Oak Tree and Sneaky Gall Wasps

The Gall wasps are a member of the Cynipoidea family distributed through Europe and North America, typically varying in size between 1 to 8 mm. The wasp is entwined in a parasitic symbiotic relationship with the oak tree. They may infect multiple species of trees though here we will be focusing on the oak tree. A single oak tree can be infested with up to 70 different species of gall wasp which can be harmful to the oak tree. (Askew, 1984)

In spring the female wasps search for sites in which to inject their eggs. The base of a successfully fertilised oak tree flower is the ideal location for a female wasp to inject her eggs; the female thrusts her ovipositor into the base of the flower and injects only one egg. The genetics of the acorn are affected by the young wasps as immature acorns grow into galls. The gall is a protective growth that is caused by the immature larval secretions and form a large array of distorted structures. Each species of wasp has characteristically different shape and sized galls which the young larvae feed on; this causes the galls to become shrivelled and brown.

Figure 1. Acorn infected with a single mature gall. Photographer: Justine Aw (2013)

Autumn promotes the loss of the oak trees leaves and acorns along with the galls. The winter following brings harsh freezing condition covering the galls in ice until the beginning of spring. The ice melts and the adult gall wasps emerge after 9 months trapped inside and only live for a few weeks to seek another oak to lay eggs (Stone et al., 2002).

Another type of parasitic gall wasp Megastigmus trisulcus has evolved to take advantage of this phenomenon and instead of producing its own galls it steals them from other species of gall wasp. It is specialised with a highly equipped drill, the tip being covered in metallic zinc for a sharp cutting edge. It is then inserted into the gall, directly into the larvae of the other wasp with a microscopic egg injected into the host larvae. The parasitic larvae then eventually kills the host and take its place in the gall for the duration of the seasons until spring and emerges as the host would have (Flett et al., 2011).

This is a great example of evolutionary adaptation and survival success. One can only imagine the kind of environmental pressures that would have prompted such a specialised piece of equipment. It is clear that these three species have a high dependency on the others for their own survival.

References:

ASKEW, R. 1984. 8. The Biology of Gall Wasps.

FLETT, H., AUSTRALIA, H. & BOARD, M. V. C. 2011. Establishment of Citrus Gall Wasp Parasites in the Murray Valley Region, Horticulture Australia.

STONE, G. N., SCHÖNROGGE, K., ATKINSON, R. J., BELLIDO, D. & PUJADE-VILLAR, J. 2002. The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual review of entomology, 47, 633-668.

Figure 1. AW, J. 2009.  Acorn infected with a single mature gall. Accessed 22/03/2015 from http://www.notcot.com/archives/2013/08/gall-wasp.php

Video with thanks to BBCWorldwide. https://www.youtube.com/watch?v=CzXccvoJThI

Cunning Crabs

Survival in the marine environment is a daily struggle for a large range of prey item species. The pressures placed against sea creatures drive them to evolve and adapt skills and various methods of survival. A species of hermit crab (Eupagurus bernhardus) has formed a symbiotic relationship with a species of sea anemone (Calliactis parasitica) as a strategy of perception and protection against predators. These omnivorous detritivores range from the arctic water of Iceland, extending south as far as Portugal occupying small rock pools and oceanic waters to depths of 140 meters; larger individuals tend to be located in deeper waters (Hazlett, 1981).

Figure1. Hermit crab Eupagurus bernhardus paired with Calliactis parasitica. Photographer: Anonymous (2004)

Hermit crabs are a tasty treat to hungry predators such as the octopus which exhibits a large bird like beak with extreme crushing force that can pierce the shell of a hermit crabs with ease. Hermit crabs have formed an unsuspecting mutualistic relationship with the anemones which was first distinguished by scientists as parasitism, but later corrected to mutualism. The reason this relationship is successful is the sting of the anemone which deters the octopus from further attempting to eat the crab. Once the tentacles trap the hermit crab and anemones, they receive a nasty deterring sting, and the octopus lets go and the crab remains unharmed. These two species are considered permanent partners once paired, and is uncommon to see them apart from one another (Ross, 1960).

Figure2. Hermit crab camouflaged by anemones.Photographer: Anonymous (2006)

These cunning crabs poses perfected methods of persuasion; a coaxing tickle is used to release the anemone’s pedal disc which is used to lock onto substrates and surfaces, (McFarlane, 1969). The crab then strategically places the anemone onto its shell until it re-adheres ensuring an evenly distributed and balanced weight is maintained on its shell; it is common for the exterior of the shell to be covered in multiple anemones for maximised protection and camouflage. As the hermit crabs increase in size they require larger accommodating shells and transfer these anemones across to new homes. (Brooks, 1989)

The anemones greatly benefit from this relationship from the mobility factor allowing access to a larger range of food sources. The anemones main source of food are scraps from the hermit crab as they float past. The below video demonstrates the crabs ability to acquire anemones and its defensive benefits.

http://www.tubechop.com/watch/5401055 

References:

BROOKS, W. R. 1989. Hermit crabs alter sea anemone placement patterns for shell balance and reduced predation. Journal of Experimental Marine Biology and Ecology, 132, 109-121.

HAZLETT, B. A. 1981. The behavioral ecology of hermit crabs. Annual Review of Ecology and Systematics, 1-22.

MCFARLANE, I. 1969. Co-ordination of pedal-disk detachment in the sea anemone Calliactis parasitica. Journal of Experimental Biology, 51, 387-396.

ROSS, D. 1960. The association between the hermit crab Eupagurus bernhardus (L.) and the sea anemone Calliactis parasitica (Couch).  Proceedings of the Zoological Society of London. Wiley Online Library, 43 57.

Figure 1. Anonymous (2004). Hermit crab Eupagurus bernhardus paired with Calliactis parasitica. http://wildlife-archipelago.gr/wordpress/marine-inverts/common-hermit-crab-pagurus-bernhardus/, retrieved 10/03/2015.

Figure 2. Anonymous (2006). Hermit crab camouflaged by anemones. www.alamy.com, retreived 10/03/2015.