The genus Anoplodactylus is the most speciose member of the family Phoxichilidiidae that also includes the genera Phoxichilidium and Pycnosomia. Commonly known as ‘sea spiders’, Pycnogonida are chelicerate arthropods that are exclusively marine invertebrates (Lehmann, Heß, & Melzer, 2014) totalling 1334 species, out of which 136 are known species of Anoplodactylus (C. Arango & Wheeler, 2007). Anoplodactylus can be found in waters across the globe across all latitudes and depths from the littoral zone down to the abyssal zone. The larval young are endoparasitic and grow within species of hydrozoans. Because they are slow moving, they generally feed on sessile organisms such as hydrozoans. Their relatively simple bauplan has led to some interesting and unique adaptations such as gut peristalsis taking on the role of the circulatory and respiratory systems. Because sea spiders are so small, slow moving and cryptic, they are among the least studied marine species.

Size and Colouration

     Members of the Anoplodactylus genus are generally small with body sizes typically ranging from 0.6 mm to 6 mm when measured from the tip of the proboscis to the posterior end of the abdomen (Lucena, Araújo, & Christoffersen, 2015). The legs are two and a half times to three times as long as the body and account for the main mass. Colours range from normally green (A. angulatus) (Figure 1A), off-white or colourless (A. petiolatus) (Figure 1B), pale straw or colourless (A. pygmaeus) (Figure 1C) (King & Crapp, 1971), green or blue (A. virescens) (Figure 1D) or even multicoloured (A. evansi) (Figure 1E) (Arnaud & Bamber, 1988). Males are generally smaller than females (Bain & Govedich, 2004).

External Morphology

     Anoplodactylus have bodies that are generally slim and of a small size (Figure 2A). This is commonly seen in species that inhabit shallower depths, while species that live in deeper depths tend to be larger. Anoplodactylus spp. are characterised by the absence of palps, simplicity of ovigers in males (five or six segment ovigerous legs), complete absence of ovigers in females, and the lack of developed strigilis (terminal section of oviger) (Figure 2C) (C. Arango & Wheeler, 2007).

     All adult Anoplodactylus have an anterior proboscis and an anterodorsal ocular tubercle that bears two pairs of eyes (Figure 2D). The first limb-pair, known as chelifores, are located dorsally to the generally round and tubular proboscis that is followed by the ovigers in males that are used to carry developing eggs. Located posterior to the ovigers are four pairs of walking legs borne on lateral processes of the body segments (Figure 2A) (Georg Brenneis, Bogomolova, Arango, & Krapp, 2017). Each leg is comprised of nine articles: coxae 1, 2 and 3; femur (featuring the male cement gland); tibiae 1 and 2; tarsus; propodus; and terminal claw (from proximal to distal) (Figure 2B). The legs house long diverticula of the midgut and gonads that are displaced far into the legs (Georg Brenneis et al., 2017). Gonopores are located on the ventral surface of coxa 2 (Figure 2B). On the last walking leg segment, the anus is borne on the distal end of an unsegmented anal tubercle (Figure 2A).

     Species that live in specialised environments show the appropriate adaptations. Abyssal species are usually blind while species that live in shallower environments with very strong wave action, e.g., fringing or barrier reefs, have condensed body forms, short robust legs with bowed propodus, and strong claws (A. arescus) (Arnaud & Bamber, 1988).

Feeding and Defence

     Most pycnogonids are parasites, i.e., they do not usually directly kill their host unlike a predator that will kill and consume all or most of the organism (Rohde, 2005). Although, some species have been described to prey on and consume entire animals,  A. petiolatus has been observed to consume annelids whole (Lotz, 1968). Other species have been described as herbivorous (Wyer & King, 1974) or detritivorous (Soler-Membrives, Arango, Cuadrado, & Munilla, 2013). Larvae are obligate endoparasites of hydrozoans and are active feeders (Georg Brenneis et al., 2017).

     Members of the Phoxichilidiidae family, which include Anoplodactylus have been found by Dietz, Dömel, Leese, Lehmann, and Melzer (2018) to definitively feed on sponges, hydroids, medusae, bryozoans, molluscs, annelids, crustaceans and echinoderms. A. evansi has been reported to attack and feed on small individuals of 13 species of opisthobranchs by hunting them on benthic algae (Rogers, Nys, & Steinberg, 2000). Other species such as A. carvalhoi and A. longiceps only feed on the cerata of nudibranchs (Figure 3) (C. P. Arango & Brodie, 2003; Piel, 1991).

     Anoplodactylus blend in well with their environments and given their heavily chitinised exoskeletons and little muscle content, do not make for an attractive source of food. Thus, they do not posses any physical or chemical defences. Records of the gut contents of sub-Antarctic king crabs and north Atlantic shrimps suggest that they appear to be ingested incidentally rather than actively, and do not constitute majorly in the diet of any predator (Arnaud & Bamber, 1988).

Locomotion

     Anoplodactylus possess four pairs of walking legs. Its eight-legged locomotion allow for variable speed; forward, backwards and sideways movement; and low turning radius on uneven substrates (Roberts, Mahon, & Halanych, 2016). However, low muscle mass relative to body mass results in generally slow movement with walking speeds measuring at between 0.05 to 0.2 cm/sec with a variable and non-symmetrical gait pattern (Roberts et al., 2016).

Habitat

     Predominantly inhabit and show the most diversity in shallow waters occurring in either tropical or temperature waters but have also been recorded in Antarctic latitudes and below depths of 1000 m  (Claudia P. Arango, 2003b; Lucena et al., 2015). Some live parasitically in polyps of Hydractinia echinate, Podocoryne carnea and Campanularia flexuosa (King & Crapp, 1971).

     There are nine species of Anoplodactylus known from waters deeper than 500 meters. These are: A. arnaudae (to 860 m), A. australis (to 549 m), A. mamillosus (to 732 m), A. neglectus (to 2926 m), A. oculatus (to 850 m), A. pelagicus (to 549 m), A. petiolatus (to 1180 m), and A. typhlops (to 3620 m) (Child, 1982).

     A. cribellatus and A. arescus live in unstable sands in the infralittoral, i.e., the region of shallow water closest to the shore, excluding the intertidal zone (Arnaud & Bamber, 1988). A. petiolatus frequent muddy bottoms of up to 22% silt in the Bay of Biscay and 50% silt off the coast of Northumberland. A. massiliensis inhabit muddy shelf sea bed in deeper waters of the Mediterranean (Arnaud & Bamber, 1988).

Behaviour

     Anoplodactylus are slow-moving given their hard cuticles and relatively small musculature. In the lab, the collected specimen was observed to respond negatively to light; curling up its legs and compressing its body into a ball shape when exposed to the bright light of the dissecting microscope. When placed in the collection tray, the specimen was also observed to actively seek darkness, only stopping once it had reached a dark area.

     Ovigers are used for grooming, handling of food, courtship and mating, transferring of eggs from female to male, and transport of eggs by male (Bain & Govedich, 2004). The collected specimen was observed using its ovigers to pick and remove bits and pieces of detritus and debris from its body.

Natural History

     Anoplodactylus are iteroparous, capable of reproducing multiple times within a period of several years. Male and female undergo a series of courtship and mating behaviours that last for several minutes, in A. lentus it was observed to be 5 minutes (Bain & Govedich, 2004). The female first approaches the male and they meet head to head. They then mount each other so that their heads are at opposing ends with their ventral surfaces facing towards each other. When their ventral surfaces meet, the female starts laying its eggs. The mature eggs leave the ovary in the femur of the walking leg and are excreted through the gonopore. The eggs are held beneath her while the male uses its oviger to fasten onto the egg mass with the aid of secretions from the cement gland. Males usually end up with two egg masses that are fertilised externally as mature sperm are released by the testes via the gonopore (Bain & Govedich, 2004). Males have been observed with multiple egg masses of different sizes indicating multiple copulation events.

     The life span of Anoplodactylus has not been well-studied and very few species have had their complete life cycles investigated. The protonymphon larvae has long filamentous strands attached to the ends of its larval appendages (Figure 4B). These strands are initially used to cling onto the male after hatching, then used for clinging onto hydroids as well (Bain, 2003). After hatching, the protonymphon larvae crawls out from the egg mass and swims away from the oviger. After finding a suitable hydroid or stylasterid coral, it burrows into the polyp or gastrozooid respectively (Bain, 2003). Post embryonic development is subdivided into three phases: the larval, the post larval, and the juvenile phase (Georg Brenneis et al., 2017). The larval phase includes all instars that resemble the protonymphon larva (Figure 4B). The second phase, the post larval phase is characterised by the formation and differentiation of walking leg segments over the course of two moults. Finally, transition into the juvenile phase occurs on the first moult that leads into a miniature adult with all walking legs, although the fourth pair of legs may still lack a few articles (Georg Brenneis et al., 2017). Moults occurring in this phase result mainly in the increase of overall body size. Sexual maturity is indicated by visible mature oocytes in gonads of females, the bearing of egg masses by males, or presence of gonopores on the second coxae (Georg Brenneis et al., 2017).

Development

     All Anoplodactylus larvae develop endoparasitically with partially synchronous differentiation of walking legs, known as type 4 postembryonic development (Georg Brenneis et al., 2017). Other terms commonly used to refer to this type of development are “type 2” (Dogiel, 1951; Sanchez, 1959), “encysted larva” (Bain, 2003) and “encysting mode” (Burris, 2011).

     Egg sizes range from 30 μm in diameter (A. angulatus and A. erectus) to 40 μm (A. eroticus), and are the smallest reported egg size of any species of pycnogonid (Georg Brenneis et al., 2017). The hatching stage is a protonymphon larva that is less than 100 µm in overall size. The post larval instars are endoparasitic within hydrozoans and are active feeders.

Digestive System

     An anterior triradially symmetric proboscis is the main organ used for the uptake of food. The proboscis features a terminal mouth surrounded by three moveable ‘lips’, one dorsal and two ventral, and gland openings that may secrete saliva (Dietz et al., 2018; Fahrenbach & Arango, 2007). Relatively strong musculature of the proboscis allows for the sucking and pumping of mostly liquid-form food. Located proximally within the proboscis is the pharyngeal filter that is composed of bristles that are densely packed to grind and filter out any solid ingested particles (Figure 8) (Dietz et al., 2018). The lips of most species are fringed with microtrichia, however in Anoplodactylus they are mostly absent or lightly fringed (Figure 5B and Figure 5C). Chelifores and palps are the secondary organs used to aid in feeding in most pycnogonid species; these are homologous to the chelicerae and pedipalps in arachnid chelicerae (G. Brenneis, Ungerer, & Scholtz, 2008). However, in Anoplodactylus only the chelifores are present and are dorsally located. They are used for the cutting off and macerating of prey organisms; guiding them to the proboscis (Figure 5A), as well as for the generally manipulation of prey. Palps in Anoplodactylus species are reduced or lost (Figure 5D).

     The digestive system is divided into three sections: the foregut, midgut and hindgut. Food processing and filtering begins in the foregut within the proboscis with the aid of the pharyngeal filter. Digestion and absorption then occur in the midgut located within the cephalosoma and walking leg segments (Figure 8). The midgut is unique in that it has diverticula extending from the central section of the body into all the walking legs (Arnaud & Bamber, 1988). Congregation of faecal pellets occurs in the hindgut that then subsequently opens to the exterior via the anus (Soler-Membrives et al., 2013). Digestion is intracellular and occurs only by pinocytosis (Dietz et al., 2018). See feeding video.

Circulatory and Respiratory Systems

     The circulatory system consists of a simple tube-like heart that pumps haemolymph. Because the heart is quite simple, the haemolymph that is pumped by the heart is mostly confined to the trunk and proximal leg segments (Georg Brenneis & Scholtz, 2014; Woods et al., 2017).

     Pycnogonids lack any specialised gas exchange structures. They respire by directly taking up oxygen and releasing carbon dioxide across the cuticle instead (Woods et al., 2017). For internal oxygen transport, counter current flows generated by gut peristalsis transports oxygen from distal parts of the legs to the central trunk and muscle-filled proboscis where oxygen demand is greatest (Woods et al., 2017). See peristalsis video.

Reproductive System

     Gonads are housed within the walking legs in the femur (Figure 6). In females, mature eggs are released from the ovary and forced towards the gonopore during copulation. The oviger, with the aid of the cement gland, aids in fastening the eggs ventrally onto the male for incubation (Figure 7) (Arnaud & Bamber, 1988). Gynandromorphs have been well recorded within Anoplodactylus with egg-bearing specimens recorded with well-developed ovaries and testes (Figure 6B) (Arnaud & Bamber, 1988).

Nervous System

     The pycnogonid central nervous system (CNS) is composed of several separate ganglia, unlike the fused CNS in euchelicerates, and is subdivided into an anterodorsal brain and the ventral nerve cord (Figure 8). The brain is spherical to ellipsoid in shape, the segmental units that contribute to the brain are the protocerebral region and deutocerebral neuromere (Georg Brenneis, 2015). Pycnogonids have the most complex stomatogastric nervous system of all extant species of arthropods (Figure 8) (Georg Brenneis, 2015).

Local Distribution

     Anoplodactylus can be found all along the coasts of Australia and Tasmania, mostly concentrated along the eastern seaboard of Australia (Figure 9).

Global Distribution

     Anoplodactylus are cosmopolitan and are found in every ocean (Figure 10). Species have been recorded in Brazil (Lucena et al., 2015), the Mediterranean (Galli et al., 2019; Lehmann et al., 2014; Soler-Membrives et al., 2013; Zool et al., 2013), the British Isles (King & Crapp, 1971; Wyer & King, 1974), Australia (Claudia P. Arango, 2003b; Atlas of Living Australia, 2020), etc.

Evolution

     The classification of Pycnogonida families has traditionally been based on morphological characters; primarily the absence of presence of cephalic appendages in adults, i.e., chelifores, palps, and ovigers (Claudia P. Arango, 2003a). It had previously been assumed that the direction of evolution within the group was that of a reductive trend. However, it has been shown that there is a possibility that reduction and loss of appendages occurred as parallel evolutionary events (Claudia P. Arango, 2002).

     Anoplodactylus is a robust monophyletic group that is well supported by its sister group Endeis (Figure 11). Together they form the Endeis–Anoplodactylus grouping that is supported by four morphological characters: the absence of palps, simplicity of ovigers in males, complete absence of ovigers in females, and the lack of developed strigilis (C. Arango & Wheeler, 2007).

Fossil Record

     Few pycnogonids appear in the extremely sparse fossil record due to their delicate forms and non-biomineralised cuticles (Siveter, Sutton, Briggs, & Siveter, 2004). The earliest possible record was that of a larva found in Upper Cambrian Orsten deposits dating back 500 million years ago (Waloszek & Dunlop, 2002), while the earliest adult was recorded in the Silurian dating back about 425 million years ago (Siveter et al., 2004). As of 2020, only 10 species have been described in the fossil record dating from the Upper Cambrian, Silurian, Devonian and Jurassic geological periods (Bamber et al., 2020).

Classification and Systematics

Phylum: Arthropoda (von Siebold, 1848)

Subphylum: Chelicerata (Heymons, 1901)

Class: Pycnogonida (Latreille, 1810)

Order: Pantopoda (Gerstaecker, 1863)

Suborder: Eupantopodida (Fry, 1978)

Superfamily: Phoxichilidoidea (Sars, 1891)

Family: Phoxichilidiidae (Sars, 1891)

Genus: Anoplodactylus (Wilson, 1878)

     Considered a small group of atypical arthropods, Pycnogonida are not as widely studied as other classes in the animal kingdom and as such are neglected in marine studies. They usually occur infrequently and in low abundance; tend to be cryptic and well camouflaged; and are without any economic importance (C. Arango & Wheeler, 2007; Claudia P. Arango, 2003a). Thus, information on population and abundance of Anoplodactylus spp. is often sparse and incomplete.

Conservation Status

     Seven species of Anoplodactylus were assessed by Membrives, Valbuena-Ureña, and Claudia P Arango (2018) in a conservation assessment of Pycnogonida in the Iberian Sea. Their results showed that two species were found to be critically endangered (A. oculatus and A. robustus), while the remaining five were found to be of least concern (A. typhlops, A. arnaudae, A. pygmaeus, A. virescens and A. petiolatus).

Pollution

     Anoplodactylus have been shown to be generally tolerant of pollution. Domestic waste and organic pollution have been shown to support populations of A. pygmaeus and A. petiolatus, with populations thriving only where there is pollution (Arnaud & Bamber, 1988). Power plant cooling water effluent has also been found to support populations of A. virescens. Generally, Anoplodactylus are not directly affected by pollution but by the effects of pollution on their food sources (Arnaud & Bamber, 1988).

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