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Pycnogonida


Clement Mottier 2020

Summary
The class Pycnogonida (Latreille, 1810) who includes all members of the order Pantopoda (Gerstaecker, 1863), is a taxonomic group of marine arthopods closely similar to the chelicerate taxa although not directly being part of it. The phylogeny of Pantopoda is still in discussion nowadays (Dunlop JA & Arango CP 2005). Genetic studies suggest that they are a sister group of Chelicerata and a basal Euarthropoda.
This class includes more than 1300 species, exclusively living in marine and estuarine environments although some were recently observed in anchialine habitats. (Alvarez F. & Ojeda M. 2018) They thrive in all oceans around the world and in a spectacular range of ecosystems from the shallow coast water to a depth of more than 7'000 meters.
Pycnogonids typically have four pairs of legs but some species can have up to six pairs. 
Most of pycnogonids are either predators or scavengers but some species use their proboscis to attack sea anemones which are much larger than potential prey. Because of their size, these anemones mostly survive the aggression from the sea spiders which makes them parasites instead of predators.
Due to their wide range of habitats, pycnogonids evolved many different body sizes. The species living in shallow waters are quite small (between 1mm and a few centimeters) but species found at great depth or in the Antartic can reach up to 70cm. 
Their bauplan is caracterised by long legs and a very thin body, so thin than some of their vital organs have ramifications in their legs, which is quite unique in the animal kindom.
In this presentation I will summarise the general knowledge about these mysterious creatures in the most structured way possible. Many details are still unknown and there is a lot more to discover about them.
We have to take into account that Pycnogonida is a class that includes many species, therefore any physiological or ecological information in this factsheet can and probably does have exceptions.
Figures 1, 2 & 3 are examples of the wide diversity of sea spiders anatomy.
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Figure 1
2
Figure 2
3
Figure 3

Physical Description
Pycnogonids are extremely diversified, with a size range from a few millimeters up to 70cm for the species living in abyssal ecosystems or in Antartic waters. Their colour varies a lot depending on the kind of ecosystem they need to blend in. They are usually well camouflaged in the coral reefs, under rocks or in the algae. Like any arthropod, their body is made out of repetitive segments who developed specific features to serve different functions. Starting from the anterior side, their prosoma (head & thorax segment or cephalothorax) holds a proboscis, a specialised organ that allows them to suck fluids out of soft-bodied animals. They also have four eyes placed on a tubercle and three pairs of appendices : one pair of chelifores, one of palps and one of ovigerous legs.
The proboscis is an elongated hollow and unsegmented appendage with a muscular structure around the tube. Its function is to penetrate the soft tissues of a prey and suck out nutritive substances. The muscles motion, reduce all solid materials in the food pulp integrated down to a subcellular level which will be later digested. The mouth is located at the end and has a triangular shape. Most of the feeding apparatus is shaped with a tertiary structure with three antimeres. The proboscis in most sea spiders has a reduced mobility on the dorsoventral and lateral axis. 
The chelifores of pycnogonids may be homologous to chelicerae in Chelicerata. This is suggested by the fact that they are affiliated with the deuterocerebrum (Brenneis G. et al. 2008) and the patterns in larval Hox gene expression (Jager M. et al. 2006). Their structure is often scissor-like and their function is to manipulate the environment, especially preys. 
The palps are sensitive appendages which allow the sea spiders to touch and integrate information about their environment. They have sensory hair that detect vibrations.
Some species have diminished or lacking chelifores and palps in the adult form. Those species usually have a much more developed proboscis with a greater mobility.
Ovigerous legs are an appendage specific to pycnogonids. They serve three main purposes : 
- Self cleaning
- Protection and transportation of the eggs and possibly larvae after hatching.
- Courtship
The second function is fulfilled by the male in which the ovigerous legs are especially developed.(Lobato Vila I. 2015)

The eyes, called ocelli, like in any chelicerate are simple as opposed to composed eyes in many other arthropods. They are located on the dorsal side in a group of four. Some abyssal species lack eyes. 
The three pairs of appendages on the cephalothorax are highly variable in size, shape and structure between different species of pycnogonids. This variation was used before phylogenetic to classify this species families. The second main part of a pycnogonid body is the abdomen also named opisthosoma. All sea spiders have a particularly atrophied abdomen. It is thinner than the legs and in a short tubular shape. At the end of the abdomen is the anus.
Pycnogonids usually have eight walking legs but some species can have up to twelve due to a polymerization of their body segments.(Lobato Vila I. 2015). The leg structure is made out of eight segments from body to the extremity : 
- The coxa
- The first trochanter that holds the gonopores
- The second trochanter
- The femur
- The patella
- The tibia
- The two tarsi
- The terminal claw
Again, this description is valid for most species of already described pycnogonids but has exceptions. 
The legs are almost as thick as the body. The names Pycnogonida and Pantopoda respectively mean “With a lot of knees” and “Totally made of legs” (Lobato Vila I. 2015), which seems accurate given that the legs take a vast majority of the whole mass of the animal. The gonopores located on the first trochanter are part of the reproductive system which is going to be described more in details in the Anatomy part.
Their whole body is covered by a non-calcareous exoskeleton that they must shed regularly in order to grow like any other arthropod.
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Figure 4

Ecology
FEEDING AND DEFENSE 

Pycnogonids are either predators, scavengers or parasites. Some species can use multiple foraging techniques and some even have been observed feeding on algae. They use their proboscis to suck fluids out of soft-bodied invertebrates, mostly cnidarians. (Hedgpeth J.W. 1960) Preys are first detected by the palps which serve as sensory organs for vibration and taste and the eyes when the sea spider has any. Then the pycnogonid uses its chelifores to capture immobilize and/or chop off its prey. The end of the proboscis is then inserted in the prey and muscles motion build pressure to extract the food pulp, it also serves the purpose of grinding the solid parts down to a subcellular size which can be chemically digested. The parasite specialized species attack sea anemones which are commonly much larger than the sea spider. For that reason, even with multiple sea spiders attacks, the anemone will be able to survive. Although hunting mostly cnidarians, some also prey on sponges, annelids, molluscs, bryozoans (Figure 5) and others. Their diet is mostly restricted to sessile fauna and algae. Some families of pycnogonids show adapted morphology of their chelifores, palps and proboscis which indicate a specialisation for a particular food source (Dietz L. , et al. 2018).
A particular study (Richards P.R. & Fry W.G. 1978) based on the observations of Nymphon australe and Nymphon orcadense, two pycnogonids living in Antarctic waters showed that those two particular species were able to modify their metabolic rate and food intake to become sustainable filter feeders, directly taking food particles from the sea water rather than preys. It was suggested that this peculiar evolution was useful to those sea spiders in an ecosystem where food is scarce and starving period long and unpredictable.

Pycnogonida biology is understudied and their cryptic nature makes them very difficult to observe. Their defense systems are quite unknown. It seems reasonable to think that their camouflage abilities are their main way to escape predators. There is also an allusion in (Sherwood et al 1998) to a specie of sea spider : Stylopallene longicauda who sequesters amathine alkaloids from a bryozoan named Amathia wilsoni which it preys on. It is possible that these alkaloids are used for chemical defense. Some species are also able to separate themselves from one of their leg and survive by using autotomy. This defense mechanism is similar to a lizard loosing its tail(Lobato Vila I. 2015).

LOCOMOTION AND DISPERSION 

Pycnogonids are benthic mobile animals. They use their legs to walk on the bottom of the sea. Some are found in soft sediments substrate and other were observed swimming by using their leg in an umbrella-like pulsing motion. The larvae can help dispersion by being parasites to larger animals or by staying in a lecitothrophic planctonic stage for a certain amount of time before settling down on a substrate. This fact could explain why pycnogonida is so widely dispersed around the globe while being incapable to travel long distances by itself.
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Figure 5

Life History and Behaviour

REPRODUCTION AND LARVAL DEVELOPMENT

There is relatively few information on the reproductive system. Some elements are common to described species in which reproduction was observed. All known species have separate sexes. The female holds unfertilized eggs in her femurs. The male uses its more developed ovigerous legs to stimulate the female during a courtship ritual. The eggs are then released through the gonopores located on the first trochanters of the legs and are externally fertilized by the male. The first case of parental postzygotic care in arthropods was described during the 19th century in a specie from the family Phoxichilidiidae (Pycnogonida). The male was carrying eggsacks on a pair of specialised legs (Sars, 1891). It is known nowadays than most sea spiders provide paternal care, which explains why the ovigerous legs are mostly developed in males since they used them to carry the eggsacks and during courtship (Figure 6). After fertilization, the male gather the eggs and secrete a sticky substance from glands in its femora (Brenneis G. et al. 2017) to glue them together and attach them to its ovigerous legs. For some taxa, a polygamous reproduction has been documented (e.g. Achelia Simplissima)(Burris ZP. 2011). The male will carry the eggs until they hatch, some species have been observed taking care of the offspring even after hatching, some males keep the younglings on them until they are able to walk by themselves. Some species are known to reproduce multiple times during their lifespan while other die after one single mating (Mercier A. et al. 2015) (Brenneis G. et al. 2017). The larval stage has a different shape and development depending on the taxa. The most common hatching phase is the protonymphon larva (Figure 7). It has an unsegmented body, a proboscis and three pairs of appendages located on both side of the cephalothorax : The larval chelifores and two other pairs of limbs which hold a similar place and neural innervation as the palps and ovigers of the adult form but differ in function, they are named respectively palpals and ovigeral larval limbs (Brenneis G. et al. 2017). The legs are not visible yet but the first walking leg segment (sometimes even the second one) already have anlagen on the inside of the body. The chelifores hold several glands which secrete a substance that facilitate the attachment of the larvae, either to the ovigerous legs of the male or its invertebrate host during the parasitic step of its life-cycle. These parasitic phases occur in many species and can be either endoparasitic or ectoparasitic. The larva will go through successive molts and progressively develop its four pairs of walking legs. This developmental step is called “Post-larval phase” and will go on until the juvenile resembles a miniature version of the adult. A more detailed description of all the developmental pathways is accessible on Brenneis G. et al. 2017. Figure 8 is a good summary of the different known modes of postembryonic development in Pycnogonida.

BEHAVIOUR

There is a very limited amount of information on the sea spiders behaviour, other than their feeding and mating habits This is both due to the little size of the most common ones and their cryptic nature. We must also take into account that this class holds very little interests for human other than the scientific knowledge and the potential discoveries concerning the phylogeny of chelicerata. The pycnogonids are not a reliable food source, their parasitic phase doesn't affect species that are of interest and they hold no merchant value. All those factor have a significant effect on the financing and the number of studies concerning these animals. 
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Figure 6
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Figure 7
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Figure 8

Anatomy and Physiology

DIGESTIVE SYSTEM

The digestive system starts with the mouth. It is a triangle shaped orifice located at the end of triradially symmetric proboscis.(Dietz L. et al. 2018) The mouth is surrounded by three movable lips of widely diversified shape (Figure 9) and glands secreting saliva(Fahrenbach WH et al. 2007). The proboscis is made to penetrate the soft tissues of the prey and suck out nutritious fluid by muscles motion. The proximal extremity of the proboscis contains the “oyster basket” or pharyngeal filter. It is a densely packed tissue made out of bristles, its purpose is to filter the fluid ingested and grind the solid residual particles(Dietz L. et al. 2018). Taxon-specific differences in the hair structure and the pharynx armatures are described in Wagner et al. 2016. (Figure 10) But since feeding behaviour and specification is different for nearly all taxa and are not well-known, there is still no way to make a correlation between these anatomical differences and their respective diet.

The internal digestive system is separated in three parts : 

- The foregut

- The midgut

- The hindgut

The foregut takes place in the proboscis, it is where the food is processed and filtered (via the pharyngeal filter). It is relatively short, compared to the other parts. The midgut is where the food is chemically digested and absorbed. This part of the gut is probably the most particular features of the pycnogonids because it has diverticula extending into the walking limbs and the chelifores, in some species, those diverticula can almost reach the tip of these appendages (Dietz L. et al. 2018). The three parts of the digestive tube are separated by tripartite valves. The hind gut is also relatively short and opens to the outside of the animal via an anus at the end of the abdomen. 
When the food enters the midgut, it is already broken down to a subcellular level. Peristalsis moves the pulp through the tube and pushes it into the limb diverticula(Richards P.R.& Fry W.G 1978). The lumen swells and the epithelium is extended to its maximum elasticity to increase contact surface

The epithelium of the midgut is composed of three distinct cellular type:

- Absorption cells

- Gland cells

- Embryo cells

Gland cells produce acidophilic globules full of enzymes. These globules are released by exocytose into the lumen at the start of food intake. Their purpose is to break down the food to a size than can be absorbed by the cells.
The absorption cells are located on the walls of the lumen which show a lot of microtriches to increase the contact surface, absorb a maximum of nutrients and digest them intracellularly. Ancient studies of pycnogonids digestion (Schlottke E. 1933) led to the conclusion that the absorption cells detached themselves from the epithelial tissue and moved freely in the lumen to catch as much nutrient as possible while still undergoing intracellular digestion, before reattaching themselves to their epithelial substrate. Although this information has not yet been formally revoked, Richards P.R. & Fry W.G. 1978 suggested that this statement was due to an illusion caused by the sections orientation that showed tips of larger microtriches as independent cells. Later examination under electronic microscope suggested that both secretory cells and absorption cells could be different stages of the same cell type. 

Embryo cells are a supportive tissue that holds the rest of the epithelium together and transfers nutrients from the absorption cells to the rest of the organism via the hemolymph circulation.

After digestion is completed, the tripartite valve at the end of the midgut opens and let the remaining fluid pass to the hindgut. It is then discharged on the outside by the anus.

RESPIRATORY AND CIRCULATORY SYSTEMS

The anatomy of sea spiders is quite unique among the animal kingdom, so are their respiratory and circulatory systems. They don’t have any specialized gas exchange organs. Their O2 intake is directly made through their cuticle, gases are released in the hemolymph via diffusion. Recent studies suggest that waste like CO2 are excreted in molts or via the digestive system. The circulation of gases within the body of the sea spiders is driven by four main motors: 

- Diffusion

- Heart contractions and heart-driven circulation

- Leg movement

- Diverticula peristalsis

Diffusion is a passive and universal way for gases to travel through tissues. The same phenomenon occurs in pycnogonids but, as opposed as other arthropods which have a specialized gas exchange structure, it happens on the whole surface of the animal, mostly on the legs (as shown in Woods H.A. et al. 2017). Their morphology is well-suited for that mode of O2 intake, the shape of their body maximize contact surface with the water relatively to their volume

The heart and circulatory system of the pycnogonids is comparable to many other arthropods, a simple open system with a simple muscle pumping and moving the hemolymph around the body. The difference is in the relative importance of that system for hemolymph circulation. Woods H.A. et al. 2017 shows in the study of 12 species that “heart-driven flows were generally confined to the trunk and proximal segments of the leg”.

Leg movement also has a role minor role in circulation. In another experiment, Woods H.A. et al. showed by restricting the movement of sea spiders legs that the impact on hemolymph distribution existed but wasn’t significative. 
The main driver of hemolymph circulation was discovered to be the peristalsis of the diverticula of the midgut. While digesting, the midgut starts a peristaltic movement to move food pulp around its lumen, since those diverticula can extend in almost all the body space, their peristaltic movement induce a motion in hemolymph and permit the oxygenated fluid to be distributed all around the body. The reason is that the leg volume is fixed by a rigid exoskeleton and cannot be changed. This same volume is filled with two fluid compartments: The gut and the hemolymph. Since the peristalsis of the gut changes its volume in a wave motion, any augmentation or diminution of the gut volume must induce an opposed modification of the hemolymph volume. Species differ in their peristalsis motion’s strength. In pycnogonids species with weak peristalsis (e.g. Nymphon australe) a single wave of gut movement could push the hemolymph on a distance corresponding to a quarter of a leg segment. Species with stronger peristalsis (e.g. Colossendeis megalonyx) could move hemolymph up to more than one leg segments. The function of gut peristalsis in gases exchange is also confirmed by the fact that its intensity and frequency varies depending on the pressure of O2 and the temperature.

NERVOUS AND SENSORY SYSTEM

Pycnogonids usually have four eyes, each of the same, disposed on a tubercle located on the dorsal side of the cephalon that can be of various shape and size. Some species also have ectopic or rudimentary eyes in addition to the four fully functional ones. There is also an example of sea spider genus (Stylopallene) which reportedly have eight eyes regrouped by pairs. It is presumed to be a subdivision of the original four eyes. (Staples, 2005,2014)(Lehmann T. et al., 2017). Species that live at abyssal depth can also be eyeless. Sea spiders that have functional eyes don’t seem to hold any kind of other photosensitive structure. “The eyes are grouped to a left and right pair innervated to the visual neuropils of the left and the right protocerebrum side, respectively.”(Lehmann T. et al., 2017) 

Many pycnogonid taxa reportedly have “lateral sense organs” located on the side of the eye tubercles. It is currently still unknown which kind of sensory organs they are and what is their purpose. They were first described in 1881 by Dohrn who thought they could be auditory organs. Other theories such as thermosensitive or chemosensitive structure were suggested based on more modern observations by Richter 1982. 

Palps and chelifores are used as sensory and motor appendages to interact with food and substrate. The “tool kit” of the sea spiders varies a lot between different taxa. Some adult form have severely reduced palps or chelifores, some lack those appendages altogether. The specific function of each frontal limb varies with its shape and their presence or absence is directly depending on the diet specialization of the species studied.

Since pycnogonids are slow moving animals with a preference for camouflage and hidden lifestyle in dark habitats, it is fair to suggest that their main sense are not related to light stimuli. Many arthropods exhibit exteroceptive organs called sensilla. Their structure is highly uniform: sensory cells with ciliary dendrites surrounded by sheath cells. Although such organs have been observed on sea spiders, present studies only concern external observation. Therefore, no conclusion can be made on the importance of those sensory organs for pycnogonids interaction with their ecosystem. Lehmann T. et al. (2017) suggest that, since the morphology of those organs is similar to those found in all euarthropod taxa, it could be a plesiomorphy. More studies have yet to be made in order to confirm that assumption.

The internal nervous system of sea spiders is rather simple. It is composed of a main neural cord with a succession of ganglia that innervate corresponding segments (Figure 11). The protonymphon larvae (Figure 12) exhibits an even much simpler nervous system with two pairs of ganglia (Supraenteric and subenteric) arranged in a circle around the esophagus and innervating the only appendages present at that stage of life :

- The eyes

- The larval proboscis

- The chelifores

- The palps

- The larval ovigers

The ganglia are linked by circumenteric connectives and are prolongated by a ventral nerve cord which will form the other ganglia during the polymerization of the future body segments of the adult form.

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Figure 9
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Figure 10
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Figure 11
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Figure 12

Biogeographic Distribution
Pycnogonids are found in every ocean and sea on the planet. There is a large number of observations in the Mediterranean sea (Kocak C. 2019) and specimens have been described as well in the Indian ocean (Wang J. et al. 2020) Atlantic (Lucena R.A. et al. 2019) and Pacific (Franciso Araya J. 2016) and in Arctic and Antarctic (Dietz L. et al. 2019) waters. The cryptic nature of the sea spiders makes it difficult to estimate the density of population in a given place and few studies have been made on the subject.
Given that the order Pantopoda includes more than 1300 described species, describing a specific habitat would be futile. Sea spiders have been found in shallow waters near the coast, estuarine habitats, anchialine habitats (Alvarez F., Ojeda M., 2018) coral reefs (Lucena R.A. et al. 2019), hydrothermal source in abyss (Wang J. et al. 2018), from the icy waters of the poles to the tropical seas. However, it is fair to say that sea spiders leave on the benthos no matter in which ecosystem and at which depth.  

Evolution and Systematics
The phylogeny of Pycnogonida is, without a doubt, the most disputed yet interesting topic about that class.
The status and understanding of the evolution of sea spiders is a cornerstone necessary for the understanding of the phylogeny of all arthropods. Multiple theories exist. Some suggest that pycnogonds are a basal group of the euarthropod and have their own lineage. Some of them support that Pycnogonida is part of the subphylum Chelicerata. Recent studies (Brenneis G. et al. 2008) (Jager M. et al. 2006) proved by showing similarities between Hox gene expression patterns and nervous innervation of the chelifores, specific to pycnogonids, and the chelicerae of Chelicerata, that these two kind of appendages were homologous and therefore a plesiomorphy. That information confirms the relation between pycnogonids and chelicerate but other possible theories are still not excluded. The most common classification I found considered the class Pycnogonida to be a part of the subphylum Chelicerata while still being distinct from Euchelicerata.
Figure 13 summarize this last theory.
A universal classification within the class Pycnogonida has yet to be established. The order Pantopoda includes all known species, but past that taxa, there is no universally accepted classification of the families and the genera.
The fact that there is probably numerous species of sea spiders that are yet to be discovered is both an issue for present classification and a potential solution for future universally accepted classification.
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Figure 13

Conservation and Threats
There is, to this day, no data available to assess the conservation status of pycnogonids. The number of observations is highly variable between different taxa and the lack of information on population density makes it even harder to guess. However, the large repartition and the wide range of habitats occupied by sea spiders leads us to think that there is presently no threat to their conservation.

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