For a species with a diploid number of \(18,\) indicate how many chromosomes will be present in the somatic nuclei of individuals that are haploid, tetraploid, trisomic, and monosomic.

The term haploid refers to a cell that contains one complete set of chromosomes. Organisms use haploid cells during sexual reproduction; in humans, these are the sperm and egg cells. When the diploid number for a species is 18, this signifies that a diploid cell has 18 pairs of chromosomes, making for a total of 36 individual chromosomes. To find the haploid number, you would halve the diploid number:
\[ \text{Haploid number} = \frac{\text{Diploid number}}{2} \]
In this case, it results in:
\[ \text{Haploid number} = \frac{18}{2} = 9 \]
Therefore, a haploid cell for this species would have 9 chromosomes. Haploid cells are crucial for maintaining genetic diversity through meiosis, where they combine with another haploid cell to create a genetically unique diploid organism.

A tetraploid organism has four times the number of chromosomes than what is found in a haploid cell. This is a form of polyploidy, a condition where cells have more than two paired sets of chromosomes. For a species with a diploid number of 18, calculating the number of chromosomes in a tetraploid cell involves multiplying the diploid number by two:
\[ \text{Chromosome number} = 2 \times \text{Diploid number} \]
Thus:
\[ \text{Chromosome number} = 2 \times 18 = 36 \]
A tetraploid cell has 36 chromosomes. Tetraploidy can occur naturally, especially in the plant kingdom, and can lead to larger cell sizes, potentially influencing the organism's overall size and characteristics.

The term trisomic describes a condition where an organism or cell has an extra chromosome in addition to the normal diploid set. This adds a third copy of a particular chromosome, hence the prefix 'tri'. In a species with a diploid number of 18, a trisomic cell would carry one additional chromosome:
\[ \text{Chromosome number} = \text{Diploid number} + 1 \]
Applying this formula gives us:
\[ \text{Chromosome number} = 18 + 1 = 19 \]
Therefore, a trisomic individual would have 19 chromosomes in their somatic cells. Trisomy can cause genetic disorders, such as Down syndrome in humans, which is the result of an extra copy of chromosome 21.

Monosomy is a chromosomal abnormality where there is a missing chromosome from the normal diploid set. In a species that usually has 18 chromosomes in its diploid cells, a monosomic cell has one less:
\[ \text{Chromosome number} = \text{Diploid number} - 1 \]
Making the calculation,
\[ \text{Chromosome number} = 18 - 1 = 17 \]
A monosomic organism would have 17 chromosomes in each somatic cell. Monosomy can be detrimental to an organism's development and, in humans, is often fatal or causes developmental disorders, such as Turner syndrome, which involves having only one X chromosome.