3.     Morphology and Biodiversity

The diversity of fish is larger than for any other vertebrate group. Not only are there more species of fish (over 25,000) than of all other vertebrates together, but also the range of body shapes and sizes of fish is larger than for mammals, birds or reptiles. Consequently, the range of habitat occupied is larger as well.

3.1.                     Diversity of Indo-Pacific shore fishes

The triangle formed by Indonesia, the Philippines and New Guinea, previously referred to as the ‘East Indies’, form the center of marine fish biodiversity in the Indo-Pacific, with about 2,800 species naturally occurring there. These numbers drop with distance from this center to, e.g., about 500 species in Hawaii and 120 species in the Easter Islands. The number of endemic species, i.e., fishes that do not occur outside a given area, increases with distance from the center, supporting the hypothesis that species evolved in the outer region and accumulated in the center. Another hypothesis holds that species evolved in the rich and stable habitats of the East Indies and were carried to the periphery by currents. Randall gives five explanations for fish biodiversity in the Indo-Pacific:

·        Sea surface temperatures in the East Indies were more stable during the glacial periods and thus extinction rates were lower than at the periphery;

·        Shelf area in the East Indies is much larger than that of the periphery, again making extinctions less likely;

·        Dispersal of shore fishes to remote islands occurs during the planktonic larval phase (which lasts from several days to several weeks). However, the larval phase of many species is not long enough for long stretches of open ocean water, thus restricting their distribution;

·        Existing current patterns support dispersal of fish larvae from, as well as convergence of larvae of species that have evolved in the periphery towards the East Indies;

·        During the last 700,000 years, there have been at least three ice age events that reduced the water level in the East Indies and separated populations long enough for speciation to occur.

3.1.1.      Exercise 3.1

Tasks for the student:

·         FishBase provides maps with species numbers by FAO area (all), country (all) and ecosystem (several). It also provides maps with indications of how many species have been collected within the cells of a grid system. Discuss the pros and cons of these approaches in mapping species diversity. [Hint: see biodiversity maps at www.fishbase.org/search.cfm].

·         Use Randall’s five explanations to discuss the pros and cons of the ‘dispersal from center versus the ‘immigration from periphery’ hypotheses.

Biodiversity-related topics in FishBase:

Distribution:                The FAOAREAS Table; The COUNTRIES Table; The COUNTREF Table; The OCCURRENCES Table

                                Plot occurrence records, families by FAO area, species by FAO area, species by climate, etc. using the Biodiversity Maps routines in www.fishbase.org/search.cfm.

3.2.                     Diversity of shapes and sizes

The shapes of fish are also extremely diverse, and include – besides the torpedo shape perceived as ‘typical’ for fishes and termed ‘fusiform’– shapes ranging from the serpentine (in the Anguilliformes and other orders) to the avian (in ‘flying fishes’), with Latimera chalumnae sporting limbs resembling, though not being used as, those of land-based tetrapods.

Shape and other morphological features are the key characteristics used to date for classifying fishes, and hence understanding their classification requires a basic overview of the basic shapes of fishes, as can be obtained from the outline drawings included in FishBase, for each of the existing 500 fish families.

Size is the most important attribute of individual organisms; it determines what can be their food, and the extent to which they can be the prey of other organisms. Size also determines how much food an animal requires to eat, how fast it can swim, and to a large extent, where it can live.

The maximum size of fish can range from one centimeter in Philippine gobies, e.g., Pandaka pygmea to 13-15 meters in the Whale shark, Rhincodon typus. This diversity of size allowed widely different environments to be colonized, ranging from temporary puddles to the central gyres of the open ocean. However, colonizing these environments required other adaptations, involving growth and mortality rates, and their various correlates, discussed below.

3.2.1.      Exercise 3.2

Tasks for the student:

·         Based on the data in Table 3.1 estimate the parameter a and b of a length-weight relationship of the form W = a × L^b, usable for predicting weights from length. The procedure to apply is the linear regression routine in Excel or another spreadsheet software after taking the logarithm of the length and weight observations. This implies plotting log (W) vs. log (L) in a regression of the form: log (W) = log (a) + b log (L), wherein log (a) is the intercept and b the slope or regression coefficient. Results should be presented with estimates of the precision of the a and b estimates. [Hint: The regression function in Excel is found in the Data Analysis option under the Tools menu.]

3.2.2.      Table 3.1

Mean fork length-at-age of the St. Lawrence River population of muskellunge, Esox masquinongy, adapted from Scott and Crossman (1973, p. 367).

^a) hypothetical data

3.3.                     Diversity of brain sizes

The brain size per body weight of adult animals is related to the sensory and behavioral capabilities of the species to which they belong. For example, fishes with well-developed electrosensing capabilities are known to have large brains. On the other hand, the brain is the body organ with the highest energy and oxygen demand, and thus, fishes as well as other animals have evolved brain sizes that are neither too small nor too large respective to the niches they occupy in nature. [Note as an aside that it is not true that people (at least most) use only 10% of their brain’s capacity].

3.3.1.      Exercise 3.3

Tasks for the student:

·         Based on your general knowledge about the fish and their habitat, rank the following groups according to their brain size: coral reef fish, deep sea fish, herrings, sharks, coelacanths. Explain in a few sentences why you ranked each group as you did.

·         For the groups listed above, find typical examples, look at their brain size compared to other fishes, and use these data to test  your hypothesis about their respective groups. [Hint: common names often contain parts of group names a group’s scientific name].

Brain size-related topics covered in FishBase:

3.4.                      Diversity of growth and mortality

In spite of the wide diversity of fish sizes mentioned above, clear patterns do emerge. One is that tropical fish tend to be smaller and faster-growing than their cold-water counterparts and that their natural mortality tends to be higher. This is due to high temperature elevating the metabolic rates of tropical fish relative to their cold-water counterparts (Pauly 1998).

This can be expressed by the parameters of the curve most commonly used to represent the growth of fish, the von Bertalanffy growth equation, which has the form:

L_t = L_¥ [ 1 – e ^{–K (t – t}₀⁾ ]                        … 4.1)

where L_t is the mean length predicted at age (t); L_¥ (‘L-infinity’), the mean size the fish would reach if they were to grow indefinitely; K is the rate at which L_¥ is approached (with dimension 1/time); and t₀ is the (usually negative) age the fish would have at length zero if they always grew as predicted by the equation (which they don’t).

Thus, when one estimates the parameter of the von Bertalanffy growth equation in tropical fish, one usually ends up with relatively low values of L_¥ and high values of K, at least as compared with their cold water analogs.

The L_¥ values estimated for fishes range from 1 cm in some short-lived gobies to around 14 m in long-lived whale sharks, as can be expected given to their maximum size (see above). Correspondingly, the natural mortalities experienced by fish, which are a function of their sizes, range from values which exterminate an entire cohort in a few months, e.g., the round herring, to 50 and more years in the lake sturgeon and 150 years in the orange roughy. These enormous differences in life span allow fish to respond differently to habitat variations. Small, short-lived fish track such variations, for example, when growing up in temporary puddles and laying desiccation-proof eggs before they dry up, thus being able to live through dry periods, or by spawning every year, but producing a successful cohort only one every 5-10 years (as may happen in such long-lived fish as cod).

3.4.1.      Exercise 3.4

Tasks for the student:

·         Estimate the von Bertalanffy growth parameters (L_¥, K and t₀) for the muskellunge based on the age-length data pairs in Table 3.1 . [Hints: The von Bertalanffy equation can be linearized through the expression L_t+1 = a + b × L_t wherein L_t and L_t+1 are the length at successive ages. Once a and b have been estimated by linear regression (see Exercise 3.2 ), L_¥ and K can be estimated from L_¥ = a/(1-b) and K = -ln (b); t₀ can then be obtained by solving the von Bertalanffy equation for a few L_t and t data pairs and averaging the solutions. Alternatively, a non-linear fitting routine, such as ‘Solver’, built in Excel can be used to solve simultaneously for L_¥, K and t₀ (see the Excel manual on how to use Solver).]

·         Identify 2 families, one tropical, one temperate, whose representatives share similar maximum sizes, and compare the distribution of their growth parameters on an auximetric grid.

·         Estimate value of natural mortality (M) from the relative abundance in Table 3.1 (4^th column). [Hints: M can be estimated as the slope (with sign changed) of the regression of ln (N) = a + b × t, where N is the number of fish in a cohort, and t their age. See Exercise 3.2 on how to do a linear regression, following transformation (i.e., linearization) of the input data.]

·         Compare natural mortality (M) estimates for 10 species of tropical fish, ranging between 50 and 100 centimeters maximum length, with 10 species of fish with similar sizes from cold waters and test for a temperature effect. [Hint: temperature and M values maybe found in the Life-history tool page.]

Size, growth and mortality-related topics covered in FishBase:

3.5.                      Diversity of habitats: inferences from occurrence records

Fish inhabit more diverse habitats than any other group of vertebrates, ranging from Himalayan or Andean brooks at 4000 meters to abyssal depth at 10 kilometers, thus spanning an extremely high range of pressures. The range of temperatures that can be tolerated is also very large, from minus 2^oC as for the Antarctic fish, Pagothenia borchgrevinki (which sport anti-freeze substances in their blood; see Eastman and Devries 1985); to up to 40^o C for Oreochromis alcalicus, which lives at the edge of a hot spring in Lake Nakuru in Kenya. (This does not consider the temperature tolerance of deep-sea vent fishes, which have not yet been studied in detail).

Because fish occur only in habitats which they can tolerate, and tend to be abundant in those habitats to which they are best adapted, occurrence records kept by museums can be used to reconstruct the habitat preferences of fishes whose ecology is otherwise unknown. Such records have been named bioquads because they refer to biodiversity and consist of four key elements: (a) the name of the organism; (b) the place where it was caught; (c) the source or person who sampled or identified it; and (d) the date. FishBase makes wide use of bioquads for documenting the distribution of fish and this can be emulated by ichthyology students who may assemble bioquads from FishBase and other sources, notably the Internet. (see Appendix A for sources of bioquads).

3.5.1.      Exercise 3.5

Task for the student:

·         Select a species in FishBase and print a point map as well as the point information. See whether you can find additional points in ichthyological museum collections (see Appendix A). Identify problematic records. Infer from the habitat (i.e., occurrence records) or the ecological requirements of that species. [Note: links to point maps are available from the Species Summary page. Point information details are shown by clicking on a point (or dot) in a map.]

Distribution and occurrence-related topics covered in FishBase:

3.6.                      Diversity of color and sexual selection

Fish are beautiful; they have strange body shapes and vivid colors, the latter a major reason why people keep them in aquaria. Color patterns in fish have been long misunderstood. Some pre-Darwinian authors thought that god had given fish such marvelous colors so that predators would find it easier to see and catch them. We know, since Darwin, that such coloring, if it serves any function at all, must benefit directly the individual sporting it and not their predators. This is now obvious in the many color patterns that camouflage their owner, or confuse predators, by, e.g., displaying large eyes in the wrong places. Darwin also proposed a reason why non-camouflaging, striking coloring should exist, and that is sexual selection.

Essentially, the males entice the females to choose them by displaying nicer colors than other males; they compete in terms of their ‘beauty’, this being related to good genes (remember Darwin did not know of genes and that part of his theory was very hard on him). Recently, the Zahavi’s complemented Darwin’s version of sexual selection through a new concept, the handicap principle, which takes into account that the colors and other adornments which males use to entice females to choose them are costly to produce (Zahavi and Zahavi 1997). Hence, the color and other adornments represent a handicap and the males capable of displaying these attributes thus must have really good genes for other life-supporting traits. We may call this ‘truth in advertisement.’

The idea is that sporting highly symmetrical patterns, as, for example, in Pomacanthus imperator, implies that the fish in question had a harmonious development since development problems, due to genetic problems, parasites or disease (also indicative of ‘bad genes’) would always lead to asymmetries. Also, for colors that do not necessarily camouflage the fish, sporting them indicates that the fish in question has been able to evade predators. Some fish, however, imitate the color patterns of other species to fool prey or predators (mimicry).

3.6.1.      Exercise 3.6

Task for the student:

·         Read chapter XII, ‘Secondary sexual characters of fishes, amphibians and reptiles’, in Charles Darwin’s Descent of Man, vol.2. Give a one-page summary of the argument and re-express the main line of Darwin’s argument using fish other than the ones in that chapter.

·         Give examples from FishBase for species that use color patterns for a) camouflage, b) predator confusion, c) sexual selection.

·         Give one example of mimicry in fishes. Explain the benefits gained. [Hint: common names of such species often contain the word ‘mimic’].

Morphology-related topics covered in FishBase:

3.7.                      Diversity of food and feeding habits

Given the diversity of their sizes and habitats, it is obvious that fish should also have a wide diversity of food and feeding habits. Thus fish range from feeding on microscopic phyto- and zooplankton to engulfing entire adult fishes, such as is done by Whale sharks or gulpers, respectively. Attempts to link fish to their ecosystems have led to a huge literature on their food and feeding habits. Unfortunately, some of this is useless because it is reported in the wrong units, i.e., frequency of occurrence of certain items in a number of stomachs sampled. Still, there are enough studies in which the proper units have been used (contribution in weight, energy or volume to total stomach contents) for a clear idea to emerge of what fish generally eat in their typical habitat. Given knowledge of the average trophic level of their diet items, the trophic level of fish whose stomach content has been studied can thus be computed, which allows evaluation of the position the consumers occupy in the food web.

3.7.1.      Exercise 3.7

Task for the student:

·         Find published studies on the diet composition of three different species of fish: one mainly herbivore; one omnivore, and one typical carnivore.

·         Compute their trophic levels using the classification of diet items and trophic level in Error! Not a valid bookmark self-reference..

Food and feeding habits-related topics covered in FishBase:

3.7.2.      Table 3.2

Hierarchy of food items, simplified from the FishBase table used to compute trophic levels (TL) from diet composition data. Therein, the TL of a consumer is 1 + (mean TL of the prey items).

a)       in FishBase, these food items have distinct trophic levels (and associated standard errors), not presented here.