Nereiphylla is a genus of the marine worm class: Polychaeta. First described by Blainville in 1828, Neriephylla belongs to the family Phyllodocidae, distinguished due to their bright coloration and unique morphological characteristics. This carnivorous genus is a relatively new discovery having only few documented findings of specimens. Most describe the genus as inhabiting the oceanic benthos (Pleijel 1991). One such specimen was discovered among coral rubble from the reefs of Heron Island Queensland. This species concept provides an in depth description of the specimen classified as Nereiphylla sp.,  with particular focus on the importance and anatomy of its distinctive sense organs. 

Kingdom: Animalia

Phylum: Annelida

Class: Polychaeta

Family: Phyllodocidae

Subfamily: Notophyllinae

Genus: Nereiphylla 

The specimen was placed within this genus using the key and individual descriptions produced by Pleijel (1991). Further classification of the organism was conducted through liaising with Dr Robin Wilson, a Polychaete expert from the Museum of Victoria. 

The specimen was measured at >1mm in length and only clearly visible through a high-powered microscope. It’s coloration before becoming fixed was a bright, deep yellow (Figure 2). After fixation in 4% PFA, the coloration changed to brown and red (Figure 1). 

The Nereiphylla sp. head region is distinguished by four visible antennae and no apparent median antennae (Pleijel 1991). The prostomium is more or less rounded with antennae at the furthest anterior point. There are two distinguishable eyes before the antennae (Figure 3).

The first 2 segments are described by Pleijel (1991) as being fused, however the specimen in question lacks this fusion that is an important feature of the genus. Protruding from the first and second segments are tentacular cirri that are cylindrical at the base and pointed at the top (Figure 4). Segment 1 has 1 pair, segment 2 has 2 pairs and segment 3 has 1 pair (Welch & Dutch 2014). Interestingly, one pair of cirri is darker in color and larger than the other pairs making them highly visible. Further along, segments display parapodia with attached cirri and setae. 

Parapodia is uniramous with no visible notopodia or neuropodia (Pleijel 1991). Setae protrudes from the parapodia (Figure 6) which is shown to be a derived trait throughout the Genus and the Phyllodocidae family. Dorsal cirri from parapodia is cordiform (Figure 5) or heart shaped and relatively larger than ventral cirri (Pleijel 1991). 

The tail region has no median papilla however, the pygidial cirri is fairly cylindrical (Pleijel 1991). 

Phyllodocidae usually inhabit oceanic waters less than 200m in depth (Rouse & Pleijel 2001). However, some specimens have been discovered in areas above and below this depth particularity in the inter-tidal areas. 

The current specimen was found among coral boulders from Heron Island reefs, Queensland, Australia (Figure 7). The habitat where the current specimen was found is close to that of the first recorded Neriephylla blainville (1828) in the Philippines. Thus, it can be assumed that Neriephylla sp. inhabit warm, tropical waters. 

Phyllodocids move via muscle and segment contraction and movement of parapodia (Tzetlin & Filippova 2005). To allow this movement, the body of Nereiphylla sp. in particular consists of a complex musculature including longitudinal muscle, muscular walls and cells (Video 1).

There is an array of research that supports the findings that the Phyllodocidae family lack any circular muscle, instead longitudinal muscles control most of movement (Figure 9). These muscles occur along the length of the body in bands, specifically, four muscle bands with half ventrally through the body and the others dorsally (Tzetlin & Filippova 2005). The figure depicts a section of longitudinal muscle seen in Phyllodocidae family (Figure 8). It is visible that each part of the muscle band is in contact with the extracellular matrix.

The palps of the Nereiphylla specimen has its own specialized musculature. This musculature assists in the palps ability to move at the anterior region and sense if an area is suitable to be followed by the rest of the body. Presence of epithelia-muscular cells allow the thin appendage to extend outwards for sensory actions (Rhode 1991).  

Video 1: Nereiphylla sp. movement under muscle relaxant

Phyllodocidae species are either opportunistic scavengers or predatory carnivores. Neriephylla sp. is most likely a carnivorous feeder due to the extent of the sensory organs that assist in distinguishing prey from predator (Rodrigo et al. 2015).  After capturing prey, Phyllodocid species use a non-specialized gut that digests food. The non-specialization allows the organisms to change the composition of the gut in regards to the amount of food it consumes. The whole digestive process primarily occurs within vacuoles that have enzymes to break down components (Rodrigo et al. 2015).

Interestingly, some phyllodocid genus: Phyllodoce mucosa are known to be carrion-feeders meaning they feed on dead organisms. Phyllodoce mucosa are generally found in the inter-tidal zones where conditions allow a large amount of dead invertebrates to sink to the benthos thus attracting the worms (Lee et al. 2004). As the specimen in study was found in similar habitats, it can be likely that such feeding behavior may be used by Nereiphylla sp. 

Within polychaetes, asexual reproduction is far more common than that of sexual reproduction (Rouse & Pleijel 2001). The reproductive method, cycle and factors of mating have only been studied in Phyllodocids of Phyllodoce mucosa and Eulalia viridis. Both species are gonochoristic. Males are observed to form a group of more than three individuals with a single female for mating. The males produce a mucous bag onto the sediment in which both sperm and eggs (from a single female) are deposited into. Therefore, the eggs are fertilized externally on the sediment (Rouse, Pleijel 2001). Knowing that benthic phylldocids are most common, it can be suggested that Nereiphylla sp. may use this same mode of reproduction, utilizing its ability to manoeuvre along the sediment with its specialized sensory organs. 

Success and time of mating is observed within Eulalia Viridis females. Olive (1980) found that environmental temperature is the factor which has the greatest effect upon the reproductive cycle in females. Over a year of testing, most reproductive success, that is the number of fertilized and hatched eggs, were found in months where the water was <14 degrees Celsius (Olive 1980). It can be assumed if environmental temperature affects all Phyllodocidae genus', then Neriephylla sp. benefit greatly especially since their habitat is that of warm tropical waters.  

Larvae of phyllodocidae has been observed as existing in a trochophore form (Lacalli 1985). This means that the larvae are free-swimming and can occur as pelagic or benthic. Lacalli (1985) observed several larval forms and considered the frontal organ, eyes and a pair of lateral sensory organs as the main components of the Phyllodocid larvae (Figure 10). Nereiphylla sp. has not been observed in the larval form, however it is most likely that trochophore larval stage does exist in its life history knowing its common occurrence in Polychaete families(Rouse & Pleijel 2001). Furthermore, the existence of primary sensory organs such as prominent eyes in the larval form suggest this is carried on into the adult life-stage as is easily distinguishable. 

Respiration and gas exchange in polychaetes generally occur through the parapodia. In those organims with biramous parapodia, respiration occurs through the upper most segment. As Nereiphylla sp. has uniramous parapodia, respiration occurs through the whole parapodia surface via blood vessels (Rouse & Pleijel 2001). 

Polychaetes are generally predatory and carnivorous and therefore need to utilize a variety of sense organs to establish food collection and hunting (Purschke 2005). Sense organs generally occur in the anterior region as palps, Tentacular cirri, antennae, specialized nervous system and eyes. Sensory cells exist within these organs for facilitating reactions to chemical and sensory stimuli. 

Phyllodicid species have a unique nervous system not seen in other polychaetes. In many Phyllodocid species, the median antennae acts as a mid-brain. Neriephylla sp. has no median antennae, thus their mid-brain is situated as ganglia clustered with the fore-brain (Figure 12). The fore-brain in Nereiphylla’s case, consists of clusters of tiny nerve cells (and ganglia) around the frontal antennae(Orrhage & Eibye-Jacobsen 1997). 

The brain is concentrated at the antennae; however, the nervous system extends up to segment five of the worm. This occurs through various connections of nerves and ganglia that differ between Phyllodocid genus'. Within Nereiphylla and closely relation Notophyllum species, connective fibers that attach hemispheres of the brain (fibrillar commissures) exist further anteriorly compared to other genus’ (Figure 11). Orrhage and Eibye-Jacobsen (1997) describes this characteristic as allowing the species to have a higher degree of cephalization. Increased cephalization is displayed in the current specimen by the distinguishable head region with distinctive eyes and rounded prostomium. 

Nuchal organs in Polychaetes are important sensory parts of the anterior region. They exist in pairs as pits within the head area and assist as chemo-receptors. Thus, such organs are shown to assist in seeking food and even in reproduction (Purschke 1997). Research has been conducted on a particular species of Neriephylla closely related to Nereiphylla blainville , Nereiphylla lutea. Nuchal organs exist as ciliated cells on top of the tentacular cirri of Nereiphylla lutea instead of the general paired pit formation (Orrhage & Eibye-Jacobsen 1997). Thus, there is potential evidence to suggest Neriphylla sp. has this nuchal organ present on its prominent tentacular cirri (Figure 13). 

A unique characteristic of Neriephylla sp. is the presence of the large, prominent eyes only seen in a handful of Phyllodocidae genus’.  Rhode (1991) describes Phyllodocidae as either having simple ocelli or complex eyes. Simple ocelli were only observed in one genus and complex eyes in that of Eteone and Anataitides (Figure 14).  Complex eyes were described as eyes that were situated dorsally on the prostomium of the organism. The eyes themselves had both pigment and sensory cells as well as a lens covering the eye (Rhode 1991). Due to the sheer size and prominence of the eyes of the Nereiphylla sp. specimen, it is considered to have complex eyes instead of simple ocelli. These complex eyes house sensory cells that have attached microvilli. In Phyllodocidae, microvilli are coiled increasing the surface area of the sensory structure (Rhode 1991). Photoreceptors exist within the microvilli and are light-absorbing pigments that allow effective response to stimuli (Purschke 2005).

To allow for feeding, tentacles known as palps exist within Phyllodicidae (Figure 15). These palps extend along the surface when the organism is moving (Rhode 1991). Ciliated sensory cells exist on these palps; they increase the sensory area so that the organism can interpret various stimuli along the benthos when moving (Purschke 2005). Furthermore, the movement of the palps themselves assist in surveying an area before the organism moves its whole body further on (Rhode 1991). 

The Phyllodocidae family were first discovered and described by Orsted in 1843, the species Eulalia viridis and Phyllodoce maculata were described earlier by Linnaeus in 1767 (Rouse & Pleijel 2001). Many other genus' and sub-families were described in the nineteenth century following Orsted's discovery. Nereiphylla as a genus was discovered by Blainville in 1828 and was placed under the sub-family Notophyllinae. However, Notophyllinae and its placement on the Phyllodicidae phylogenetic tree has been continuously debated (Rouse & Pleijel 2001). 

The phylogeny of the Phyllodocidae family in general has been very widely debated and most recent findings are suggested through assumptions. Pleijel (1990) describes the group as being monophyletic considering the group evolved from benthic polychaetes. Pleijel also suggests the family consists of three main subfamilies of Notophyllinae, Phyllodocinae and Eteoninae (Figure 17). The conclusion was brought about by assessing apomorphies. In the case of Notophyllinae, these apomorphies include characteristics such as lack of median antennae, anterior ganglia, fusion of first and second segments and rounded or cordiform dorsal cirri (Pleijel 1990).

Pleijel, F 1991, 'Phylogeny and classification of the Phyllodocidae (Polychaeta)', Zoologica Scripta, vol. 20, no. 3, pp. 225-261. 

Welch, K & Dutch, M 2014, Puget Sound Polychaetes: Family Phyllodocidae, Polychaete Workshop Report, Department of Ecology, State of Washington.

Rouse, GW & Pleijel, F 2001, Polychaetes, Oxford University Press, New York. 

Tzetlin, AB, Filippova, AV 2005, 'Muscular system in polychaetes (Annelida)', Hydrobiologica, vol.535, no.1, pp. 113-126. 

Rhode, B 1991, 'Ultrastructure of Prostomial Photoreceptors in Four Marine Polychaete Species (Annelida)', Journal of Morphology, vol. 209, no.2, pp. 177-188.

Rodrigo, AP, Costa, MH, Costa, PM 2015, 'Microanatomical alterations in the gut of an marine polychaete (Eulalia viridis, Errantia: Phyllodocidae) during the digestive process', Microscopy and Microanalysis, vol.21, no.6, pp. 12-13.

Lee, CG, Huettel, M, Hong, JS, Reise, K 2004, 'Carrion-feeding on the sediment surface at nocturnal low tides by the polychaete Phyllodoce mucosa', Marine Biology, vol. 145, no. 3, pp. 575-583.

Olive, PJW 1980, 'Control Of The Reproductive Cycle In Female Eulalia Viridis (Polychaeta: Phyllodocidae)', Journal of the Marine Biological Association of the United Kingdom, vol. 61, no.4, pp. 941-958.

Lacalli, TC 1985, 'Prototroch structure and innervation in the trochophore larva of Phyllodoce (Polychaeta)', Canadian Journal of Zoology, vol. 64, no. 1, pp. 176-184.

Purschke, G 2005, 'Sense organs in polychaetes (Annelida)', Hydrobiologia, vol. 535, no.1, pp. 53-78.

Orrhage, L, Eibye-Jacobsen, D 1998, 'On the Anatomy of the Central Nervous System of Phyllodocidae (Polychaeta) and the Phylogeny of Phyllodocis Genera: a New Alternative', Acta Zoologica, vol.79, no.3, pp. 215-234.

Global Diversity Information Facility 2015, Nereiphylla Blainville, 1828, viewed 27/05/2016,
<http://www.gbif.org/species/114254465>

Special thanks to Robin Wilson, Museum of Victoria for assistance with the classification of the specimen.

Special thanks to Eunice Wong for assistance with photographing and classifying the specimen.