Giant larvae in metazoans
The phenomenon of oversized larval forms has been reported from various metazoan groups. Yet, in many cases ‘giant’ is a matter of relation. An overview of giant larvae can be seen in Tab. 1.
Table 1. Overview of giant larvae with larval terms and reported maximum sizes of their respective group or close relatives.
As pointed out above, larvae of flying insects (Pterygota) are in their final larval stage often as large, sometimes larger, than the adult. Yet, as almost all insects have such comparably large larvae it is somehow difficult to consider any of them as a giant. Comparably larger larval size is mostly coupled to larger adult size.
Larvae of corals, sea anemones and others (Cnidaria) – planula – have an average maximum size of about 1 mm (Leloup, 1932). Yet, also specimens of up to 11 mm have been reported. Some of the even larger specimens with larva-like morphology already possess gonads (Molodtsova 2004; Stampar et al., 2015) and are therefore no longer larvae in the meaning of being immature.
The planktic larvae of marine snails and slugs (Gastropoda) – veliger – are usually below 1 mm in size before settling to a pelagic life. Yet, in some groups significantly larger forms are known. Veliger larvae of strombiids, coniids and cypraeiids have extremely elongated structures, the velum lobes. With these structures they reach sizes of about 5 mm (Hickman, 1999). Even larger forms of about 6–7 mm have been reported by Dawydoff (1940).
The early larval stage of ringed or segmented worms (Annelida) is plesiomorphically the trochophora. These are mostly below one millimeter in size before they metamorphose into forms with few body segments that carry appendages (chaetigers; often three such segments). Exceptions are special forms of phyllodocid larvae. Here the trunk grows significantly longer from the trochophora before undergoing metamorphosis. The spherical anterior region (hence the original trochophora) can reach sizes of up to 2 mm; the trunk with up to 120 rudimentary segments can reach 10 mm. Hence, the total length of these larvae reaches up to 12 mm (Tzetlin, 1998).
Larvae of peanut worms (Sipunculida) – pelagosphaera – have an average size of 300 µm (Rice, 1967). Yet also significantly larger forms of up to 3.2 mm can sometimes be found in the plankton of open ocean regions (Rice, 1973).
Larvae of horseshoe worms (Phoronida) – actinotrocha – reach in general a maximum size of 0.7–0.9 mm. An unusually large phoronid larva has been reported by Temereva et al. (2006). This larval specimen was 3.5 mm long, thus 4–5 times larger than a “normal” actinotrocha larva.
Larvae of echinoderms (Echinodermata) are generally small, below 1 mm (e.g. Pawson, 1971). Yet, certain larvae of abyssal sea cucumbers (Holothuroidea) – auricularia – can reach sizes between 3 and 15 mm (Ohshima, 1911; Mortensen, 1913; 1921; Garstang, 1939). Also, the larva of the deep-sea starfish Luidia sarsi (Asteroidea) – bipinnaria – can reach body lengths of up to 25–35 mm (Domanski, 1984).
Larvae of acorn worms (Enteropneusta, Hemichordata) – tornaria – reach usually about 0.5–1 mm (Stiasny, 1928). Giant tornaria-like larvae (Planctosphaera pelagica) with a length of up to 28 mm have been found in the Atlantic and Pacific Ocean (Spengel, 1932; Hadfield and Young, 1983). Thus the found giant larvae are at least 20 times bigger than the “normal” larvae of Hemichordata. Yet, it is still controversial if Planctosphaera pelagica represents an ingroup of Enteropneusta (Hadfield and Young, 1983) or a separate group of hemichordates (Van der Horst, 1936).
Larvae of teleost fishes (Teleostei) are often quite large; few centimeters length is not uncommon. A very notable size is reached by larval eels (Anguilliformes) – leptocephalus – which regularly reach 300 mm in length (Miller, 2009), but sometimes even giant larvae longer than 1800 mm have been reported (Aron and McCrery, 1958; Tabeta, 1970; Kurogi et al., 2016).
Amphibian tadpoles (Lissamphibia) are all large compared to many other metazoan larvae, being in the range of several centimeters. Tadpoles of the frog Pseudis paradoxa reach sizes of up to 230 mm (Emerson, 1988). Also other species of Pseudis can reach quite a large tadpole sizes with up to 180 mm (Fabrezi et al., 2009). In these species the larva is also significantly larger than the adult. Fossil tadpoles with a size of up to 150 mm have been reported from the Miocene (Roček et al., 2006) and from the Lower Cretaceous (Chipman and Tchernov, 2002).