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Theridion grallator

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Theridion grallator
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Araneae
Infraorder: Araneomorphae
Family: Theridiidae
Genus: Theridion
Species:
T. grallator
Binomial name
Theridion grallator
Simon, 1900[1]

Theridion grallator, also known as the Hawaiian happy-face spider, is a spider in the family Theridiidae that resides on the Hawaiian Islands. T. grallator obtains its vernacular name of “Hawaiian happy-face spider” from the unique patterns superimposed on its abdomen, specifically those that may resemble a human smiling face.[2] Its Hawaiian name is nananana makakiʻi (face-patterned spider).[3] The specific epithet grallator is Latin for "stilt walker", a reference to the species' long, spindly legs. T. grallator is particularly notable because of its wide range of polymorphisms that may be studied to allow a better understanding of evolutionary mechanisms.

Description

Female

T. grallator is a small spider with a body size less than 5 millimeters long. It has characteristically long and slender legs along with a translucent yellow body.[4] These distinctly long legs lead T. grallator to have the most divergent bodily morphology out of all the members of its clade - this unique characteristic occurred as a result of an ecological or behavioral shift.[4]

Its abdomen is often pale translucent yellow and can also contain a variety of red, white, and/or black superimposed patterns.[5] Certain morphs have a pattern resembling a smiley face or a grinning clown face on their yellow body. Each spider has a unique pattern, and the patterns differ from island to island. Some lack abdominal markings altogether.[6] Abdominal color changes from translucent yellow to green or orange, depending on diet.[7]

Color morphs

A key characteristic of T. grallator is the presence of a large variety of abdominal color morphs.[7] The ratio of unpatterned to patterned morphs are relatively constant throughout the year; the ratios are also constant between and within populations regardless of climate and elevation, indicating some form of selection is acting to maintain these proportions.[8] Although across all of the Hawaiian islands, there is a similar frequency of the discrete morphs,[9] there are different genetic bases for these morphs between islands.[10] The various morphs are assigned to a series of broad categories that characterize the abdominal color and/or its patterned patches; the categories include: Yellow, Red front, Red back, Red front and back, Red lines, Red ring, Black ring, Red/black ring, Red blob, Red/black blob, and White.[5]

Genetic factors underlying color morphs

These color polymorphisms follow simple Mendelian genetics. The most common morph is Yellow as it makes up 70 percent of populations.[10] Genetic studies of these morphs have shown that the Yellow morph, which is also known as the ‘unpatterned’ morph, is recessive to all patterned morphs. Within patterned morphs, the amount of pigment present in the abdomen is correlated with the dominance of the associated allele.[10] The alleles that are associated with black, red, or white pigments are arranged in a hierarchical structure and exhibit dominant effects.[10] In addition, unpatterned morphs are recessive to patterned morphs. Lastly, White is dominant to nearly all morphs.[10] The White morph is produced by a massive deposit of guanine below the hypodermis, which is a structure derived from the ectoderm.[11] These guanine deposits and their distribution within the body are under the control of a major gene loci in T. grallator. Guanine is found only under the red and black hypodermal pigments that form the various morph patterns.[12] Exceptions from simple Mendelian genetics have been observed; for example, White and Red lines exhibit codominance.[11] In regards to distribution of morphs amongst sexes, there appears to be no sex-linkage in these traits.[4]

Red front morph

Population structure, speciation, and phylogeny

Upon analysis of genitalia patterns, at least nine species in the Hawaiian islands have been identified to be members of the T. grallator-clade.[2] This clade is believed to have been colonized from the Americas and is closely related to the genus Exalbidion.[2]

Most of the Hawaiian Theridion are believed to be closely related except for T. actitarase, which contains a number of common traits with the related Rugathodes genus. Similar traits include the palpal organ and certain genitalia features. There is another Theridion species, which remains unnamed, that also displays features that are distinct from most Hawaiian Theridion. However, this unnamed species does contain a few characteristics that resemble the T. grallator , namely, its long legs and abdominal shape. Thus, this unnamed Theridion species may have evolved under similar evolutionary pressures as T. grallator.[2] Despite some variations in the bodily appearance of the Theridion species, there still remains a uniformity in sexual behavior. In addition, there is also a highly uniform web-building behavior and structure.[2] Overall, there has been much debate on how to organize clades and construct an appropriate phylogenetic structure of Theridiidae as work is still being done to properly classify these species.

In addition, the genetic bases of the abdominal color morphs of the T. grallator vary by island despite the actual abdominal color morphs having an identical appearance throughout the islands. On Maui, the color morphs of T. grallator were shown to have originated from one locus while those on Hawai’i were shown to have at least two unlinked loci involved in the color polymorphisms. In addition, on Maui, all polymorphisms are attributed to individual alleles while on Hawai’i, there are two pairs of color morphs that are believed to depend on one single locus that is differentially expressed in males and females.[11] One pair of these differentially expressed morphs are the Yellow and Red fronts, where the morph manifests phenotypically as Yellow in females but Red in males. Similarly, the Red blob and Red ring in Hawai’i populations are shown to have a varied manifestation between the sexes with the Red blob in females and Red ring in males.[10] In addition, these differences in phenotypes are most likely due to differential expression and not sex-linkage.[13]

Evolution and selection

The closest relatives of T. grallator are other Hawaiian species, such as T. posticatum and T. kauaiense. This "T. grallator clade" may be more closely related to the genus Exalbidion than to any other species currently classified in the genus Theridion.[14]

The evolutionary significance of the color polymorphisms of T. grallator is elusive, but there are selection pressures acting on the various morph proportions. The Yellow morph is the most common, sometimes with proportions of about 70% of the total population, with the remaining portion of the population displaying a variety of the patterned morphs. This high skew toward the Yellow morph indicates there must be evolutionary significance involved in this specific polymorphism. The predominant theory to explain this skew is predator selection. Because T. grallator resides on the underside of green leaves, the Yellow morph provides them a degree of conspicuousness under the sunlight, allowing them to better evade predators when they are situated on their residence leaves. However, there still exists advantages to the other color polymorphisms despite their lower observed frequencies. This can also be explained in terms of predation. Females benefit much more from the Yellow morph because they are largely sedentary, residing on their leaves most of the time. On the other hand, the male T. grallator is much more mobile and spends much of its time on the ground, searching for mates. Without the shield of the leaf, the Yellow morph will not always be the most beneficial to males; some rarer patterned morphs provide an increased level of conspicuousness and thus allows these males to evade predators. Thus, when the Yellow morph reaches a frequency higher than normal, the Yellow morph females may shift their preference to these conspicuous patterned males. Until this patterned morph no longer provides an advantage from predators, females will continue to place their preference on these patterned morphs.[10]

T. grallator on Clermontia arborescens

Distribution and habitat

The colonization routes of Theridion grallator on the Hawaiian archipelago. Dark purple lines indicate colonization occurring in conjunction with island age. Light purple indicates a reverse colonization. T. grallator is not present on Kauai or Niihau so colonization may have occurred from there, or the nearest continent.

T. grallator is endemic to the Hawaiian archipelago, and sparsely distributed populations have been reported from Oʻahu, Molokaʻi, Maui and the island of Hawaiʻi in rainforests at elevations of 300–2,000 m (980–6,560 ft).[4]

T. grallator inhabits wet and mesic environments.[4] Wet environments are defined as having an annual rainfall from 200 to 350 centimeters and mesic are defined as having an annual rainfall of 100 to 200 centimeters. These spiders are found in the forests of the Hawaiian Islands.[4] More specifically, they have been recorded to be found on the islands of O’ahu, Moloka’i, Maui, and Hawai’i.[15] They prefer to reside on the underside of plant leaves such as the native Broussaisia arguta and Clermontia arborescens and the introduced Hedychium coronarium. H. coronarium is a particularly tactical plant to reside on as its large, slippery leaves allow T. grallator to better evade predation.[4]

Diet

T. grallator spiders may change color depending on their diet. This color change may occur because of the translucent quality of their abdomens. Experimental studies have shown that color pigments can be retained in the abdomen for two to six days, and the color generally shifts from translucent yellow to orange.[7] Once the food is digested and excreted, the color of the abdomen returns to its original translucent pale yellow.

T. grallator spiders do not utilize webs to capture prey, so they do not follow the sit-and-wait method of web-building spiders. Instead, they will forage freely, often traveling to nearby leaves to capture insects. During prey capture, T. grallator spiders use their silk. Common prey include Dolichopodidae and Drosophilidae; there is no correlation between prey preference and resident leaf species. However, depending on the species of the resident leaf, T. grallator may exhibit different predator behavior. For example, on Hedychium leaves, these spiders are more aggressive toward prey despite often having a lower prey capture rate as compared to residence on other species of plants.[16]

Webs

T. grallator lives beneath the leaves of plants, where they spin a much reduced web.[7] The T. grallator produces a web that is two-dimensional and usually found on the underside of leaf and occasionally in the crevices of trees. T. grallator webs are often very flimsy and even tangled, which is very typical of the Theridiid spiders. T. grallator builds small webs that are much flimsier than the webs built by most Theridiidae. Webs are not highly utilized, which may be the result of evolutionary pressures of Hawaii’s climate that yielded these webs disadvantageous. The high level of rainfall damages the glue of the web’s silk threads, leading to ineffective prey capture.[17] Instead of using the web as a prey-detection medium, T. grallator detects prey through vibrations that are transmitted by the prey species through the resident leaf. Spiders are then able to discern the location and orientation of these prey.

Often, the building of small webs is associated with a specialization in prey type, but this is not observed to be the case in T. grallator. During the day, T. grallator spiders tightly cling on to underside of leaves to evade predation by gleaning birds.[18] At night, when diurnal predatory birds are asleep, these spiders will hang by silk threads under the leaf. Although T. grallator exhibits only minimal use of webs, they can use their silk to capture prey. T. grallator will sense prey based off of vibrations and will orient itself near the prey of interest, turn around rapidly and toss its silk onto the prey to unravel it. The silk consists of a sticky substance that will allow for efficient prey capture.[17] In addition, maternal T. grallator spiders may use webs to guard their egg sacs or store the prey they have caught for their young.[18]  

Reproduction and Life cycle

During the last molt of a female T. grallator, a mature male may be found to share a leaf with her. Once the female completes its molt, the male will copulate with her.[10] A few weeks after copulation, a female will deposit her egg sacs and will remain closely attached to the egg sacs by a short silk thread until the eggs have hatched. When the egg sacs are ready to hatch, the maternal female T. grallator will loosen the silk that is wrapped around the eggs to allow the spiderlings to emerge.[18]

T. grallator populations seasonally fluctuate in terms of spider size and sex make-up. During winter months, specifically October to March, there is a higher proportion of smaller sized and immature spiders. In the spring, specifically May to August, there is an increased number of adults in the population with the majority of these adults being maternal females. In fact, up to 85% of a population can consist of maternal females with egg sacs in these later months.[18]

Mating

Female/male interactions

Mature males actively move through forest vegetation seeking out females, which tend to be more sedentary. Courtship depends primarily on vibrations and olfaction.[10] For example, males may carry out a courtship dance that involves somatic movements and web-plucking; these vibrations during the courting performance are assessed by potential female mates. Copulation occurs at night, while both spiders hang from the underside of the leaf. Males die soon after mating, but females live longer, and guard their eggs until they hatch, and catch prey for their young.[18]

In addition, there has been observed to be a rare-male advantage phenomenon during mating. Females may prefer a rarer male morph due to many reasons; for example a less common morph may better evade predation. This rare morph may then be selected for and will increase in number until it no longer provides the inconspicuous advantage from predators - an example of apostatic selection, which is a type of negative frequency-dependent selection.[10] The advantage will be eliminated when predators begin to recognize this rarer pattern and thus will begin to target these patterned morphs. This phenomena of the rare-male mating advantage is believed to act more strongly on reproductive males than females because males are much more mobile during reproductive season.

Because T. grallator belongs to a family of spiders with very low levels of visual acuity, the preference for these rarer patterned morphs is not attributed to physical attractiveness but instead to this advantage from predators. In fact, due to their poor vision, males court females using vibratory and olfactory signals.[10]

Parental care

Egg guarding

A maternal female T. grallator is notably aggressive against intruders right after the birth of her young while she is guarding her egg sac.[18] She must protect her young from predation, parasitic wasps, and the possibility of the resident leaf dropping. Once the spiderlings have hatched, the maternal female will continue to defend and care for her young. The mother will demonstrate exceptional maternal care as she communally feeds all the spiderlings and protects them from predators; spiderlings remain on the same leaf with their mother for approximately 40 to 100 days. Spiderlings are unable to catch their own prey during this first period of their life and will die in the absence of the mother.[18]  The mother wraps all prey that she catches in her silk and is never observed to consume the prey itself.

This aggressive guarding behavior improves reproductive success. Observational studies show that if a maternal T. grallator dies or abandons her egg sac, the egg sac will be captured by a predator in less than a week. When a maternal T. grallator guards and remains with her egg sac, experimental studies have shown there to be a 57.2% hatching success rate. Although this value is not high, it does signify the advantage in egg sac guarding. Also, this rate shows how susceptible a maternal T. grallator and her egg sac are to predators.[18]  

Adoption

Mothers take on foster egg sacs with acceptance. When the spiderlings are transferred between broods, the new mothers are observed to ‘adopt’ these spiderlings into their family and care for them as if they were their own.[18] In nature, adoption of spiderlings may occur if the related mother has been lost. Lost mothers are generally victims to predation or old age. These spiderlings of the lost mother either leave their resident leaf by dropping down a silk thread or they will climb down the stem or stalk of the plant. The spiderlings may attempt to survive on their own but often may migrate to other leaves and join another brood - mothers are very receptive in adopting spiderlings, regardless of the color morph. In addition, the lack of competition within a brood contributes to the ease of acceptance of adopted spiderlings.[18]

Parent-offspring conflict

Parent-offspring conflict, an idea first introduced by Robert L. Trivers in 1974, may be observed in the costs of mothers guarding their spiderlings. When a maternal female T. grallator has a second brood, she must remain with the first brood for a period of time after hatching because of the spiderlings' inability to feed themselves. Thus, the second brood may be compromised due to the need for parental investment by the first brood.[18]

Social behavior

Adult females are usually found to be sedentary and located on the underside of leaves while males are often more mobile as they may move about in the search of mates. Thus, due to male mobility, they often become more conspicuous to predators.[10] Gravid females and females guarding egg sacs will never share a leaf with other adult T. grallator.[18]

There has been no observation of competition for food resources between members of the same brood. Siblicide and cannibalism have not been observed either.[18]

Enemies

T. grallator experiences high rates of parasitism by wasps in the Baeus genus.[19] These wasps have been found to parasitize others spiders as well including Clubiona robusta.[18] Parasitism contributes to a high rate of egg mortality. A possible explanation for the high rates of parasitism to these spiders may be the wasp's small egg size.[18] Mothers may have a hard time detecting if their egg-sacs have been parasitized. Baeus parasitic behavior occurs even when the mother guards her eggs.

T. grallator with the unique pattern on abdomen

Protective coloration and behavior

It has been hypothesized that the variety of polymorphisms present in T. grallator allows an evolutionary benefit to evade predation. Spiders with depigmentation or polymorphic colors and patterns can avoid predation by birds that use a search image, when scanning for prey. A search image may be a particularly abundant color morph, and predators will use this as an identification of possible prey.[7]

T. grallator hunts mainly from dusk, through the evening, to dawn. By day, the spiders assume a remarkable posture that flattens the body against the residence leaf, with legs splayed out and all joints held flat against the leaf.

References

  1. ^ "Taxon details Theridion grallator Simon, 1900", World Spider Catalog, Natural History Museum Bern, retrieved 2016-01-30
  2. ^ a b c d e Arnedo, Mquel A.; Agnarsson, Ingi; Gillespie, Rosemary G. (July 2007). "Molecular insights into the phylogenetic structure of the spider genus Theridion (Araneae, Theridiidae) and the origin of the Hawaiian Theridion-like fauna". Zoologica Scripta. 36 (4): 337–352. doi:10.1111/j.1463-6409.2007.00280.x. ISSN 0300-3256.
  3. ^ Hawaiian Dictionaries
  4. ^ a b c d e f g Gillespie, Rosemary G; Tabashnik, Bruce E (August 1990). "Maintaining a happy face: stable colour polymorphism in the spider Theiridion grallator (Araneae, Theridiidae)". Heredity. 65 (1): 67–74. doi:10.1038/hdy.1990.71. ISSN 0018-067X.
  5. ^ a b Gon, Samuel M. (1985). Comparative Behavioral Ecology of the Spider Theridion Grallator (Araneae: Theridiidae) in the Hawaiian Archipelago. University of California, Davis.
  6. ^ Rosemary G. Gillespie & Bruce E. Tabashink (1989). "What makes a happy face? Determinants of colour pattern in the Hawaiian happy face spider Theridion grallator (Araneae, Theridiidae)". Heredity. 62 (3): 355–363. doi:10.1038/hdy.1989.50.
  7. ^ a b c d e Gillespie, Rosemary G. (1989). "Diet-Induced Color Change in the Hawaiian Happy-Face Spider Theridion grallator, (Araneae, Theridiidae)". The Journal of Arachnology. 17 (2): 171–177. ISSN 0161-8202.
  8. ^ Gillespie, Rosemary G.; Oxford, Geoffrey S. (1998). "Selection on the Color Polymorphism in Hawaiian Happy-Face Spiders: Evidence from Genetic Structure and Temporal Fluctuations". Evolution. 52 (3): 775–783. doi:10.1111/j.1558-5646.1998.tb03701.x. ISSN 1558-5646.
  9. ^ Gillespie, Rosemary G.; Oxford, Geoffrey S. (June 1998). "Selection on the Color Polymorphism in Hawaiian Happy-Face Spiders: Evidence from Genetic Structure and Temporal Fluctuations". Evolution. 52 (3): 775. doi:10.2307/2411271.
  10. ^ a b c d e f g h i j k l Oxford, Geoff S.; Gillespie, Rosemary G. (2001). "Portraits of Evolution: Studies of Coloration in Hawaiian Spiders". BioScience. 51 (7): 521. doi:10.1641/0006-3568(2001)051[0521:POESOC]2.0.CO;2. ISSN 0006-3568.
  11. ^ a b c Oxford, G S; Gillespie, R G (March 1996). "Quantum shifts in the genetic control of a colour polymorphism in Theridion grallator (Araneae: Theridiidae), the Hawaiian happy-face spider". Heredity. 76 (3): 249–256. doi:10.1038/hdy.1996.38. ISSN 0018-067X.
  12. ^ Oxford, G.S. (1997). "Guanine as a colorant in spiders: development, genetics, phylogenetics and ecology". Proceedings of the 17th European Colloquium of Arachnology.
  13. ^ Knoflach, Barbara (April 1998). "Mating in Theridion varians Hahn and related species (Araneae: Theridiidae)". Journal of Natural History. 32 (4): 545–604. doi:10.1080/00222939800770301. ISSN 0022-2933.
  14. ^ Miquel A. Arnedo, Ingi Agnarsson & Rosemary G. Gillespie (2007). "Molecular insights into the phylogenetic structure of the spider genus Theridion (Araneae, Theridiidae) and the origin of the Hawaiian Theridion-like fauna" (PDF). Zoologica Scripta. 36 (4): 337–352. doi:10.1111/j.1463-6409.2007.00280.x.
  15. ^ Oxford, G S; Gillespie, R G (March 1996). "Genetics of a colour polymorphism in Theridion grallator (Araneae: Theridiidae), the Hawaiian happy-face spider, from Greater Maui". Heredity. 76 (3): 238–248. doi:10.1038/hdy.1996.37. ISSN 0018-067X.
  16. ^ Gillespie, Rosemary G.; Tabashnik, Bruce E. (1994-11-01). "Foraging Behavior of the Hawaiian Happy Face Spider (Araneae: Theridiidae)". Annals of the Entomological Society of America. 87 (6): 815–822. doi:10.1093/aesa/87.6.815. ISSN 0013-8746.
  17. ^ a b Gillespie, Rosemary G.; Tabashnik, Bruce E. (1994-11-01). "Foraging Behavior of the Hawaiian Happy Face Spider (Araneae: Theridiidae)". Annals of the Entomological Society of America. 87 (6): 815–822. doi:10.1093/aesa/87.6.815. ISSN 0013-8746.
  18. ^ a b c d e f g h i j k l m n o Gillespie, Rosemary G. (April 1990). "Costs and Benefits of Brood Care in the Hawaiian Happy Face Spider Theridion grallator (Araneae, Theridiidae)". American Midland Naturalist. 123 (2): 236. doi:10.2307/2426552.
  19. ^ MARGARÍA, CECILIA B.; LOIÁCONO, MARTA S.; GONZAGA, MARCELO O. (2006-03-30). "Two new species of Baeus (Hymenoptera: Scelionidae) from Southeastern Brazil parasitoids of Anelosimus (Araneae: Theridiidae)". Zootaxa. 1162 (1): 45. doi:10.11646/zootaxa.1162.1.4. ISSN 1175-5334.
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