Schindleria praematura

Schindler (1930), described the first species from the Hawaiian Islands as Hemiramphus praematurus because he assumed that the species was a larval hemiramphid. The second species –also described by Schindler (1931)—was observed from the Hawaiian Islands and described as Hemiramphus pietschmanni. Giltay (1934), found a single specimen of H. praematurus off New Guinea, and concluded that the species did not show a close relation to other known hemiramphids. He therefore combined both species Schindler found into a new genus called Schindleria and a new family Schindleriidae.^[8] And the third species, Schindleria brevipinguis, was described by Watson et Walker, 2004 off north-eastern Australia ^[9] To date, these three species along with eight others (totaling 11 species) are the only described members of the family, although studies have suggested that this family probably contains many species that have yet to be discovered and classified.^[9] However, the classification of Schindleria praematura moved from the family Schindleriidae to the family Gobiidae in 2009 based on the molecular phylogenetic study ^[10] and was later supported by Nelson’s et al.’s classification in 2016.^[11] Schindleriidae is also a member of the suborder Gobioidei.

Schindleria (Gobiidae) are the world’s smallest and fastest maturing marine vertebrates. They are likely the most extreme case of paedomorphism among teleost fishes. Adult S. praematura have an average body length of around 10-20 mm resembling the body length of most fishes early larval stage.^[7] Some of the larval characteristics observed in sexually mature individuals include functional pronephros –(most primitive form of a kidney for excretory organ functions), a transparent body and a largely unossified skeleton (not yet converted to bone or lacking a bony structure)^[12]

Identification of the species as Schindleria praematura (Schindler) is based on diagnostic features including but not limited to 13 segmented caudal rays, the gut extending to about two-thirds of the standard length, between 18 to 22 dorsal fin rays, and 11 to 14 anal fin rays, which originate immediately behind the urogenital opening.^[13][14] The entire body, including the head, is uniformly whitish, elongated, and compressed. The average length of head is about 10.5% of the total length. The black pigment cap on the swim bladder is visible through the body wall, and their fins are translucent. (Figure 1.) The iris of the eye is black and capped dorsally with an iridescent silvery layer spotted with numerous melanophores.^[6] Eye diameter is about equal to the length of the snout, 25% in head length, and two or three rows of small teeth are present in the jaws.^[13] (Figure 2.) S. praematura are sexually dimorphic. Among females, the most distinctive feature is the pre-dorsal fin length and the caudal fin length, while among males, the snout length was overall most distinctive. There were different urogenital papilla (UG) shape between males and females. Males of S. praematura had a long and narrow UG with the opening on the tip. In female S. praematura, the UG was rounded and shorter compared to the males’, but was distinctly wider and had two narrow projections laterally to the UG opening.^[15]

Schindleriidae, a family of small, paedomorphic, marine gobioid fishes (Johnson & Brothers, 1993), is widely distributed in warm-water reef habitats of the Indian and Pacific Oceans ^[16] Schindleria’s biogeographic range is extremely large, inhabiting a wide range of environmentally very different tropical waters at and off coral reefs (figure 1), from the submarine ridges of Nazca and Sala y Gomez, off of South America, to Hawaii, South Africa, and the Red Sea.^[15] But more specifically, Schindleria praematura is confined to Indo-West Pacific coral reefs and is distributed from southern Japan ^[17] and the South China Sea ^[12] to the Great Barrier Reef (Australia), Papua New Guinea, Palau and the Marshall Islands. This species has also been observed at Easter Island, Madagascar in the western Indian Ocean and Southern African regions inside the Kosi Mouth Estuary, KwaZulu-Natal South Africa.^[18] Additionally, the first record of the species in the Red Sea was made by El-Regal and Kon, 2019 which significantly extends the known distribution of this species northward in the Indo-West Pacific.^[8] (Figure 3.)

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Mature S. praematura displays some characters which could be regarded as larval (linear form of heart, lack of surface body pigment, developing vertebral column) and others which are debatably larval (pronephros, small body size). Depending on how the combination of larval and sexual characters are viewed Schindleria might be described as either paedogenetic (essentially larvae with precocious gonads) or neotenic (adults in which larval characters have been retained- called peadomorphosis) with often the latter being described in most literature.^[16] Schindleria praematura is unusual among gobies in that it is planktonic; not only does it retain a larval morphology, but it also exhibits a larval ecology, never settling out of the plankton as do most gobies. Schindleria praematura populations have been recorded near islands in the tropical Pacific and Indian oceans, not too far out in open waters, and perform the diel migrations typical of plankton, rising to the surface at night to feed and returning to the bottom during the day.^[15][17] Schindleria praematura primarily feeds on zooplankton. More specifically its diet is known to include small marine invertebrates such as crustaceans, and mollusks. Copepods are a large part of their diet. It feeds in the water column near the seabed on drifting organisms.^[6]

Schindleriidae, now referred to as Gobiidae, are one of the few families that can use atoll lagoons throughout their larval stage and are therefore called “lagoon completers”. Hence, Schindleria may remain pelagic for most of their very short lifespan of less than three months and may use the reef only during a brief period for reproduction.^[7] A study around the Hawaiian island assumes these fish migrate daily around the reef matrix. They move to shallower depths in the lagoons at night. This migration has also been suggested to occur at smaller spatial scales, where vertical daily migrations take place between the epibenthos during the day and the water column during the night. The reason for migration behavior in Schindleria is still unknown, but it is likely that the two species S. praematura and S. pietschemanni differ in their migration behavior and the spatial scale at which migration is taking place around the Hawaiian coasts. S. pietschmanni was found to be more abundant close to shore while S. praematura was more abundant offshore, and the latter was more frequently caught distant from the reef matrix and the former in closer vicinity to the reef. This supports the idea of differing habitat and/or ecology in these two species due to intraspecific competition for limited resources.^[15]

The first description of the swimming behavior of Schindleria was studied by, Robitzch, V., Olivier, D., & Ahnelt, H. (2022). They determined that Schindleria can switch between two swimming modes: the anguilliform—a type of movement used by all fish larvae; and the subcarangiform—a type of movement found in many late larvae and juvenile fishes (Fig. 3). Schindleria also used a "C-start" to initiate swimming or change swimming direction, another typical movement amongst larval fish stages. The pectoral fins were kept close to the body during the movements.^[7] (Figure 4.)

Schindleria may also have a relatively higher anguilliform swimming performance compared to other similar sized fishes, which may be related to its particularly large pectoral fins and its long, less rudimentary dorsal and anal fins in comparison to other larvae. The two median fins may further increase stability, aid against roll, and augment thrust during the anguilliform swimming; overall, increasing speed in Schindleria’s anguilliform swimming.^[7]

This complex and advanced swimming ability the study recorded in Schindleria, despite its small and progenic body construction, suggest that the development of unique morphological features has been advantageous in this short-lived taxon. Thus, Schindleria must have taken advantage of performing one or the other swimming mode depending on the purpose of the swimming behavior and evolved the ability to switch between them.^[7]

For example, subcarangiform swimming may be required for higher swimming speed (during escape or foraging) or the maintenance of swimming speed over prolonged swimming periods (during migrations to coral reefs for reproduction), while the anguilliform mode may be more efficient during slower swimming movements or maneuverability that are less energetically demanding.^[7]

Additionally, although it was believed that Schindleria spent most of its time in reefs, Robitzch, V., Olivier, D., & Ahnelt, H. (2022) suggests that due to morphological features such as body shape, its small size, its paedomorphic nature, and low Reynold's number this fish may favor anguilliform swimming to remain in the water column and migrate long distances in the pelagic zone while being energy efficient. Thus, many Schindleria may remain pelagic for most of their very short lifespan (of less than three months) and may use the reef only during a brief period of a few days for reproduction.^[7][15] On the other hand, fast swimming speeds are easier to reach with a subcarangiform swimming, for which Schindleria may have evolved its unique teleost structures and further developed few of its rudimentary fins despite progenesis.^[7] (Figure 4.)