamita danae does not appear to be a suitable biological control agent for Xenostrobus securis in estuarine areas due to its low survival rate.
The predatory crab Thalamita danae exerts similar predation pressure on the invasive mussel Xenostrobus securis and the native mussel Brachidontes variabilis of the same size, suggesting that the crab could prevent the establishment of Xenostrobus securis in Hong Kong.
However, variations in the physical environment strongly influence the predation pressure of crabs on invasive mussels, with the survival and consumption rates of crabs decreasing significantly at low salinity.
Although the invasive mussels can be eaten by Thalamita danae in regions with salinity levels above 15 ‰, both this and previous studies show that local predators (swimming crabs and whelks) are unable to effectively hunt the invasive mussels in more estuarine environments (Astudillo et al. 2018).
Therefore, there is an urgent need to investigate the biological control potential of other predators in estuarine and freshwater habitats to control invasive mussel populations in these environments.
Those who take the time to read the English article will find much more information about Thalamita danae and its prey.
Our heartfelt thanks for the first photo to John Witton who was able to take the great photo of Thalamita danae around Chong Samet, Thailand!
Synonyms:
Thalamita prymna var. proxima Montgomery, 1931 · unaccepted > junior subjective synonym
Thalamita stimpsoni A. Milne-Edwards, 1861 · unaccepted > junior subjective synonym
Thranita danae (Stimpson, 1858) · unaccepted > superseded combination
