2
Reproductive Behavior: Parents

Having completed their role as spawners, the partners will often take their leave of each other, abandoning their eggs. However, some continue their common or solitary lives by taking care of their eggs and then eventually their offspring, who are vulnerable to predation. It is appropriate to protect these and sometimes to feed them with nutrients and respiratory gases, as well as to educate them in order to enable them not only to survive but also to prepare for a life full of pitfalls.

Many ovuliparous species do not wait for mating to take on these parental tasks. In anticipation, they prepare formidable welcoming structures that will be appreciated by their sexual partners and which will count in their favor during the period of sexual selection (Volume 2, section 1.1.1). The construction of spawning nests (Volume 2, section 2.1.1) thus contributes greatly to the success in love of males of various species. These males are capable of responding to a multitude of demands from their female partners, and their parental phase begins early, only to be completed when the young have survived.

Some of developments that follow could perfectly well find a place in Chapter 1, if we were to respect to the letter the chronological order of reproductive events. However, the early construction of nests and the late practice of parental care (respectively before and after spawning) constitute a set of activities with a common purpose for the benefit of eggs, embryos, larvae and juveniles, which justifies the choice we have made to locate them in this Chapter 2.

2.1. The post-spawning phase: the future of the offspring

2.1.1. Nest builders

Nesting areas or spawning grounds are chosen by spawners in order to ensure for the clutches (eggs, then larvae) the conditions most favorable to their survival: accessible food resources, protection against predators, optimum temperatures ensuring their development and growth, etc. These areas of reproduction are generally traditional, located close to their feeding habitats (Volume 1, section 1.2) or alternatively located at more or less significant distances, which require spawners to undertake reproductive migration of a larger or smaller distance. They are occupied by successive generations of spawners who perpetuate the "traditions" specific to each species, who have used them to their benefit since time immemorial.

The interest of some of these breeding areas lies in the fact that "spawning nests" are constructed by the spawners, acting as builders of more or less developed structures that promote the protection of eggs, and then sometimes also larvae.

2.1.1.1. Architects and urbanists anxious to please their demanding companions

Birds are often taken as models of inventive nesting capacities in vertebrates. Yet sometimes, fish have nothing to envy of them. They were even, from a chronological point of view in the course of evolution, the initiators.

There are behaviors prefiguring nesting, such as those of silversides, such as Atherina boyeri, who seek out branching algae such as Gracilaria on which to hang their eggs which will then be immediately abandoned by the spawners. Other species are content to choose an overhang of rock to which to attach their clutches without making specific accommodations, clutches which they leave behind.

When there is preparation of a spawning space or construction of a nest intended to host the offspring of couples, it is generally the male who does this work. These nests have a dual function: they protect the clutches and the fry resulting from their hatching, thanks to their robustness (resistance to mechanical shock), their ease of defending (visually accessible location) by the male owner of the nest, the difficulty of penetration by predators (orientation and size of openings), but they are also attractive to the females who are invited to enter there to deposit their oocytes. The latter are generally demanding and prefer the largest nests which are capable of hosting a large number of eggs, the best located and protected in relation to the hydraulic currents, the best concealed from the view of potential predators and whose low opening is the most obstructive to penetration. Males must therefore integrate all these constraints into their urban planning and in the choice of their architecture. They seek to make best use of the mineral, vegetable or animal resources of their near environment, always showing creativity and never reproducing nests identically, in a stereotypical manner. They take into account the local hydraulic constraints, the availability of materials as well as forecasts of the activities of guardianship that are complementary to their efforts of nesting (Volume 2, section 2.1.1).

Note the case of the addition of a final touch brought to the nests by the male builders: some gobiids cover the internal walls of their nests, before attracting females, with semen rich in sperm embedded in a mucus rich in nutrients with bactericidal and fungicidal components.

2.1.1.2. Mineral nests

Some species use natural cavities available in their habitats. The male of the barred-chin blenny Rhabdoblennius ellipes does not build a nest in the strict sense, merely using cavities dug into the coastal rocks by shellfish or burrowing worms. The choice of cavity must respond to a security objective: having an entry carefully adapted to the cephalic size of the nesting fish in such a way as to prohibit any space allowing the entry of a potential predator. This nest being perfectly matched to the size of its guardian, the latter changes habitat during its growth. It is from this entryway that he courts passing females, by nods of the head, in order to invite them to enter the nest. Females' preference, upon their first visit - one-step decision - or after several consecutive visits in the case of sequential choice, is both a function of the quality of the nest and that of its guardian, those with a well-developed cephalic ridge (Volume 2, section 1.1.1) having the most success.

The benthic* sculpins endemic to Lake Baikal Cottus kneri and C. kessleri use rocky, hard bottoms and look for overhanging stones for spawning nests. Competition between them is severe and the number of favorable microhabitats often insufficient, so that it is common to find nests used jointly by the two species. Males of C. kneri are however able to discern the foreign eggs of C. kessleri and cannibalize them.

Males of the Amur goby Rhinogobius sp. of Japanese water courses build nests by digging in the sand or gravel below a stone, with a space intended to accommodate the clutches of the multiple females that they attract into this nest. Their reproductive success, measured by the number of eggs deposited, depends on both the size of the nest, in favor of larger ones, and their topographic position in the current. Those located in more rapid currents are the most appreciated for two reasons: they demonstrate the constructor's great swimming skills and they offer the clutches better oxygenation. These are the large, oldest males of 3 years who, although constituting only 36% of the population, are entrusted with reproductive success by becoming predominantly, by 70%, the guardians of clutches. The males of the American hornyhead chub Nocomis biguttatus build, in the upper Mississippi, nests at depth, in a moderate current. These nests in the shape of a dome are the result of an accumulation of approximately 3,000 pebbles and stones for a total weight of 11 kg. These are of small diameter (6-8 mm) in order to reduce the energy cost of transport, but dense in order to avoid a risk of dispersion by the current while ensuring a good protection of clutches as well as their oxygenation by means of percolation* by a current of water. Collection of materials, performed by mouth, is done over a radius of several meters. Totaling up the comings and goings, the path traveled in transporting pebbles for the construction of such a nest would be around 25 km, remarkable in a small fish of 15 cm in length. The neighboring species N. micropogon does even better: a nest of 40 kg of materials.

Male cichlid fish, such as tilapia Oreochromis niloticus, use the substrate of African lakes to make nests where the clutches will be laid. They choose, following buccal inspection, the most favorable type of substrate, preferably homogeneous, soft and whose elements are transportable - sand or gravel. They then proceed to preparation of the nest, digging and moving the mineral elements with their mouth so as to create a regularly circular depression. Vertical constructions in the sand which are clearly more elaborate in the form of towers ending in a platform are produced by the utaka cichlids of Lake Malawi of the genus Copadichromis. Their height, diameter and the inclination of the walls of these small volcanoes that are used during courtship behavior and for spawning vary according to a dozen construction plans which are rigorously specific. They thereby demonstrate a certain innate sense of geometry, which is a good way for the fine conservative females to recognize males of their species. These characteristics thus serve as a signal of recognition for spawners of each species in order to induce homospecific couplings. Females use the criteria of...