Enoplognatha ovata & E. latimana
Distribution and habitat:
Both species are usually associated with low-growing vegetation and shrubs; occasionally the lower branches of trees. In continental Europe the two species are frequently found occupying the same habitat as mixed-species populations. In the British Isles, however, E. latimana seems to be confined to more open, hotter and drier environments such as sand dunes, coastal cliffs and heathlands (e.g. in the Breckland of East Anglia). (Other, similar species, such as E. penelope, also occur on the continent.)
General life history:
Genetic variation and its implications:
Enoplognatha ovata has been the subject of a number of studies over the last 80 years, most of which have concerned the striking colour and pattern variation found in virtually all populations (see (ref.) for a list of historical publications). The variation comprises three major forms (morphs): lineata with a plain yellow/cream opisthosoma, redimita which has, in addition, a pair of dorsolateral carmine stripes and ovata in which the entire dorsal surface is carmine. These morphs are inherited and comprise a genetic polymorphism. The vast majority of populations contain both the lineata and redimita morphs; the ovata morph is more sporadic. Black spotting and when, during development, the red pigments appear in morphs redimita and ovata are also under separate genetic control. All of this genetic variation is also found in E. latimana, implying that it was present in the common ancestor of the two species. The redimita colour morph is often at a lower frequency in E. latimana, and the ovata morph especially so.
Most genetic work has focused on the colour variation in E. ovata in an attempt to elucidate what controls the frequencies of the three morphs within populations. The evidence that the polymorphism is maintained by natural selection is overwhelming, although there can be great variation in morph frequencies between populations only tens of metres apart, some of which is a result of pure chance. The adaptive significance of the polymorphism is unknown but may relate to the way predators search for prey. There are two principal mechanisms that could operate. First, predators might develop a search image for prey with a particular appearance and, at the same time, ignore other prey that look different (a process called apostatic selection). Thus, if lineata is common in a population predators (such as birds) would come across this colour morph frequently and learn to associate it with food. They would actively seek out lineata individuals but at the same time tend to overlook other morphs, such as redimita. So lineata is selected against and falls in frequency and redimita is selected for and becomes more numerous. At some stage, when redimita is very common, predators will switch their search image and actively seek out this morph as food, consequently under-predating lineata. In this way both colour morphs will be maintained in the population but at constantly varying frequencies.
A second mechanism that can select for variability in populations involves dietary wariness. Like humans faced with a bowl of bright purple mashed potato, predators may reject a novel prey item (for example, a colour morph they haven't seen before) for the first few encounters. However, over time, they will get used to this new morph and eventually fully incorporate it into their diets. Computer simulation models show that if novel colour morphs are rejected at the first encounter but from the second encounter onwards are fully incorporated into the diet, very large number of morphs can be maintained.
Remarkably, distantly related species within the family Theridiidae have independently evolved polymorphisms that include colour morphs virtually identical to those exhibited by E. ovata and E. latimana. This is probably a consequence of selection for individuals looking distinct from one another (as explained above) plus developmental constraints on the range of colour patterns available.
Geoff Oxford - July 2013
(All photos by Geoff Oxford)
- Roberts, M. J. (1995) Collins Field Guide to Spiders of Britain and Northern Europe. Harper Collins, London, UK.
- Hippa, H. & Oksala, I. 1982. Definition and revision of the Enoplognatha ovata (Clerck) group (Araneae: Theridiidae). Insect Systematics & Evolution, 13: 213-222
- Oxford, G. S. & Reillo, P. R. 1994. The world distributions of species within the Enoplognatha ovata group (Araneae: Theridiidae): implications for their evolution and for previous research. Bull. Br. Arachnol. Soc., 9: 226-232. Download pdf of paper.
- Oxford, G. S. & Reillo, P. R. 1993. Trans-continental visible morph-frequency variation at homologous loci in two species of spider, Enoplognatha ovata s.s. and E. latimana. Biol. J. Linn. Soc., 50: 235-253. Download pdf of paper.
- Oxford, G. S. 1992. Enoplognatha ovata and E. latimana: a comparison of their phenologies and genetics in Norfolk populations. Bull. Br. Arachnol. Soc., 9: 13-18. Download pdf of paper.
- Oxford, G. S. 2005. Genetic drift within a protected polymorphism: enigmatic variation in color-morph frequencies in the candy-striped spider, Enoplognatha ovata. Evolution, 59: 2170-2184. Download pdf of paper.
- Franks, D. W. & Oxford, G. S. 2009. The evolution of exuberant visible polymorphisms. Evolution, 63: 2697-2706. Download pdf of paper.
- Oxford, G. S. (2009) An exuberant, undescribed colour polymorphism in Theridion californicum (Araneae, Theridiidae): implications for a theridiid pattern ground plan and the convergent evolution of visible morphs. Biol. J. Linn. Soc., 96: 23-34.
Note: BAS cannot verify the accuracy of any information on any external links
Links to other sites with info on or images of Enoplognatha ovata or E. latimana:
- Jorgen Lissner's Theridiid pages: http://jorgenlissner.dk/Theridiidae.aspx
- Ed Nieuwenhuys' Araneid pages: http://ednieuw.home.xs4all.nl/Spiders/Theridiidae/Theridiidae.htm
- Czech spiders - E. ovata: http://www.pavouci-cz.eu/Pavouci.php?str=Enoplognatha_ovata
- Czech spiders - E. latimana: http://www.pavouci-cz.eu/Pavouci.php?str=Enoplognatha_latimana
- Spiders of Europe - E. ovata: http://www.araneae.unibe.ch/data/951/Enoplognatha_ovata
- Spiders of Europe - E. latimana: http://www.araneae.unibe.ch/data/681/Enoplognatha_latimana