General Characteristics

Adults usually grow up to around 1cm (body length). ⁴

Figure 2: Photo of Desis martensi specimen with anatomy labelled.

Photo credits: Ria Tan. Permission Granted. Annotated by Kieron Gabriel Ng.

Its cephalothorax and chelicera are smooth and coloured with a deep maroon hue, while its abdomen and eight legs are covered in numerous fine hairs and are grey in colour, with an occasional pinkish tint.⁴

Etymology

The species is named after German marine zoologist Dr Eduard von Martens, who discovered and collected the species in Singapore in 1861. It was then described by German entomologist and arachnologist Dr. Ludwig Carl Christian Koch in 1872. ¹ Desis martensi has no official common name listed, however it has been called the 'marine spider' or 'reef spider' by various sources (although naming across sources is inconsistent). ^{4 5}

Distribution

Desis martensi is native to Singapore, with its distribution limited to the Singapore strait. They are the only marine spider in Singapore. ⁵

Figure 5: Flickr map illustrating recent locations on Singapore where Desis martensi was photographed.

Taken from Wild Singapore's Flickr.com tags. Edited by Kieron Gabriel Ng.

Desis martensi is mostly sighted on the southern shores of Singapore: Sentosa (where it was first discovered), Labrador Nature Reserve's rocky shores, and the Southern Islands, with one recent sighting on Pulau Ubin. Additionally there were a few older records of sightings in the other areas along the coasts of Singapore: Tuas, East Coast Park (Not shown). 

One source has also claimed that they have been sighted along some coastlines in the Indonesian archipelago. However, this is unverified.⁵   

Conservation

Status

Desis martensi has not yet been evaluated by the IUCN red list.

However, it is listed as 'Vulnerable' in the Singapore Red Data Book 2008. ³

The species itself has little to no direct interaction with human beings, apart from being spotted by people exploring the intertidal zone. However, it is indirectly affected by the actions of human beings. The main threats to Desis martensi populations include land reclamation and beach enhancement projects. ³ As of 2018, many of the locations that these spiders have been sighted at have been closed off to the public such as the Labrador Park rocky shores, or Sentosa Reefs.

Importance to Singapore

Very few scientific studies have been carried out on Desis martensi. However, it holds great scientific importance as one of the few marine spider species present, and insights could also be gleamed from studying it's developmental processes or morphological adaptations to surviving in the intertidal habitat. ⁶ Thus, it is important that we protect the habitats that this spider is found in and attempt to reduce the impact of detrimental anthropogenic effects on populations such as land reclamation or pollution. 

Potential Future Efforts

As this is one of the few marine spiders in the world, and is a marine spider found exclusively in Singapore, efforts to raise public and stakeholder awareness would prove to be quite successful. As Desis martensi does not have an official common name, giving the spider a Singapore-related common name such as 'Singapore Marine Spider' or 'Singapore Reef Spider' might raise national interest and aid in conservation efforts as well.  

As the conservation status was only evaluated locally in the Singapore Red Book, it could also be useful for IUCN to conduct an evaluation as well. 

Habitat

Desis martensi is classified as a marine spider and can be found in the inter-tidal zone, in habitats such as rocky shores or coral reefs, hiding inside hollow corals or rocks, which they seal up using waterproof silk, during high tide.⁷ These holes are thought to be a result of Lithophaga sp. (date mussels), which bore into rock or corals.¹

Desis martensi are thought to not have permanent 'nests', and might switch holes depending on prey availability. Bristowe, 1931 proposed that they would simple drag prey to the nearest available Lithophaga sp. hole after capture. ⁷

During low tide, they are often found scurrying around the surface of corals or rocks, or scuttling across the water surface, trying to find prey before the tide rises again. ¹

Like all spiders, Desis martensi also possesses spinnerets and is able to spin webs. However, rather than for prey capture, Desis martensi utilize their webs to line the inside and outside of their nests in corals or rocks, or for sealing up holes during high tide to create air chambers.¹

Furthermore, although Desis martensi is found in the intertidal habitat, and is considered to be a marine spider, it does not have the ability to respire by taking in dissolved oxygen from seawater like many other aquatic arthropods do. Thus it relies on creating air pockets or air chambers in hollow rocks or corals with waterproof webbing in order to survive high tide.^{8 9 1} However, it has been observed to immediately gather an air bubble when suddenly submerged in order for respiration. Similarly, fully submerging the spider will cause it to panic and attempt to reach the surface.^{10 9}

Biology

Feeding Habits

Desis martensi, like all spiders, are carnivores, and have been observed to consume small marine invertebrates the are abundant across the intertidal zone, such as sea slaters (Figure 6 and 7). It has also been observed venturing onto the shore to hunt prey such as crickets (Figure 8).  ⁴ They have also been observed to consume amphipods and small crabs. ⁷

These spiders are venomous, and are able to inject a potent toxin into their targets which paralyses them, making it easier to carry them off to nearby holes for safe consumption.¹¹

Predators

It is unknown what organisms prey on Desis martensi. However, some reports have shown that they are possibly eaten by octopi. ⁷

Locomotion

Desis martensi have been observed to scurry about like normal spiders on solid substrate or in shallow water (Figure 9). They possess high manoeuvrability and speed that comes with octopedal locomotion, which is useful for traversing the uneven terrain of the intertidal habitat.¹² They have also been observed to jump from rock to rock if the tide gets too high. ⁷

Figure 9: Video of active Desis martensi specimens observed on St John's Island in Singapore.

Video credit: Kieron Gabriel Ng.

Furthermore, Desis martensi are able to effortlessly skate across the surface of the water due to the presence of long hairs on each of their legs that prevent them from breaking the surface tension of the water (Figure 10).⁷

Reproduction

Desis martensi have a complex mating ritual where the males engage in 'sparring' behaviour with females. ¹³ Males aim to grip the chelicerae of the females with their own chelicerae, and pivot her onto her back, whereby he will mount her and begin the insemination process; inserting both palps successively to inject sperm (Figure 11).¹³

Figure 11: Drawing of Desis martensi copulation position from Bristowe, 1931. The male spider (above) is shaded in black while the female spider (below) is in white.

After mating is completed, females hide in a crevice, spin a thick waterproof silk cocoon, and seal their eggs inside it.¹³ Each female spider is able to lay from thirty to fifty yellowish eggs at once. ¹³ The female stays with her young from the time she lays her eggs to (possibly) around their third moult.  ¹³

Diagnostic

Spiders from the family Desidae can be identified with the following dichotomous key (Figure 12).

Figure 12: Dichotomous key of littoral spider families. Taken from Cheng, 1976.¹⁴

The behaviour of these spiders with regards to using silk to seal up holes made by Lithophaga sp. is also unique to two littoral spider families - Desidae and Agelenidae. ¹⁴

Spiders in the genus Desis are all fully marine spiders, and they generally have very similar morphological appearances due to living in similar environments, such as their elongated hairy legs that allow them to skate across the surface of the water, as well as the general characteristics of spiders in the family Desidae.¹⁵ They are also characterised by their large, forward-projecting chelicerae, widely-spaced spinnerets and a large colulus (vestigial reduced cribellum). ¹⁴

Taxonomy & Systematics

Description

Original Description (German) (L. Koch, 1872)

The original full-length description by L. Koch in 1872 (in German) can be found here. (T4254-Desismartensi-file-1.pdf)² The description is from pages 347-351.

English Description (Workman, 1986)

Workman, 1986 provides another description of the spider in English, with diagrammatical representation (Figure 14).

Figure 14: Description of Desis martensi by Workman, 1986.⁹

Species Concept

It is unclear what species concept L. Koch used, however as the original description by L Koch was in 1872, he likely backed up his description of Desis martensi using the morphological species concept or typological species concept (based on the morphology of the spider) as the description was carried out before the biological species concept (or most other species concepts) were published. 

Type Information

The Desis martensi type specimen was collected from Singapore; on coral reefs surrounding the land mass that is presently known as Sentosa.⁵

It is unknown where the holotype currently is. However, syntype specimens could possibly be at the British Museum as there is information that specimens collected from Singapore were sent there.¹

The type of the genus Desis is the species Desis maxillosa described by Fabricus, 1793. ¹⁶

History

Dr. Eduard von Martens first discovered Desis martensi in October 1861 after he and a colleague (Dr. Johnswick) broke open fragments of coral in the shores of Singapore. They were stunned to see several small spiders scurrying away, and initially assumed that they had brought the spiders into the intertidal zone on their clothes.¹ Dr. von Martens then sent specimens to Dr. Ludwig Carl Christian Koch who described it in 1872. ¹ Desis martensi was initially described as 'Desis martensii' by Koch. The second 'i' was dropped in subsequent publications mentioning the spider. It it not known why. ²

Phylogeny

Genus Desis

Desis martensi is one of fourteen species in the genus Desis (Figure 15). 

Figure 15: Diagrammatic representation of Desis species. Information from World Register of Marine Species (WoRMS). Diagram done by Kieron Gabriel Ng. ¹⁷

Additionally, some databases also list Desis hartmeyeri (Simon, 1909) in the genus Desis. However, WoRMS lists it as nomen dubium (doubtful/ uncertain name). ¹⁸ Furthermore, some postulate that up to one third of the aforementioned species may be synonyms. More research is needed in order to resolve this. ¹⁴

Family Desidae

The genus Desis is situated in the family Desidae (Walckenaer, 1837).¹⁹ Although the genus Desis may be one of the more well known littoral spider genera, the family Desidae is known to be one of the most difficult to characterize taxonomically. ¹⁴

Figures 16 and 17 are taken from Wheeler et al., 2017. Leg tissue samples from 932 spider species were collected and sequenced. After PCR, the sequences were aligned with CLUSTALW, with gaps treated as missing data. Parsimony analysis was carried out with MUSCLE for multiple sequence alignment (MSA) followed by analysis in TNT. Maximum likelihood analysis was carried out with RAxML after MSAs were carried out with MAFFT. Bootstrap trees were then obtained. Bayesian analysis was carried out using ExaBayes 1.4.1. (posterior probabilities were used).

For ease of understanding, one maximum likelihood tree for the Marronoid clade and one specifically for the Desidae family are shown below.²⁰ The paper refers to clade support below 50% as weak, between 50-65% as low, between 66-79% as moderate, between 80-94% as good, and between 95-100% as strong.  

Figure 16: Maximum Likelihood tree of Marronoid clade and other groups (Entelegynae: Eresidae, Deinopidae, Uloboridae, Oecobioidea, Titanoecoidea and Zodarioidea) with bootstrap values on groups. The Desidae family is at the bottom of the diagram (in purple). Taken from Wheeler et al., 2017. Edited by Kieron Gabriel Ng 

Figure 17:  Maximum Likelihood tree of Desidae family with bootstrap values on groups. Zoomed-in from Figure 16. Taken from Wheeler et al., 2017. Edited by Kieron Gabriel Ng 

Wheeler et al., 2017 found that the genus Desis (represented by Desis formidabilis; at the top of Figure 17) was quite distinct, and was the most divergent, from the other genera in the family Desidae (e.g. compared to sister groups Barahna and Poaka). ²⁰ Furthermore, there were generally very weak (below 50%) or low (between 50-65%) bootstrap support values across all 5 subfamilies in the family Desidae (Figure 17), with support increasing for distal branches (although many of the distal branches had 100% support). Overall, this led them to conclude that despite years of effort and study by scientists, the family Desidae remains one of the more broadly circumscribed and less well supported spider families.²⁰

Proposed Future Research

Future research specifically targeting the Desis genus might prove useful. As many of the known species now are thought to be synonyms, and because the genus Desis is highly divergent from the rest of the genera in the family Desidae, a comprehensive phylogenetic study of all 14 species would serve to advance scientific knowledge. Furthermore, focusing on only the Desis genus might yield more specific and accurate results than the Wheeler et al., 2017 study, which analyzed 932 species of spiders. 

There is also not much existing data on the various Desis species (whether it be morphological, genetic, or behavioural) in public databases, so this could be remedied with future studies. For example, Genbank only has 1 sequence from Desis formidabilis' Histone-3. 

Future research into the evolutionary history of littoral spiders in general could also help to determine when and how the various genera or families became adapted to the marine environment.