In 1865 G.O. Sars (1837-1927) was asked by the Norwegian authorities to study the biology of Gadus morhua in order to understand the fluctuations of cod fisheries in the Lofoten area. Sars carefully studied, for the first time, the complete life history of cod, from pelagic eggs and pelagic larvae to juveniles and adults. The pioneer work of Sars stimulated interest in pelagic fish eggs and larvae. Soon it was realized that most species of commercial interest had planktonic eggs and larvae.
Systematic sampling of fish eggs was initiated by the German planktonologist Vitor Hensen (1835-1924). Hensen devised special plankton nets to capture pelagic fish eggs in a quantitative way. During the last two decades of the 19th century, fish eggs and early larvae were reared under controlled conditions to determine the main characters that would permit their identification in plankton samples. This early work was mainly pursued in England, Italy and Germany. Among these pioneers were J.T. Cunningham, E.W.L. Holt, W.C. M'Intosh, W.C. Prince, A.T. Masterman in the United Kingdom, C. Emery, L. Facciola, F. Raffaele in Italy and E. Ehrenbaum, FR. Heincke in Germany.
Older stages of fish larvae were seldom obtained by rearing fish eggs. The improvement of plankton nets and research vessels was a major step forward in the early 20th century. German researchers (V. Hensen, C. Apstein) were responsible for most of the early work on performance and quantification of plankton nets. C.G.J. Petersen in Denmark designed a very effective young fish trawl.
E. Ehrenbaum (1861-1942) published a comprehensive account of these studies that became a standard reference for the identification of early life history stages of marine fish in the North-eastern Atlantic. This book was published in two volumes, one dated 1905 and the other 1909.
A series of papers published by C.G.J. Petersen and J. Schmidt emerged from the cooperative research undertaken by the research steamer "Thor" off Iceland and the Faeroe Islands in 1903 and 1904. Petersen described the early life history stages of flatfishes. Schmidt dealt mainly with the genus Gadus. These contributions are landmarks even by present standards. Schmidt gives an excellent description of his technique for describing the early life history stages:
"The order of procedure has been, to begin with such older stages as were so far developed, that they could be identified from characters similar to those which mark the adult fish. Then, earlier and still earlier stages were taken and compared with the older, and the determinations were in this manner, if the material was rich enough, successfully carried down to the youngest, post-larval stages, attention being directed to certain outstanding characters whose successive development could be followed thought the development series. The method thus employed for the determination of the unknown, pelagic fish-young might be called the series-method and it stands in contrast to the hatching-method in that, instead of making certain starting-point with the egg and following its further development, it begins at the opposite end and follows the development backwards. The condition for being able to use the series-method is, that a large material has to be at disposal, but in such cases it will lead to certain results, especially if the material contains whole series of the species".
Schmidt used for the first time meristic counts especially of anal and dorsal fins to verify each series. The pigmentation pattern was also extensively used for discriminating among the described gadoid species. J. Schmidt is also well known for his work on the European eel, especially for establishing the its breeding ground.
The Danish researcher A.V. Tåning described a great number of larval myctophids and sternoptychids. His larval studies of myctophids preceded his major contribution in adult taxonomy. V. Ege made very important contributions on larvae of meso- and bathypelagic species. E. Bertelsen was the first author to include larval characters in the clarification of the taxonomy of ceratioid fishes.
A few ichthyoplanktonologists are well known for their collective works. L. Sanzo published 65 contributions between 1905 and 1940 that lead to the publication of the monograph "Uova, larve e stadî giovanili di Teleostei" that appeared in the series "Fauna e Flora del Golfo di Napoli". This monograph was published over a 25-year period in four volumes. The monograph used extensively the material sampled by Lo Bianco, however his name does not appear as author of any section.
The study of fish eggs and larvae can thus be roughly divided into two major periods: (i) rearing work for the description of early developmental stages, non quantitative sampling at sea mainly for studies of identification, life history and overall distribution of eggs and early larvae; (ii) quantitative surveys for estimations of abundance as a measure of parent stock size and subsequent recruitment, studies on the ecology of fish eggs and larvae at sea and experimental contributions to the physiology of the early life history stages.
There are perhaps three principal reasons why ichthyoplankton surveys are made: (i) Surveys are often directed towards a single target species (or a group of closely related species) in order to use the distribution and abundance of the pelagic eggs to obtain an estimate of the biomass of the adult spawning population; (ii) Larvae of the target species are studied in order to estimate the success of the year brood resulting from its spawn and hopefully to understand the factors underlying fluctuations e survival; (iii) Surveys are also used to evaluate fish resources in general. The plankton net collects the eggs and larvae of all kinds of fishes with pelagic eggs and/or larval stages. It provides important information on exploited as well as unexploited resources. With few exceptions, it provides information on the whole spectrum of fish in the area being surveyed.
As mentioned before, field investigations of fish eggs and larvae originated in the late 1800s. The motivations for investigations have changed little over this period, being mainly the assessment of adult spawning biomass and distribution, and the desire to understand how environmental variations and changes in the abundance of other species interact to regulate the abundance in particular fish populations. The factors affecting recruitment, in particular those that affect the survival of fish eggs and larvae are of key importance in this context.
The process of recruitment, in spite of about 100 years of research, is still not fully understood. Trophic relations are implicated as a major influence on early fish life dynamics and are embodied in the "critical period" and "match-mismatch" hypothesis. Spatial characteristics are also considered of importance in the alternative "member-vagrant" and "ocean triad" (enrichment, concentration, transport/retention) hypotheses.
Most marine fishes spawn pelagic eggs that are fertilized externally and float individually near the sea surface. These eggs range from 0.5 to 5.5 mm in diameter. The embryonic period can be divided into three stages: early (fertilization to bastopore closure); middle (from blastopore closure to the time the tail begins to curve laterally away from the embryonic axis) and; late (from the time the tail is curved away from the embryonic axis to hatching).
Within a single species there is little variation in egg characters (size, number and size of oil globules, chorion surface, yolk, pigmentation, and morphology of the developing embryo) (Anatomic and morphometric features of early stages of fish eggs). Development time is highly related to temperature and is species-specific. The majority of pelagic eggs are spherical with chorion diameters close to 1 mm. Species in some groups produce eggs with ellipsoidal chorions (e.g. Engraulidae). Demersal eggs tend to be spherical (e.g. Blenniidae), flattened (e.g. Blenniidae) or urn-shaped (e.g. Gobiidae). The chorion can be smooth or ornamented with spines and filaments (e.g. Belonidae, Atherinidae), hexagonal or polygonal networks of different sizes (e.g. Callinonymidae, Macrouridae) or a single protuberance or swelling (e.g. Centrachantidae). The space between the chorion and the yolk mass (perivitelline space) is usually small, but in some groups can be considerably large (e.g. Clupeidae, Anguiliforms). The yolk can be segmented or homogenous. In some groups the yolk is initially homogenous becoming segmented in late stages of embryonic development. Yolk segmentation can be a useful taxonomical character. The presence or absence of oil globules, size, number and position, are also important taxonomic characters. About 60 % of the species with described eggs have a single oil globule, 15 % have multiple oil globules and 25 % have none.
Individuals with a yolk-sac or remnants of yolk are referred as yolk-sac larvae or newly hatched larvae. Those that have used all their yolk are referred as larvae and early or late larvae. Finally those that are in the process of changing from larvae to juveniles are referred as transforming or transformation stage specimens. The larval period subsequent to the yolk-sac stage falls into three stages related to flexion of the notochord during caudal fin development. These three stages are termed preflexion-, flexion- and postflexion-stage larvae (Early life history stages of Diplodus sargus.).
The size and length at hatch varies among fish species, being generally related to egg or yolk diameter. Yolk size, in newly hatched larvae, is also related to egg size and to the amount of yolk used before hatching. Usually the body length at the time of hatching is about 2.5 to 3 times the diameter of the yolk. Newly hatched larvae frequently have an unformed mouth, unpigmented eyes and undeveloped pectoral fins. A prominent median finfold (primordial fin) is also present extending from the top of the head, around the caudal region and forward to the posterior margin of the yolk (Anatomic and morphometric features of yolk-sac larvae.).
The shape of the yolk sac varies greatly from round to elongate in species with elongated guts (e.g. Clupeoids). When present, the oil globule can be located anteriorly or posteriorly in the yolk. Multiple oil globules can be aggregated or evenly distributed. Location of the oil or oils globules is an important taxonomic character. Overall pigmentation is also very important as far as identification is concerned. Melanophores are the main pigments used for the identification of yolk-sac larvae. Other pigments may be present but most will be lost in preserved (formalin or alcohol) specimens. At the end of the yolk-sac stage the mouth and gut are formed and the anus is open at or close to the margin of the primordial fin (e.g. Gadoids). The eyes become pigmented and the major organs and sensory systems, essential for capturing preys, become functional.
The larval stage, as mentioned before, can be divided into three different sub-stages, based on the degree of flexion of the terminal section of the notochord.
Marine teleost larvae are extremely diverse reflecting an array of specializations in form, pigment pattern and behaviour. Body length at the beginning of the larval stage is about 4 to 5 times the diameter of the egg (the majority of first-feeding larvae are 3 to 6 mm in length). Most species reach maximum larval size within the 10-30 mm range. The basic organ systems are developed during the larval period. One obvious feature is the myomeres that correspond roughly to the number of vertebrae in the adult. Gut shape and length are also important features. Gut is elongated in lower telosts (clupeiforms, salmoniformes, stommiforms). On myctophids it varies from short to long. The gut is coiled in gadoids and higher teleosts. The relative size of the head, jaws and eyes are important taxonomic characters. The sequence of fin formation can be a useful character at specific or higher taxonomic levels. Larval teeth and head spines vary a lot in size, shape and pattern (Anatomic and morphometric features of fish larvae.).
Pigmentation provides a wealth of information. Preserved specimens are usually limited to melanistic pigment. Melanophore location, size, shape, pattern and sequence of formation are very important taxonomic features. Many species have unique pigmentation patterns that typify higher taxa. Several pigmentation patterns have evolved independently, such as rows of melanophores above or below the gut, over the gas bladder, on the ventral margin of the tail, over the head, along the dorsal margin of the body or covering the entire larva (Melanophore pigmentation of fish larvae.).
The larval stage is followed by a transformation stage. This stage is characterized by changes in general shape and structural detail that can be gradual to abrupt. In the majority of fish species, larval shape and form is very different from that of the juvenile.
Identification of early stages of fishes is not an easy task. Fish eggs and larvae are usually small requiring the use of a good stereoscopic microscope and adequate lighting. In a single ichthyoplankton sample there is usually a large variety of sizes, shapes and pigmentation patterns. Generally dichotomous keys can not be used since most of the important taxonomic characters change dramatically over the course of the development. Very few species have distinct features that can be recognized throughout the entire early life history. Is a given area a large proportion of the fish eggs and larvae may be unknown or undescribed.
Identifying a fish larva is very different from identifying an adult specimen. Its shape, size, stage of development, pigmentation pattern and myomere count are very important features. New ways of manipulating data are needed to reach identification. This has been called the "look alike" system. It consists of a simple procedure: one should be able to identify a fish larva to the family level by matching the characteristics of a specimen as far as general shape and striking features are concerned.
Illustrations of representative larvae of families of fishes occurring off the Iberian Peninsula.|
Illustrations of representative larvae of families of fishes occurring off the Iberian Peninsula.
Illustrations of representative larvae of families of fishes occurring off the Iberian Peninsula. RÉ, P., I. MENESES (2008). Early stages of marine fishes occurring in the Iberian Peninsula. IPIMAR/IMAR. This guide is intended for the identification of the Early Life History (ELH) stages of fishes collected by plankton nets from the marine and estuarine waters of the Iberian Peninsula (Eastern North Atlantic Ocean). The coverage area extends from latitude 34º - 45º north, to longitude 6º - 14º west. The basic characteristics of the eggs and larvae of 104 species belonging to 45 families are described. The emphasis has been placed on the most diagnostic or easily observed characters in order to facilitate comparisons between taxa. The descriptive accounts of this guide follow the format of previous ELH guides. Nomenclature follows Eschmeyer (1998) except for more recent changes. Within families, genera are listed in alphabetical order. Species descriptions are given only for species for which some ELH stages are known. Each species account includes the same basic information (written information on the left hand page and figures on the facing right hand page). Written information includes meristic data (fin-ray counts in adults and myomere counts), life history information (range, habitat, spawning season, ELH pattern), main references and ELH descriptions (eggs and larvae). Measurements of larvae usually refer to standard lengths. Many published illustrations have been redrawn mainly to provide certain uniformity throughout the guide. Sources of illustrations are given for every Plate. The contents of the present guide represent the current knowledge on the development of ELH stages of fishes occurring in coastal waters of the Iberian Peninsula. The authors have been involved, for more than 25 years, in ichthyoplankton research.