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Biology

Parasitic crabs steal food and the possibility of offspring from unsuspecting snails.  

 

The Paper:

Ocampo, E. H.; Nuñez, J. D.; Cledón, M.; Baeza, J. A. Parasitic castration in slipper limpets infested by the symbiotic crab Calyptraeotheres garthi. Mar Biol, 2014, 161: 2107-2120. DOI: 10.1007/s00227-014-2490-y

Background

Parasites vary as much as their hosts, causing effects ranging from death or severe sickness to behavioral changes or no noticeable impact at all. Common marine parasites include various barnacles, isopods or trematodes. Even unassuming crabs can be parasites in the marine world. This paper focuses on one species of parasitic pea crab Calyptraeotheres garthi, which live within the shells of sea snails. When snails have resident crabs they do not reproduce but the reason why is still a mystery. Parasites that affect the ability of the host to reproduce are known as parasitic castrators.

 

figpiclimpet

Three species of sea snails used in the field study.

 

Ocambo et al. explore two possible explanations through which the pea crab halts reproduction of a host sea snail: 1) energy drain and 2) interference. The researchers used three wild species of a sea snail known as the slipper limpet for field studies and one species for lab studies (Crepidula cachimilla).

Energy drain refers to parasites consuming so much of the host’s nutrients their bodies are not able to reproduce, an energetically demanding process. Interference refers to the phenomenon of parasites physically stopping host reproduction. In this case the crabs occupy the same space the snails use to hold embryos, potentially causing direct interference.

 

Methods

The authors used a variety of approaches to answer the question: How do pea crabs castrate their hosts?

Is the pea crab truly a parasitic castrator?

The authors collected wild limpets and noted the presence or absence of a parasitic crabs or embryos.   In a lab study the experimenters removed or added parasitic crabs from limpets and observed the reproductive activity of the limpets over 90 days.

How do the crabs stop reproduction?

After establishing the existence of parasitic castrators the authors investigated potential mechanisms through which the crab may impact limpet reproduction- physical interference or nutrient deprivation. The experimenters observed the feeding behavior of the crabs and the snails in the lab and measured the size or the snails with and without parasites.

 

Results

Above: proportion of limpets of that spawned and brooded eggs during the entire experimental period, which lasted 90 days in limpets with and without female crabs, and 20 days in limpets stripped of female crabs. Below: ventral view of limpets brooding eggs (on left, arrow points to egg masses) and harboring a mature female crab (on right, arrow points to the crab). Scale bars 0.5 cm

Above: proportion of limpets of that spawned and brooded eggs during the entire experimental period, which lasted 90 days in limpets with and without female crabs, and 20 days in limpets stripped of female crabs. Below: ventral view of limpets brooding eggs (on left, arrow points to egg masses) and harboring a mature female crab (on right, arrow points to the crab). Scale bars 0.5 cm

This study found parasitic pea crabs in three wild limpet species and showed that while a pea crab lived within the limpet

they did not brood embryos. Similarly, in lab studies limpets with crabs were never seen to brood embryos either. Interestingly, limpets that had their crabs removed sometimes regained reproductive abilities.

Evidence for Energy Drain:

In the lab pea crabs were observed feeding directly on the mucous-phytoplankton strings formed during limpet feeding. This demonstrates that pea crabs may indeed be effectively stealing food from the limpet hosts, providing supporting evidence for the energy drain hypothesis. On the other hand the authors did not find clear patterns of lower physical condition in parasitized limpets so other explanations must be investigated.

 

Evidence for interference:

When pea crabs are removed from limpets the hosts can promptly regain their ability to reproduce. This observation indicates that poor physical condition may not be the only explanation halting reproduction because the limpets did not need a lengthy recovery time to regain their energy.

 

Relationship between soft body weight and shell length in C. cachimilla harboring and not harboring pea crabs. The size range in which there was a significant effect of the crab on the host condition is highlighted in gray.

Relationship between soft body weight and shell length in C. cachimilla harboring and not harboring pea crabs. The size range in which there was a significant effect of the crab on the host condition is highlighted in gray.

 

 

 

Parasitized snails did not show differences in size (a proxy for overall physical condition) during spring months. During the winter months parasitized snails are smaller but during summer months snails that are home to crabs are significantly larger than their parasitized sibling snails. This pattern does not show overall lower physical condition and actually supports the concept of interference. During summer months reproducing individuals are using a lot of energy brooding and spawning eggs, parasitized individuals are eating the same amount but not using energy for reproductive tasks so have more to simply grow larger.

In conclusion, the authors found more support for the interference hypothesis. The presence of the crab is enough to halt reproduction and when removed the snail can return to normal reproductive business.

Significance

Parasites are common among all organisms and their effects can be widespread and difficult to understand. Parasitic castration is an interesting phenomenon and understanding the mechanisms behind it could help in conservation issues in species where parasitism may be a serious problem.

 

Article featured image by Vicki Burton.

Discussion

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