A few morphological features can be used in identification and taxonomy of ostracods. These include the shape of the carapace, the number and shape of appendages (including male copulatory organs) and the adductor muscle scar patterns inside the carapace. Identifying ostracods is very difficult and requires dissection and careful examination of these features under a microscope (Maddocks, 1992). There are 131 species belonging to the genus Neonesidea at present (Brandao & Horne, 2014). The Neonesidea had a carapace length ranging from 0.8 to 1mm and I could not confirm some features to enable identification to a species level. Features will be described based on what could be seen in the specimens collected and what may be normal for the genus Neonesidea in general.

Neonesidea is benthic and found living in shallow tropical and subtropical marine environments including rock or dead coral fragments, calcareous algae and sand in reef environments (Maddocks, 2013). Subfossil carapaces are often found in sediments in close proximity to reefs (Maddocks, 2013). I found 11 specimens of the same Neonesidea species of ostracod on the bottom of a container with reef rubble covered in algae (figures 8 and 9). The reef rubble had been collected from the reef flat on Heron Island.

There is little information available on the feeding preference, interaction with predators or population dynamics for Neonesidea. Generally Bairdioidea are benthic crawlers that mostly feed on detritus (Maddocks, 1992; Smith & Horne, 2002). There is no planktonic larval stage like most other crustaceans have where they disperse. Instead dispersal of Podocopida may include transport of eggs via currents or by fish (where they can pass through the gut undigested and still viable) or by the adults being swept away by currents (Smith & Horne, 2002). 

The Cypridinoidea were collected with coral rubble which suggests that these species are also benthic (although there are pelagic species within the superfamily). Some species of Cypridinoidea are infaunal (burying themselves in the sediment) by day and actively swimming by night (Vannier & Abe, 1993). In terms of their feeding, a study at Lizard Island on the Great Barrier Reef recorded numerous Cypridinidae in traps baited with fish carcasses suggesting they are scavengers (Keable, 1995). Predators of Cypridinoidea have been noted to include nektobenthic fishes and larger crustaceans (Vannier & Abe, 1993).

Reproduction and Development

Most marine ostracods are gonochoristic and reproduce sexually with internal fertilization (Maddocks, 1992). Copulation takes place in a ventral to ventral position (Katsumi & Vannier, 1991). In one species of Podocopida, it was reported that males initiated courtship and copulating by touching a female and rotating her using their fifth and sixth limbs (Katsumi & Vannier, 1991). 

Most female Podocopida (which includes Bairdiidae) lay eggs singly or in clusters, whereas most female Myodocopida (which includes Cypridinoidea) have brood care where juvenile instars are released upon hatching. Smith and Kamiya (2002) detailed the ontogeny of a Neonesidea species and noted in an unpublished thesis that females lay up to a cluster of 10 eggs. The first juvenile stage (also known as instar) hatches out much smaller in size and with fewer appendages but still resemble adults. It has been reported in that Neonesidea go through 8 set instar stages before they become adults after which they no longer moult (Cohen & Morin, 1990; Maddocks, 2013). One species that was investigated, N. oligodentata, was found to have 7 free living instar stages. They suggest that the first instar moults while still inside the egg (Smith & Kamiya, 2002). Ontogeny of Myodocopida consists of fewer instar stages before reaching adult stage (Horne et al., 2002). The life span in Podocopida is thought to range from a few months to 4 years and from 2 months to 4 years in some Myodocopida (Horne at al., 2002).

Most studies of reproductive behaviour have been on Cypridinoidea species in which some males have an unusual courtship display which involves bioluminescence (Rivers & Morin, 2008). Luminescent compounds are released by the males into the water by a special organ which results in bright blue pulses of light used to attract females (Rivers & Morin, 2008). However, these displays have mostly been recorded in shallow seagrass beds in the Caribbean area. So far there have been no recorded bioluminescent displays by Ostracoda in the Great Barrier Reef. 

Locomotion

Locomotion is another feature which is difficult to study in such small marine invertebrates. These Neonesidea are benthic crawlers and appear to use a combination of their antennules, antennae and three paired limbs (fifth, sixth and seventh). In the video below, the Neonesidea are on their sides possibly because there was no substrate to allow them to get any grip on the bottom of the petri dish. The Cypridinoidea are able to swim and move around much faster by using their antennae (like oars out each side) and developed furca for propulsion and rotation (Maddocks, 1992; Vannier & Abe, 1993). See below video. 

Carapace

The carapace is composed of chitin as well as calcium carbonate. There is a thin outer chitinous layer, a calcified layer in the middle and an inner chitinous layer. The body and appendages are covered in a chitinous cuticle (Maddocks, 1992; Horne et al., 2002). Myodocopid (which includes Cypridinoidea) tend to have less calcified or uncalcified carapaces and so they rarely survive as microfossils unlike the Podocopid (Maddocks, 1992; Horne et al., 2002). This may explain why the Neonesidea carapace felt much harder and was less likely to break during dissection than the Cypridinoidea.

Muscles

Ostracods only have striated muscle and its structure is similar to that of other arthropods, except that instead of an opposing-lever arrangement, it uses a hydrostatic pressure system (Maddocks, 1992). The adductor muscles run through the centre of the body and are used to close the carapace (Horne et al., 2002). 

Digestive System

All ostracods have a complete gut similar to other crustacea. In addition, all Bairdiidae have a unique masticatory organ at the top of the oesophagus. This breaks down food for the stomach and hind gut (Maddocks, 1992)(see figure 7).

Gas Exchange and Circulatory System

Neonesidea do not have gills or other special respiratory structure. They are small enough that gas exchange takes place by diffusion across the soft body (Maddocks, 1992; Horne et al., 2002). When the valves are open, vibratory plates move rapidly to create a water current over the soft body surface (Maddocks, 1992; Horne et al., 2002). A heart is absent in Podocopida (which includes Neonesidea) and is present in Cypridinidae (Maddocks, 1992).

Reproductive System

The female reproductive system consists of the ovaries and uterus in the posterior region of the carapace and genital lobes (see figure 10 close up of the genital lobe). The genital lobe houses the vagina and a coiled tube leading to the seminal receptacles (storage for sperm)(Maddocks, 1992). The male reproductive system accounts for a large portion of the body volume and is complicated. Testes and seminal vesicles are in the posterior region of the carapace. Vas deferens lead into the copulatory appendage which includes the hemipenes and clasping structures (Maddocks, 1992). The male copulatory appendage varies in each species (especially in Podocopida) and in some cases can be used to identify taxa (see figure 7 for example of hemipenis of Neonesidea). In male Cypridinidae there is an additional organ called a zeneker's organ which is an ejaculatory pump. Reproductive systems are paired in both males and females (Maddocks, 1992).  

Nervous System and Sensory Organs

The central nervous system in ostracods is highly organised and consists of a cerebrum and different ganglia (Maddocks, 1992). In some Neonesidea there is a type 1 naupliar eye which is separate from the carapace and lacks a lens (Tanaka, 2005). I was unable to locate an eye in my specimens. However, it was found in specimens of five different species of Neonesidea described by Maddocks (2013) who noted the eye is small and unpigmented. The eye should be positioned near the base of the antennules and it is thought that it is only used to determine the presence and direction of light (Tanaka, 2005). It would make sense for Neonesidea to have an eye as they live in the photic zone. 

Cypridinidae have a pair of lateral compound eyes in addition to the naupliar eye (see figures 5 and 6 where the eye marked is the compound eye). The naupliar eye is capable of light detection as in Neonesidea. The compound eyes are thought to be capable of light detection as well as image formation (Huvard, 1990; Huvard, 1990b). Most research on these compound eyes have been on species that produce bioluminescent displays. It is thought that the compound eyes play a role in detecting luminescent courtship signalling displays in those species (Huvard, 1990; Huvard, 1990b). 

Neonesidea is common in reef environments and has a circumtropical distribution (Maddocks, 2013).  See figure 11 map showing points where they have been mentioned prominently in the literature. This is just as an example and is not exhaustive. Examples are from Australia, Indonesia (Mostafawi et al., 2005), Ascension Island (Maddocks, 1975), Madagascar (Maddocks, 1969), Indian Ocean region (Maddocks, 1969), Easter Island (Whatley & Jones, 1999), Florida USA, Bermuda (Maddocks & Iliffe, 1986), Belize (Maddocks, 1969) and Japan (Smith & Kamiya, 2002).

Classification

Here, I follow the classification of Neonesidea by Maddocks (2013).

Phylogeny within Ostracoda has been based on appendages (antennules and antennae) (Horne, 2005; Kaji & Tsukagoshi, 2010) as well as carapace features. Ontogeny has also been used to suggest the relationships within families (Smith & Kamiya, 2002). Some authors still have Ostracoda as a subclass to Maxillopoda, but often Ostracoda has been assigned its own class (Horne et al., 2002). There is some morphological and molecular evidence that the subclasses Podocopa and Myodocopa (which the Cypridoidea belong to) are not monophyletic (Horne, 2005). Abushik (2015) points out the difficulties of systematics in Ostracoda between extinct and extant groups as soft body features such as the appendages which are used for extant groups are rarely preserved in the microfossils. Relationships and classification remains under debate with new information and methods that arise.  Butlin and Menozzi (2000) discuss the use of ostracods in evolutionary ecology to answer questions in sexual reproduction or sexual selection considering they have complex reproductive systems. Some ostracods (from Podocopa) have giant sperm, being three times the length of the male itself (Matzke-Karasz, 2005). 

Fossil Record

Ostracods have a fossil record extending back to the Ordovician (Siveter et al., 2014). Ostracods are useful to geologists and palaeontologists in biostratigraphy and for interpreting palaeoenvironments (Kornicker, 1961). Especially those living species which have a rich fossil history. Research into the ecology of the living species and their habitat range helps with interpreting the past environments (Kornicker, 1961). We know that existing Bairdiidae have a tropical distribution and it is likely that past Bairdiidae had the same tropical distribution. The different designs of ostracod valves also provide an idea on the type and energy of the environment and chemical analysis of the shells can provide detailed information of the salinity and temperature (Frenzel & Boomer, 2005).