The possible function of giant larvae
Generally, we can distinguish between two types of giant larva: Type one are facultative giant larvae, type two are obligate giant larvae.
Type one giant larvae occur in species that usually have “normal-sized” larva, but in which from time to time giant individuals occur. Here ‘giant’ is meant in comparison to individuals of the same species. Such giant larvae must be understood as caused by external factors. A rather simple and probably widespread case for causing such instances is the simple absence of a settling trigger. Many larvae need specific chemical environmental cues that indicate an advantageous habitat for the benthic juvenile/adult. If such cues are absent, larvae can simply continue to grow without metamorphosing. Also other abiotic factors have been suggested to be important in this aspect. For example, temperature and shifts in photoperiod length seem to influence the development of tadpoles in the direction to giant tadpoles (Emerson, 1988; Fabrezi et al., 2010).
It has also been suggested that giant size of larvae may be a consequence of a physiological defect. Such larvae often already develop adult organs, e.g., primordial gonads (Temereva et al., 2006). A disruption in thyroid hormone production before metamorphosis has been suggested as reason for this phenomenon (Emerson, 1988; Shi and Hayes, 1994; Schreiber et al., 2001; Yun-Bo et al., 2001; Ogielska and Kotusz, 2004; Rot-Nikcevic and Wassersug, 2004; Roček et al., 2006).
Parasites have also been identified as causes of suppressing a metamorphosis trigger, with this leading to giant-sized larval forms. Insect larvae infected with parasites molt more often than non-parasitized larvae and die as giant larvae (Fisher, 1963). Hormones increasing the juvenile activity of the host cause this exceptional development. In this way, the parasite gets a larger host by its hormone manipulation (Dawkins, 1990).
Type two giant larvae are cases in which representatives of all individual species (or larger group) develop through larval forms that grow significantly larger than the larvae of closely related groups (Fabrezi and Goldberg, 2009). This also leads to a prolonged larval phase. Such a prolonged larval span can enhance the capability for long-distance dispersal in the planktic phase of some species of different molluscs, echinodermatans, or achelatan lobsters (Domanski, 1984).
In this context, one could think of abyssal gigantism (Herring, 2001) also as explanation for giant larvae. Mainly crustaceans have been reported to reach a larger size in deep-sea environments than their relatives in shallow waters (King and Butler, 1985; Mauchline, 1995; Chapelle and Peck, 1999). Low temperature and restricted food availability in deep seas are thought to decrease growth rates, but to increase longevity and the time span to reach sexual maturity (Nybakken, 2001). Hence, it seems to affect juvenile instead of larval development, not necessarily leading to large larvae. Abyssal gigantism has been proposed for the loriciferan Higgins larvae by Gad (2005). Yet, these forms are in fact paedomorphic adults and therefore not larvae.
Giant larvae of type two often bear structural specializations. In many giant crustacean larvae spines or extensions of the shield are necessary to increase the buoyancy (Eiler et al., 2016; Haug et al., 2016). Eel larvae deposit large amounts of glycosaminoglycans in their musculature increasing their swimming ability due to the enhanced skeletal stability (Bishop and Torres, 1999). Giant acorn worm larvae are adapted to a prolonged larval span by relatively larger feeding structures to process more food (Damas and Stiasny, 1961; Strathmann and Bonar, 1976).
Interestingly, we can even identify combined cases of type one and type two giant larvae. Eel larvae are in some species 300 mm in average and with this significantly larger than many other fish larvae and representing cases of giant larvae of type 2. Yet, among these even larger larval individuals are known of 1800 mm, with this being cases of type 1, representing a kind of super-giant larva.